draft 7/10/23 then 7/15 7/17
Abstract. The formal literature on the Ivory-billed Woodpecker, Campephilus principalis, was reviewed and analyzed to establish an estimate of total population before anthropogenic effects for the species.
Exploring past population dynamics of Ivory-bills provides more information on natural trophic processes and niches that can be economically important to forestry and silviculture. The potential value of the species has been ignored which lessened, and lessens the management response by federal, state and local governmental departments.
Ivory-billed Woodpeckers were estimated to have a population of 57,696 animals 18,000 years ago (YA). Taking the above number of pairs, 28,848, potentially producing 2.11 fledglings per year per pair (Tanner 1942) gives 60,869 fledglings estimated in an assumed average year.
Total hypothetical maximum population after an average breeding season is estimated by combining adults and fledglings; 57,696 adults plus 60,869 fledglings, equals 118,565 Ivory-bills post breeding, 18,000 YA. The population of all species of birds drops post breeding due to predation disease, age, food resources etc.
Introduction
The formal Ivory-billed Woodpecker, Campephilus principalis literature concentrates on the specie's range, description, specimen records, interactions with other species including people, life history based mainly on the Singer Tract, LA and modern presentations of sightings and evidence. In an extensive literature review few details on the total population of the species from any date or time scale other than the 1930's (Tanner, 1942) was located. Minimal information can be found about the numbers of these large woodpeckers before anthropogenic effects began in the southeast United States.
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| The only detailed study of Ivory-billed Woodpeckers. (Tanner 1942) |
The Ivory-billed Woodpecker, if at carrying capacity, is a keystone species of significant ecological importance. Primary cavity nesters, such as woodpeckers, support community biodiversity and especially secondary cavity nesters. Picidae are an integral component of forest ecology.
The species past or possible economic importance is potentially substantial, but ignored; sustainable forestry was mostly unknown in prior centuries or deemed unimportant since forests were erroneously assumed to be impeding agriculture or a source of infinite firewood, fence posts, board feet, game animals, etc.
Ivory-bills with their unique and powerful bill and body morphology begin the pivotal "Tree to Soil" cycle ~ 24 months or more before any other Picidae can remove bark from a senescent tree exposing the cambium and heart wood. An intact Ivory-billed population can hypothetically increase long term productivity by up to 5% in some southeastern US forests as primary productivity, natural decomposition and silviculture regeneration rates are accelerated.
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| NBP's artist D. Tattoli draws Ivory-bills unique bill. Green lines indicate that the IBWO has its eyes positionally lower on head than the Pileated Woodpecker |
Ivory-billed Woodpeckers are relatively efficient within the Picidae at physically removing the hard bark covering of stressed or dead trees speeding up forest cycles, while converting wood boring insects into avian biomass. The ancestor species of the three northern clade species likley had
C. principalis has been noted to eat small beetle larvae (Dendroctonus spp.) prevalent under bark of recently stressed or dead pine trees. These beetles are considered one of the most important causes of economic loss in forestry (Ungerer 1999). Ivory-bills are a natural control on forest boring insects but the USDA, US Forest Service and the USFWS are confronted with fragmented, managed forest blocks and policies that immediately default to costly pesticides, or tree removal to address insect outbreaks. A balanced approach that included Ivory-billed management over the last 60 years may have resulted in a better integrated, efficient and effective result if coordinated governmental forestry and wildlife goals are properly merged. Department wide and interdepartmental MOAs united in a sustainable goal could have greatly lessened the large costs of pesticides and increased forest output while retaining biodiversity.
The most efficient ecosystems for converting solar energy into plant material and vertebrate biomass are the complex assemblage of biodiversity in East Africa.
"The combination of volcanic soils combined with the ecological impact of the migration results in one of the most productive ecosystems on earth, sustaining the largest number of ungulates and the highest concentration of large predators in the world." Source: UNESCO--Productivity of the SerengetiThe Serengeti produces a much greater biomass of plants and animals than anthropogenically managed open spaces. The assemblage of animals there accelerates basal rates of primary production and regenerative cycles. The Ivory-billed efficiently contributed to forest regeneration for possibly a million years. The Ivory-billed is a relatively large, understudied, forest bird; a keystone species that likely can increase economic output of some US forests if properly managed.
The US Department of the Interior, the US Fish and Wildlife Service, etc., have not properly valued the potential irreversible economic impact if the species is perpetually lost. Government entities have historically and recently underfunded the research, management for, and protection of, this valuable natural resource despite being mandated to preserve economic, recreational and natural resources for present and future American generations.
Exploring past population dynamics of Ivory-bills provides more information on natural forest processes that can be economically important to forestry and silviculture; Ivory-bills are an important resource but of course being critically endangered their beneficial impact is now miniscule.
Using the data and constructs detailed here an estimate is calculated for the total Ivory-billed population circa 18,000 years ago (18K YA) before many people (Pre-Clovis or Clovis) had arrived in the SE USA. By the 19th century there were certainly less birds than 18 K YA as anthropogenic effects were substantial; habitat destruction combined with increasing and more efficient direct taking of birds had and was accelerating.
Tanner (1942, p.31) said this about recent historical times:
None of the earlier accounts of the Ivory-billed Woodpecker contained accurate or definite statements as to the abundance of the bird. Judging from the interest that naturalists and collectors had in the Ivory-bill, and the accounts they wrote, it was never common. Most writers mentioned the Ivory-bill as being a rare bird, or an uncommon one, and some heightened the difficulties they had in securing specimens.
And:
As an example in North America, there have been estimates of the American Bison population number that are derived from descriptions and direct field observations related to the size of large herds. From such recollections and accounts, historians, anthropologists, and zoologists have estimated the original or pre-settlement population size for the species.Audubon usually described the big woodpecker as being quite rare.
For the Ivory-billed, a forest interior species, we will never have an
answer on historical numbers meeting today's standards of modern wildlife
research, which demands random or systematic sampling and replication. But the estimates that have been made for the American Bison and other species show us how numbers are deduced.
Any estimate of a derivative of a dynamic complex ecosystem will have uncertainties. For example, it is not realistic to presume that before the Pre-Clovis and Clovis cultures reached the southeast United States that the Ivory-billed population was stable. Floods, droughts, large ice or log jams, beavers, insect outbreaks and fire regimes caused landscape scale ecological impacts pertinent to the Ivory-billed population demographics.
Any estimate of a derivative of a dynamic complex ecosystem will have uncertainties. For example, it is not realistic to presume that before the Pre-Clovis and Clovis cultures reached the southeast United States that the Ivory-billed population was stable. Floods, droughts, large ice or log jams, beavers, insect outbreaks and fire regimes caused landscape scale ecological impacts pertinent to the Ivory-billed population demographics.
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| Riparian forest aerial, a small portion of the Ivory-bills possible habitat today; the remaining acres are greatly reduced and of lower quality and heterogeneity. |
Methods
Three approaches have traditionally been used to estimate animal population sizes: direct observations, estimates of numbers
killed, and estimates based on carrying capacity. There is enough Ivory-billed observations to deduce a total population estimate since in its simplest form it equals the species entire range (breeding range) times the number of birds per area as reported from the field.
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| Some animals are relatively simple to count |
For the Ivory-billed, historical observations inclusive of birds killed are used as base line data yielding animals per area. Tanner and others have examined these basic densities. Ecological interpretations on what was the likely total range of the species such as typical impacts of latitude, ocean currents, plains effect, etc., on climate and carrying capacity are utilized here.
On age demographics, including the ratio of non-breeding adults to breeding in the few historical metapopulations studied, there is little data. Conclusions on age structure, with veracity, could be made by examining A. T. Wayne's extensive field notes and papers which were not available online (personal communications, 10/2022). They can be accessed by a visit to the Charleston Museum of Natural History. Combining Wayne's info et al. with a coordinated examination of most or all skins, noting plumage details, age, molt, etc. would provide some information on the numbers of animals in respective age cohorts of the species.
Here this work rationalizes, or perhaps idealizes, the following unknowns: The pertinent field studies' birds, at Wacissa and California Swamps, which were counted as breeding adults, may not have all been adults. This research, roughly negates or nets out two drivers as following: the number of adult birds that were missed in the verbal population estimates of the time equals the number of birds that were actually fledglings, but counted as adults.
Undercounting actual numbers via field encounters and number of birds shot is very likely even in formal field studies since detection rates are needed (Buckland, et al., 2010). A. T. Wayne was collecting rather than studying the birds; this makes Tanner's research work important in the context of the subject of demographics.
The temporal point chosen for this estimate, 18,000 YA, accomplishes three things that are required to increase the population estimate accuracy of a naturally functioning system with no anthropogenic in puts but is still similar to the 19th century's climatic conditions when the most pertinent field data used was gathered.
Three Requirements
1) 18,000 YA, is ~ 2,500 years after the accepted date for the glacial maximum, assumed in this research to be an interglacial period with climate somewhat similar to the 19th century with annual and seasonally ice-free land mass available to the Ivory-billed approximately the same
2) 18,000 YA is before more than negligible anthropogenic effects on Ivory-billed numbers developed
3) 18,000 YA meets prerequisites 1) and 2) and is as close as possible to 19th century condtions when the main population data was gathered
The temporal point, 18,000 YA is assumed to have had a similar climate, ocean level and seasonally ice-free summer land mass available to the Ivory-billed as was in the 19th century. By choosing 18,000 YA we matched as closely as possible the forest conditions of the three most significant studies of Ivory-billed population density. In general, these three forested areas were considered to have late seral forests with old growth characteristics similar to the habitat of 18,000 YA. Tanner, 1942 used "primitive" to describe these areas.
The temporal point, 18,000 YA is assumed to have had a similar climate, ocean level and seasonally ice-free summer land mass available to the Ivory-billed as was in the 19th century. By choosing 18,000 YA we matched as closely as possible the forest conditions of the three most significant studies of Ivory-billed population density. In general, these three forested areas were considered to have late seral forests with old growth characteristics similar to the habitat of 18,000 YA. Tanner, 1942 used "primitive" to describe these areas.
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| Congaree National Park, NBP performed point surveys in over 30 locations |
Unfortunately, Ivory-bills do not aggregate in thermals, fall out spots, migratory stopovers or roosting sites. These phenomena have been used to help determine population size and the age structure demographics of birds with large but scattered metapopulations that are difficult to monitor.
Obviously there were no surveys of any type done 18,000 YA; reliance on the various data gathered much later is temporally adjusted 180 centuries.
In this research the small number of studies together with anecdotal information strongly supported that C. principalis density had a north to south gradient which is to be expected for a species with nestlings that are obligate insectivores. This enforces known ecological drivers on population demographics of many species related to latitudinal, climate related clines (Hut, 2013) with actual observations of Ivory-billed population density. This supports the method of having two different bird densities based on the many biotic and abiotic characteristics latitudinal clines encompass, to more accurately model the Ivory-billed Woodpeckers' population.
Included are several million acres of breeding habitat outside of Tanner's generally accepted historical breeding range map as the target time here is 18,000 YA. The geographical breeding range here is also influenced by writings of P. Kalm (IBWO seasonal to SW New Jersey in the 1700s inferring MD for this analysis), J. J. Audubon (states to Maryland in the 1800s, inferring NC and VA), T. Jefferson (places the species in Virginia inferring VA and NC) and Hasbrouck (historical range into central coast of NC).
NBP during modern Ivory-billed studies employing advanced attraction techniques, accumulating data over 650 points from many locations. In a few forests, and from year to year, these locations supported the pair range estimate in this paper; NBP found that a pair of Ivory-bills could have a range of ~ 10,000 acres.
Post-breeding population estimate for Ivory-billed Woodpecker:
Obviously total population of all species of birds is maximal right after the breeding season. Populations can literally double or more in a few months; the increase is due to eggs hatching and fledglings leaving the nests.
At the Singer Tract, Tanner and Kuhn reported observing 6 Ivory-billed nests which had 9 broods from 1931-39 (Tanner, 1942 p. 81). The Ivory-bill had 19 young leave the nest from 9 broods; this is 2.11 fledglings/brood. Three broods failed. This is direct Ivory-billed field data on success of their breeding phenology. This is a small location sample size (n = 1) however the data was accumulated over several years with several nesting pairs.
Taking the above total pair number for the SE USA 18,000 YA we have 28,848 pairs potentially producing 2.11 fledglings per year per pair. This equals 60,869 fledglings.
Total population after breeding season is estimated at 57,696 adults plus 60,869 fledglings which equals 118,565 Ivory-bills.
On average these young birds will have a high mortality rate in their first year, while the adults will have a lower mortality rate. After the birds are fledged the total Ivory-bill population would have dropped until the next years breeding season due to natural mortality.
Discussion
Tanner and his Cornell professors overestimated the amount of virgin forest in the Singer Tract; this infers the same forest, 18,000 years ago, may have supported more IBs than J. J Kuhn's Singer. For the N half of the IB's range the winters are colder and trophic resources available would have been reduced and less varied comparatively to the S half of the range. Intraspecific carrying capacity proportionally drops for more northerly latitudes for many semi-tropical and temperate taxa.
References
Matthew J. Ungerer & Matthew P. Ayres & MarÃa J. Lombardero (1999). "Climate and the northern distribution limits of Dendroctonus frontalis Zimmermann (Coleoptera: Scolytidae)" (PDF). Journal of Biogeography.
Boyer W. D. (1990). Pinus palustris Mill. Longleaf Pine, Pinaceae -- Pine family Boyer
Hut R. A. (2013)
Latitudinal clines: an evolutionary view on biological rhythms
https://royalsocietypublishing.org › doi › rspb.2013.0433
Lammertink. Status and conservation of old-growth forests and endemic birds in the pine-oak zone of the Sierra Madre occidental Mexico (1996). This reference reports the total species range for the IMWO was 39,920 square miles and each individual needed 13 square Km. Imperial Woodpecker Population Estimate
by · Cited by 239 — We review literature with respect to latitudinal clines
end of virrazzi paper
Raw Data
Beavers---to Fv trapping significant before 1800
Probably because of their feeding and breeding dependence on large old trees, great slaty woodpeckers are most common in primary forests and show density reductions of over 80% in logged forests.[6] The global population is in decline because of the loss of forest cover and logging of old-growth forest throughout its range, with habitat loss being particularly rapid in Myanmar, Cambodia and Indonesia which are the countries that still hold the majority of the global population. In 2010, the great slaty woodpecker was included in the IUCN Red List in the Vulnerable category.[1]
We conclude, first, that spatial scale has indeed had an important effect on the characterization of the Panama bird community. The intrinsically patchy distributions of most forest-dwelling bird species raise the need for large-scale censuses. Second, the Panama community, compared with the two Amazonian sites, has a fundamentally different organization; it hosts nearly twice as many individual birds and is distinctly less dominated by rarity. Similar patterns of community structure appear to be present within tree and mammal communities as well. Therefore, results from the Amazonian studies cannot be generalized to all lowland Neotropical communities. We attribute differences in community structure primarily to differing biogeographic histories. The lower species richness and the greater number of total birds present in Panama appear to derive, at least in part, from two important factors: an area effect linked to the location of Panama on a narrow isthmus, and the repeated history of disturbance on multiple temporal scales in Panama.
This picture has been disputed by Noel Snyder, who contended that hunting rather than habitat loss had been the primary cause of the population decline. He argued that Tanner's population estimates were made of an already depleted population, and the home range needs were significantly smaller.[48]
synder inserts into
For species like the Ivory-billed the probability of detecting single birds or groups of birds by means of audio point surveys or visual surveys may be asymmetrical due to the specie's range of behaviors and/or habitat and topographic heterogeneity. To assess these potential biases, it is necessary to compare results obtained from various surveys with estimates derived from population models or idealized, comparative matrixes. These are possible future research topics.
To date, only four, liberally called surveys or population studies have been performed, due to the inherent difficulty in obtaining long-term population data for this and other long-lived vertebrate populations. These studies, Tanner, 1942, Cornell Univ./USFWS Arkansas Effort 2004 to 2006, Cornell Mobile Team ~ 2006 to 2008, and Virrazzi, National Biodiversity Parks, Inc. 2022 all detected small numbers of Ivory-bills.
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| Typical Point Survey Conditions |
These studies occurred after the Ivory-billed had suffered not only habitat destruction but also direct shooting for various reasons. C. principalis numbers were greatly reduced by the 20th and 21st centuries. These 21st century studies do give some limited, nuanced information about facultative habitat occupancy of the Ivory-billed 18,000 YA. National Biodiversity Parks, Inc (NBP) detected Ivory-bills in a variety of USDA forest types in three states. NBP by finding birds in different USDA defined ecological communities influences this research to assume occupancy in a variety of states and forest types of 18,000 YA. The Ivory-billed was likely not obligate to either conifer or deciduous forests; its modern preference for wet, seasonally flooded riparian habitats is a response to human predation of the Anthropocene which began ~ 15,000 YA.
Here are articles on some of NBP's field research:
SUMMARY NBP'S IBWO FIELD WORK
NBP IBWO STUDIES in CONGAREE NP
Here are articles on some of NBP's field research:
SUMMARY NBP'S IBWO FIELD WORK
NBP IBWO STUDIES in CONGAREE NP
Obviously there were no surveys of any type done 18,000 YA; reliance on the various data gathered much later is temporally adjusted 180 centuries.
This research may provide insight into how modern landscape-level simulations might eventually replace the historical estimates with more granular based ranges of possible Ivory-billed population sizes.
These maps or similar can be used to model improved granular population densities depending on variables such as forest type which each can be assigned, with ecological analysis, different population densities.
These maps or similar can be used to model improved granular population densities depending on variables such as forest type which each can be assigned, with ecological analysis, different population densities.
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| This map closely represents the amount of forest that may have existed 18,000 Years Ago |
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| USDA forest type information can improve population estimate for the Ivory-billed |
In this research the small number of studies together with anecdotal information strongly supported that C. principalis density had a north to south gradient which is to be expected for a species with nestlings that are obligate insectivores. This enforces known ecological drivers on population demographics of many species related to latitudinal, climate related clines (Hut, 2013) with actual observations of Ivory-billed population density. This supports the method of having two different bird densities based on the many biotic and abiotic characteristics latitudinal clines encompass, to more accurately model the Ivory-billed Woodpeckers' population.
The following estimated acres list is based on NBP's internal range map for each significant state's respective area that contained breeding or likely contained breeding IBWO's. It is assumed that 18,000 YA was an interglacial period similar to the 19th century today. M equals a million acres in the list below.
Included are several million acres of breeding habitat outside of Tanner's generally accepted historical breeding range map as the target time here is 18,000 YA. The geographical breeding range here is also influenced by writings of P. Kalm (IBWO seasonal to SW New Jersey in the 1700s inferring MD for this analysis), J. J. Audubon (states to Maryland in the 1800s, inferring NC and VA), T. Jefferson (places the species in Virginia inferring VA and NC) and Hasbrouck (historical range into central coast of NC).
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| Various references indicate that even in 1600 the Ivory-bills range was larger than this depiction. |
The contemplation of range treatment here is subjectively weighed by the ecological impact and moderation of the eastern coastal plain's climate by the Atlantic Ocean's Gulf Stream compared to the cold winters of the Ohio Valley and other mid-west areas due to the plains affect.
MD 1 M
VA 2 M
NC 5 M
SC 10 M
GA 10 M
FL 40 M
AL 20 M
MS 15 M
LA 30 M
TX 30
AR 15 M
OK 10 M
MO 5 M
TN 5 M
IL 2 M
IN 1 M
KY 3 M
TOTAL 203 million acres
To estimate the carrying capacity for these two equally sized cohorts, north and south, we started with Tanner's summary (Tanner, 1942, pg 32) which is based on actual field work by himself and A. T. Wayne in the 19th and 20th centuries.
"This gives us three estimates of the abundance or density of Ivory-bills in primitive areas: in Louisiana, seven pairs in 120 square miles or one pair per seventeen square miles; in California swamp in northern Florida, about six pairs in sixty square miles or one pair per ten square miles; in Wacissa swamps in northern Florida, about twelve pairs in seventy-five square miles or one pair per six and a quarter square miles."
Two separate estimates of the Ivory-bills abundance or population density, one for the northern half of the total range acreage and one for the southern half are summated to obtain a total population number.
Conclusions
Tanner's and J.J. Kuhn's Singer Tract observations and density results is used for the N half of the Ivory-bills range. One pair per seventeen square miles is the estimated density for the N half of the IB's range. Each pair occupied 10,880 acres. The N half of the Ivory-bills range is modeled at 101,500,000 acres. 101,500,000 acres divided by 10,880 acres/pair = 9,329 pairs.
This equals 9,329 pairs x 2 = 18,658 birds
For the S half the two densities Tanner deduced from A. T. Wayne's field notes are averaged for the Ivory-bill from the two Florida areas; this results in one pair per 8.125 square miles.
This equals 19,519 pairs x 2 birds/pair = 39,038 birds
Total = 18,658 birds + 39,038 birds = 57,696 Ivory-billed Woodpeckers, estimated 18,000 years ago.
The Imperial Woodpecker (IMWO) has a maximum historical population estimate of 8,000 according to the article: Status and conservation of old-growth forests and endemic birds in the pine-oak zone of the Sierra Madre occidental Mexico (1996). This reference reports the total species range for the IMWO was 39,920 square miles and each individual needed 13 square Km. Imperial Woodpecker Population Estimate
Considering the two congeners and adjusting for respective total species range in square miles yields:
IBWO 317,187 square miles
IMWO 39,920 square miles
The range of the IBWO is 7.9 times that of the IMWO and the two species respective total population estimates are 57,696 vs 8,000. The IBWO has 7.2 times the population of the IMWO in 7.9 times the area. This infers that carrying capacity of the two species per area is somewhat similar with the Ivory-billed needing about 10% more area per pair.
The world's largest woodpecker is the likely extinct Imperial Woodpecker, which averaged 58 cm (23 inches) in length and is estimated to weigh over 600 g (1.3 lb). The Ivory-billed Woodpecker is smaller, with a length of 50 cm (20 inches) and a weight of 500 g (1.1 lb).
Ecological success and species abundance is a correlate of a species' ability to persist through time and is driven by multiple factors that cause differential survival and reproduction. The abundance and density of congeners can be considered as a measure of ecological success between species.
The Imperial had pragmatically no large Picidae competitor as does the Ivory-billed in the Pileated. The Pale-billed Woodpecker was only marginally sympatric with the Imperial in Mexico. The trophic niche of the C. imperialis was therefore larger than C. principalis'; it has been observed to feed on fallen logs even at a time when sizable areas of large dbh trees were still found in Mexico. Horizontal, prone logs were a part of the forest strata they utilized. This microhabitat alone can account for a substantial increase for the Sierra Madre's forest's carrying capacity for a large and efficient Campephilus.
A careful Bayesian analysis considering several major ecological differences would likely establish that the Ivory-billed may have had a density per area similar to the larger Imperial. The literature and examination of the ecology of the respective species is consistent with population estimates deduced here.
As said the Imperial seems to have had a broader niche than the Ivory-billed. W. L. Rhien observed Imperials foraging on fallen logs in comparison to Ivory-bills (Tanner, 1964). This niche is dominated by Pileated Woodpeckers in the southeast United States.
Population estimates of 57,696 for IBWO and 8,000 for IMWO are not in opposition but confirmatory.
NBP during modern Ivory-billed studies employing advanced attraction techniques, accumulating data over 650 points from many locations. In a few forests, and from year to year, these locations supported the pair range estimate in this paper; NBP found that a pair of Ivory-bills could have a range of ~ 10,000 acres.
Post-breeding population estimate for Ivory-billed Woodpecker:
Obviously total population of all species of birds is maximal right after the breeding season. Populations can literally double or more in a few months; the increase is due to eggs hatching and fledglings leaving the nests.
At the Singer Tract, Tanner and Kuhn reported observing 6 Ivory-billed nests which had 9 broods from 1931-39 (Tanner, 1942 p. 81). The Ivory-bill had 19 young leave the nest from 9 broods; this is 2.11 fledglings/brood. Three broods failed. This is direct Ivory-billed field data on success of their breeding phenology. This is a small location sample size (n = 1) however the data was accumulated over several years with several nesting pairs.
Taking the above total pair number for the SE USA 18,000 YA we have 28,848 pairs potentially producing 2.11 fledglings per year per pair. This equals 60,869 fledglings.
Total population after breeding season is estimated at 57,696 adults plus 60,869 fledglings which equals 118,565 Ivory-bills.
On average these young birds will have a high mortality rate in their first year, while the adults will have a lower mortality rate. After the birds are fledged the total Ivory-bill population would have dropped until the next years breeding season due to natural mortality.
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| Ivory-billed nestling about 8 days before fledging |
Discussion
Modeling ecological characteristics of several different forest types in relation to an abundance or population density, could provide a more accurate estimate of the past IBWO population. There are five different major USDA forest types in the Ivory-bills habitat (see maps). The literature ranges within being adequate, uneven, silent or nonspecific on the relative carrying capacity and occupancy by IBWOs in these different forest types; however inferences can be made.
But better estimates are not a pressing conservation issue for the modern Ivory-billed; any additional justified variables will retain assumptions on the species demographic response to different woodland communities since raw data varies or does not exist for forests. The management of the Ivory-billed needs more attention.
Objectively, from the totality of the C. principalis reports, more southerly populations within the range were specifically highlighted for their relative abundance compared to all other areas (Tanner, 1942). This enforces known ecological drivers on population demographics of many species related to latitudinal, climate related clines (Hut, 2013) with actual observations of Ivory-billed population density. This supports the method of having two different bird densities based on the many biotic and abiotic characteristics latitudinal clines encompass, to more accurately model the Ivory-billed Woodpeckers' population.
Two separate estimates of the Ivory-bills abundance or population density, one for the northern half of the IBs range and one for the southern half of the total range are summated to obtain a total population number in those acres.
Tanner, used his own observations and others (J.J .Audubon, A. T. Wayne, et al.) to relate that the peninsular Florida, Gulf regions of the Suwanee River such as California Swamp, Wacissa wetlands, and Buffalo Bayou (present Houston. Tx area) had a relative abundance of Ivory-bills. These three areas are all in the southern half of the Ivory-billed's range as defined in this research:
Location --------------Latitude (S to N)
Buffalo Bayou-------29.7400373,-95.357490
California Swamp---29.7612452,-83.054545XXXX
Wacissa Swamps---30.237583,--83.984288
Singer Tract--------- 32.3016241,-91.9054949
Tanner and his Cornell professors overestimated the amount of virgin forest in the Singer Tract; this infers the same forest, 18,000 years ago, may have supported more IBs than J. J Kuhn's Singer. For the N half of the IB's range the winters are colder and trophic resources available would have been reduced and less varied comparatively to the S half of the range. Intraspecific carrying capacity proportionally drops for more northerly latitudes for many semi-tropical and temperate taxa.
The two prior inputs or factors, one, underestimating the maximum carrying capacity of the Singer Tract by Cornell's/Tanner and, two, the decreasing carrying capacity with more N latitudes, are assumed to offset each other's carrying capacity impact for the N half of the IB's range. Further research into the differences in carrying capacity in relationship to latitude in a semi-tropical zone of North America will likely result in a better estimate of carrying capacity and population but any adjustments to the final estimated numbers should be minimal.
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| Ivory-billed Woodpecker in cavity. Copyright to Academy of Natural Science VIREO |
Buckland, S. T., Studeny, A., Magurran, A. E., and Newson, S. E. (2010). “Biodiversity monitoring: the relevance of detectability,” in Biological Diversity: Frontiers in Measurement and Assessment, eds A. Magurran and B. McGill (Oxford: Oxford University Press), 25–36.
Boyer W. D. (1990). Pinus palustris Mill. Longleaf Pine, Pinaceae -- Pine family Boyer
U.S. Fish and Wildlife Service.
200x. Recovery Plan for the
Ivory-billed Woodpecker
(Campephilus principalis).
U.S. Fish and Wildlife Service,
Atlanta, Georgia. 156 pp.
Hut R. A. (2013)
Latitudinal clines: an evolutionary view on biological rhythms
https://royalsocietypublishing.org › doi › rspb.2013.0433
Lammertink. Status and conservation of old-growth forests and endemic birds in the pine-oak zone of the Sierra Madre occidental Mexico (1996). This reference reports the total species range for the IMWO was 39,920 square miles and each individual needed 13 square Km. Imperial Woodpecker Population Estimate
by · Cited by 239 — We review literature with respect to latitudinal clines
end of virrazzi paper
- Carbaryl
Raw Data
Beavers---to Fv trapping significant before 1800
Probably because of their feeding and breeding dependence on large old trees, great slaty woodpeckers are most common in primary forests and show density reductions of over 80% in logged forests.[6] The global population is in decline because of the loss of forest cover and logging of old-growth forest throughout its range, with habitat loss being particularly rapid in Myanmar, Cambodia and Indonesia which are the countries that still hold the majority of the global population. In 2010, the great slaty woodpecker was included in the IUCN Red List in the Vulnerable category.[1]
We conclude, first, that spatial scale has indeed had an important effect on the characterization of the Panama bird community. The intrinsically patchy distributions of most forest-dwelling bird species raise the need for large-scale censuses. Second, the Panama community, compared with the two Amazonian sites, has a fundamentally different organization; it hosts nearly twice as many individual birds and is distinctly less dominated by rarity. Similar patterns of community structure appear to be present within tree and mammal communities as well. Therefore, results from the Amazonian studies cannot be generalized to all lowland Neotropical communities. We attribute differences in community structure primarily to differing biogeographic histories. The lower species richness and the greater number of total birds present in Panama appear to derive, at least in part, from two important factors: an area effect linked to the location of Panama on a narrow isthmus, and the repeated history of disturbance on multiple temporal scales in Panama.
FB 9/12/22
Modern Carrying Capacity--Why the Low numbers-Critical Topic
Someone asserted there may have been 500,000 IBWO at one time. This is a high number, only possible if we do not agree on the probable or actual niche then. An unrealistic broad niche, drastically exaggerates derivatives like pre-contact, 1930's, and modern population numbers and carrying capacity. My early draft pre-contact number is 150,000 birds. (Today there may be 1.9 million PIWOS and their peak could have been 25 million birds).
PIWO may have 150 times the popalation of IBWO at one time; in the SE USA the ratio would been much less elevated.
PIWO may have 150 times the popalation of IBWO at one time; in the SE USA the ratio would been much less elevated.
Since this is a critically endangered species, actual niche and modern carrying capacity should be carefully discussed. Taking acres of land from 1800 via time tunnel and doing a simple acre to acre extrapolation, with today's low quality forest, small linear patches, and loss of genetic viability is the raw material that leads to an unrealistic, unfounded and likely erroneous modern carrying capacity numbers.
Today we have a somewhat stabilized acreage number in some areas of very slowly increasing DBH forests. Good ecological news seems to end there. Linear shaped acreage linking these areas is under pressure and these greenways are slowly to quickly, being destroyed.
Historically we have a small fraction of the original acres this species occupied, remaining forest is mainly mid-seral, fragmented habitat, greenways being attrited, global warming causing unknown and known ecological asynchronicities (diseases spread/mutate faster, invasive species in the hundreds. unprecedented destruction in the batture forest, light pollution adversely affecting coleopteran biomass, feral pigs driving snakes into canopy eating birds/eggs, known inbreeding by 1905 and much more).
Field reports are basically disappointing. The preprint paper reporting on 3 birds in LA 2019 dropping to one or two birds that dozens of people have encountered over 15 years in the same general spot. It's a small number of birds seen infrequently in a large area that have been reported over and over.
Older references point to IBs reaching highest numbers in large scale, forest mosaics of upland pines (prone to burn) with interdigitized forest bottoms (do not burn; low food for nestlings, but good for nest location due to water at base of tree). This optimal habitat landscape is basically gone in the SE USA, without basic management.
The IBWO's niche likely was and is forest areas with the highest 2-3 year post senescent standing deadwood. Fires, hurricanes, beaver, etc provided that. As the concentration of this age cohort of substrate drops carrying capacity drops below support level, as is the case in most modern areas. As the species was forced back into more inaccessible areas (forest swamps, bottoms, batture forests) it's faced with foraging in forests that have less 2 year dead wood than it needs to consistently fledge even 1 bird/year let alone 3.
Its forced to excavate (time consuming, increased time from nest) rather than the more efficient scaling; it has a morphological and behavioral advantage and predilection to scale recently dead wood.
Behaviorally the modern IBWO is not the same as the precontact animal. Due to nominal microevolution caused by severe and persistent anthropogenic taking of the least wary bird (subsistence, museum, and curiosity hunting) the bird now instinctively is "hard-wired" to prefer the most secluded section of forest which may be far from quality feeding areas for themselves or nestlings. Today the habitat landscape is dominated by short to longer, narrow river forests. The IBWO being instinctively fearful of people results in it foraging away from habitat patch/forest edge as you would if you instinctively were programmed to avoid, for example lions, which are in the fields and shade of the forest edge and they will kill you. So the aerial of a 10 mile long and 1 mile wide river forest habitat may seem to be 10 sq miles but an IBWO may perceive it as only 5 sq miles further reducing carrying capacity.
Finally the IB was already showing a loss of genetic variability before 1900. It's likely that there is some increase in embryological or hatchling mortality via various genetically based mechanisms that have increased the chances of homogygosity of deleterious alleles.
Behaviorally the modern IBWO is not the same as the precontact animal. Due to nominal microevolution caused by severe and persistent anthropogenic taking of the least wary bird (subsistence, museum, and curiosity hunting) the bird now instinctively is "hard-wired" to prefer the most secluded section of forest which may be far from quality feeding areas for themselves or nestlings. Today the habitat landscape is dominated by short to longer, narrow river forests. The IBWO being instinctively fearful of people results in it foraging away from habitat patch/forest edge as you would if you instinctively were programmed to avoid, for example lions, which are in the fields and shade of the forest edge and they will kill you. So the aerial of a 10 mile long and 1 mile wide river forest habitat may seem to be 10 sq miles but an IBWO may perceive it as only 5 sq miles further reducing carrying capacity.
Finally the IB was already showing a loss of genetic variability before 1900. It's likely that there is some increase in embryological or hatchling mortality via various genetically based mechanisms that have increased the chances of homogygosity of deleterious alleles.
Our science based surveys lasting months camping located several birds over many years. However in only one high DBH forest did we surmise the density of a pair in ~ 6K acres. Overall we encountered only 0 to 1 bird every 30K acres covered at locations that some would call good habitat by basic assessment, but not necessarily one we agree is optimal without management or fire (to increase deadwood/concentrating beetle larvae reducing nest predation).
Tanner stated pg 32 or 33 that the maximum IBWO abundance was one pair in 6 sq miles (4,000 acres). This max estimate were from forests long gone; replaced with houses, or at best substantially lower quality habitats in many, many ways. Mid seral or even older forest likely do not have enough substrate for a strong species recovery and maybe any recovery. Management may be the only hope. This is independent of how many pictures we have or do not have.
Regardless ecology is complex; the IBWO has somehow persited, microevolution is rapid and things can by definition change rapidly for the better.
Regardless ecology is complex; the IBWO has somehow persited, microevolution is rapid and things can by definition change rapidly for the better.
A few years later, James Tanner began a study of Ivory-bills as part of his Ph.D. dissertation and eventually wrote an in-depth report, funded by the National Audubon Society, that was later published as The Ivory-billed Woodpecker. His report estimated that only 22 to 24 of the birds remained in the United States.
Population number
According to the IUCN Red List, the total Ivory-billed woodpecker population size is fewer than 50 individuals and mature individuals.Records exist of the ivory-billed woodpecker farther north along the Atlantic Coast; Thomas Jefferson included it as a bird of Virginia in Notes on the State of Virginia, where it is listed as the "White bill woodpecker" with the designation of Picus principalis.[41][42] Audubon reported the bird could occasionally be found as far north as Maryland.[43] Pehr Kalm reported it was present seasonally in Swedesboro, New Jersey in the mid-18th century.[44] Farther inland, Wilson reported shooting an ivory-bill west of Winchester, Virginia.[39] Bones recovered from the Etowah Mounds in Georgia are generally believed to come from birds hunted locally.[35] Within its range, the ivory-billed woodpecker is not smoothly distributed, but highly locally concentrated in areas where the habitat is suitable and where large quantities of appropriate food may be found.[28]
An internet post on Facebook (4/29/22 below) guessed that there was 500,000 IBWOs at one time or perhaps circa 1800.
Tanner found home ranges of 1500 to 3000 acres in the Singer Tract, versus 40-50 for a Pileated pair in good habitat. It could be the Singer Tract birds were way below carrying capacity and therefore had large ranges, or it could be they really need that much space as they are, during breeding season, pretty much a specialist on boring (generally Cerambicid) beetle larvae and that is a pretty scarce resource (maybe patchy is a better descriptor than scarce). The Pileated, being more of a generalist and able to eat a wider variety of foods can subsist on a smaller territory (I have seen huge dead pines nearly cut in half by Pileateds searching for carpenter ants, for instance).
There are about 11,000 acres of old growth (not necessarily Virgin, just old growth) in Congaree NP. That would, in theory, support 3-6 breeding pairs. That is just the largest area of old growth. A place like the Choctahatchee or the Apalachicola bottoms in the FL panhandle might not have as much “old growth” forest, but still would have the type of dead and dying trees that provide food for the IBWO. There are probably close to 100,000 acres of older secondary forest in those two river systems. If they had territories at the higher end there you could easily accommodate 15-25 pairs if at capacity. White River NWR in AR has 160,000 acres, much of it very advanced second growth if not old growth. What would capacity be there? Maybe 40 or more pairs? There are dozens of other places with large enough areas of older growth to support one pair or multiple pairs - the lower Santee, the Savannah, the Altamaha, Okeefenokee and environs, the Green Swamp of FL, the Pearl, the Achafalaya, Tensas NWR, Big Thicket, etc. if they survive and can persist in second growth forests, there are probably well over a million acres of potential habitat. Carrying capacity? Who knows, maybe 200-300 pairs?
Compare that to the days of Wilson or Audubon when just the Mississippi Valley had tens of millions of acres of old growth. I would guess the original population, particularly if it sustained the kind of trophy hunting described by Audubon and Wilson, must have been at least in the 500,000 range (that is just a guess without any calculations behind it!).
synder inserts into
trevaisl of towo species
Granted, the summary remarks about commonness or abundance
of the ivory-bill found in early accounts were generalities lacking quan-
titative precision, but they were definite and meaningful remarks and
should not be simply ignored or assumed to be inaccurate. The frequency
and wide distribution of these remarks strongly suggest that the spe-
cies was quite easy to find in early times, at least in appropriate habitat.
Audubon’s remarks, quoted earlier, suggest that the ivory-bill may even
have been more abundant than the pileated woodpecker in one Louisiana
locality, and inference similar to the reports I heard from senior citizens
in central Florida and a claim that was also heard by Tanner from some
early residents of the Singer Tract region. Tanner rejected such claims,
but the reason he offered for doing so (people potentially underestimat-
ing the range sizes of birds) rests on a number of questionable assump-
tions, as discussed in Snyder (2007: 24-25).
Regardless of how one may wish to view the claims that ivory-bills
were sometimes more abundant than pileateds, it seems doubtful that
LAST OF THE IVORY-BILLS_ [ 25 ]
these claims were based on regions in which pileateds were in fact thirty-
six times as abundant as ivory-bills. And unless one might wish to dis-
miss the honesty or competence of an impressive array of naturalists
commenting on status of the ivory-bill in early times, the many accounts
of the commonness and abundance of the species simply do not suggest
that the ivory-bill was originally a rare species or that it was a narrow food
specialist, with individuals needing huge areas of forest to satisfy their
nutritional needs. Some observers (e.g., Jackson 2004) have suggested
that early reports of the abundance of the species might somehow repre-
sent major overestimates, but no persuasive evidence for this appears to
exist (see discussion in Snyder 2007: 26-27).
But what about Tanner’s documentation of a home range of roughly
6 square miles of virgin forest for one pair of ivory-bills in the Singer
Tract? Doesn’t this range size in itself give evidence for huge spatial
needs and intrinsic rarity of the species?
Well perhaps yes, and perhaps no. The answer depends on whether
the ivory-bills in this instance were fully utilizing the food resources in
their home range and whether the pair observed by Tanner might have
tolerated considerable overlap in the use of its home range by other pairs.
Tanner found no evidence that ivory-bill pairs defended exclusive territo-
ries, and extensive range overlap among pairs is something that is found
in many bird species, for example, many raptors, parrots, and seabirds.
In many of these species, individuals and pairs occupy ranges offering
a considerable excess of their actual needs for food and other resources
and, in many, it is simply neither feasible nor worthwhile for pairs to
defend their use areas and attempt to prevent extensive simultaneous use’
of the same areas by other members of their own species or other poten-
tially competing species.
For two years (1936 and 1937), Tanner (1942: 39) documented a sec-
ond pair of ivory-bills simultaneously occupying the range of his closely
documented pair in the Singer Tract, so it appears that amicable range
overlap was indeed occurring in the species, something also suggested
by observations of Scott (1888, 1898, 1903), who found eleven ivory-bills
feeding simultaneously in close proximity to an active ivory-bill nest in
west-central Florida in 1887. If range overlap was normal and extensive
for the species, the home range sizes of individual ivory-bill pairs would
tell us virtually nothing about potential densities of the species.
[26] CHAPTER ONE
Further, Tanner’s estimate of one pair per 6 square miles as a maxi-
mum density for the ivory-bill, which led to his calculation that pileateds
were thirty-six times as abundant as ivory-bills in optimal habitat, rested
on other questionable assumptions. The primary basis for this estimate
was collecting data of Arthur T. Wayne for the Wacissa Swamp region
of northwestern Florida during 1894 (fig. 7). Wayne, a native of South
Carolina, procured nineteen ivory-bills from this swamp, and Tanner
hypothesized that he might have left on the order of half a dozen ivory-
bills uncollected there, yielding a total density estimate of about one pair
per 6.25 square miles of swamp, assuming the swamp included about
75 square miles of suitable habitat. This was the highest density Tanner
calculated for any known ivory-bill habitat, past or present.
Unfortunately, Tanner did not present supporting data for his estimate
of just six ivory-bills left after Wayne’s collecting efforts. And although
the low magnitude of this estimate might seem consistent with Wayne’s
(1895) published claim that the population was in severe decline from
shooting, other unpublished information provided by Wayne (but evi-
dently not seen by Tanner) suggested that the population was neverthe-
less still quite large (see discussion in Snyder 2007: 12-13). Specifically,
in considering the general Wacissa region in letters of August 23 and
October 12, 1905, to Frank Chapman of the American Museum of Natural
History (see appendix 1), Wayne indicated that the ivory-bill was still
“abundant” after he finished collecting activities (in 1894) and that he
knew he had “left more than 100 birds in a radius of 20 square miles.”
These remarks seem to imply many more ivory-bills than Tanner pro-
posed for the same location and time period.
However, Wayne’s use of the word radius in the above quotation is
unusual and deserves comment, as radius generally refers to a linear,
not areal, measurement. Wayne may well have been describing a region
[28] CHAPTER ONE
of 20 square miles surrounding some undefined location, but alterna-
tively it seems possible that he may have envisioned a much larger region
with a radius of 20 linear miles. The latter interpretation, however, poses
several difficulties. A region with a radius of 20 linear miles, assum-
ing it might be roughly circular, would imply an area on the order of
1,260 square miles. Such a large area apparently greatly exceeded the
area Wayne might actually have assessed in his 1894 activities, judging
in part from the time it would likely take someone heavily engaged in
collecting activities to truly cover such an area, from the description of
the region given in Wayne’s 1895 paper, and from Tanner’s (1942) cal-
culation of only 75 square miles in the Wacissa region. In addition, one
hundred ivory-bills in 1,260 square miles would yield a density of only
one pair per 25 square miles, a density considerably below what anyone
would likely consider “abundant.” The same number of ivory-bills in
20 square miles, on the other hand, would be equivalent to 2.5 pairs per
square mile—a density that might plausibly qualify as indicating true
abundance because of its similarity to documented densities of pileated
woodpeckers in the very best habitat (e.g., three to six pairs per square
milein the Singer Tract, according to Tanner 1942).
Thus, taken as a whole, Wayne’s remarks make consistent sense
only if he was truly referring to a region of just 20 square miles for his
hundred-bird estimate. Moreover, assuming the validity of this interpre-
tation, the minimum density of ivory-bills left in the region would have
been fifteen times as great as Tanner’s proposed “maximum” density for
the species. Further, since Wayne collected nineteen ivory-bills from the
Wacissa region, the minimum ivory-bill density before he began collect-
ing might well have been close to three pairs per square mile, or about
eighteen times Tanner’s proposed maximum density. Even if we were
to assume that Wayne’s area estimate may have been quite rough and
that the boundaries of the region he was referring to might actually have
resembled the boundaries of Tanner’s 75-square-mile Wacissa region,
one hundred or more ivory-bills left in the region after Wayne finished
collecting would suggest a minimum density for 1894 on the order of five
times as great as Tanner’s maximum density.
Giving apparent support to substantial numbers of ivory-bills still
existing along the Wacissa after Wayne’s collecting activities ended is the
report of thirty-seven ivory-bills collected circa 1905 by J. R. Jack from the
Oreilla Swamp (see Phillips 1926: 513; Jackson 2004: 72). The location of
LAST OF THE IVORY-BILLS [ 29 ]
the Oreilla Swamp is obscure (I’ve been unable to find any other reference
to a swamp of this name), although it was apparently close to St. Marks
(as is the Wacissa). In his letter of October 12, 1905, to Frank Chapman,
Wayne stated he did not know exactly where Jack had collected his birds,
but his remarks suggest that either he or Chapman believed Jack may
have collected them from the Wacissa region, and, by implication, that
forty birds collected there would not likely have wiped out the population
(see appendix 1).
It is also important to note that the Wacissa population could already
have been substantially depleted by shooting activities of others by the
time Wayne first arrived in the region. Indeed Hasbrouck (1891) docu-
mented the shooting of ivory-bills along the Wacissa in 1890, and Wayne
(1895) himself reported the shooting of Wacissa ivory-bills by others for
food and bill trophies at the time of his collecting activities. Thus, the
original density of ivory-bills along the Wacissa before depredations
began might well have been substantially larger than the population
density of 1894, and the latter may have been far greater than Tanner’s
maximum-density estimate. These considerations raise major doubts as
to the validity of Tanner’s maximum-density estimate and his estimate
of only a single ivory-bill pair to be expected in an area occupied by thirty-
six pairs of pileateds. Both values could easily have been more than an
order of magnitude too low.
Also raising doubts as to these calculations is a density estimate pro-
vided by data of M. G. Vaiden (in Jackson 2004: 60-62) for a popula-
tion of ivory-bills still in existence in Mississippi shortly before World
War II. Vaiden reported six pairs occupying 19.2 square imiles of for-
est (a density of one pair per 3.2 square miles—nearly twice Tanner’s
maximum-density estimate). In similar fashion, Mason Spencer (in
Tanner’s field notes, May 17, 193'7) reported the existence of ten to fif-
teen ivory-bills occupying the Greenlea Bend region of the Singer Tract
until about 1932 (a region of approximately 6 square miles as mapped
in Tanner’s fig. 22), suggesting a population density that may even have
exceeded one pair per square mile (six times Tanner’s maximum den-
sity) if the birds involved were fully resident in the area. Tanner did not
discuss either of these populations’ estimates in his publications, and
no independent checks are available on their validity, but both may well
have represented already depleted, not pristine, populations. Taken at
face value, these estimates provide additional support to the conclusion
[30] CHAPTER ONE
that Tanner may have greatly underestimated the early abundance of the
species and greatly overestimated the space needs of populations.
Providing still more support for this conclusion is the apparent
improbability that any of the many early naturalists claiming early com-
monness to abundance of the species might have considered Tanner’s
proposed maximum density—one pair per 6 square miles—to have
represented commonness or abundance of the species. Tanner himself
clearly did not consider the bird common at such a density. Indeed at such
a density observers would generally have to travel many miles through
swamp forests to encounter even a single pair, given the facts that ivory-
bills were reputedly difficult to hear from more than a quarter-mile dis-
tance and that Tanner reported the birds to be silent for about half of the
daylight hours when not nesting (for a quantitative analysis of potential
encounter rates see Snyder 2007: 22).
Finally, that the ivory-bill may once have enjoyed densities consider-
ably in excess of one pair per 6 square miles is made especially plausible
by recent density data for another species of Campephilus woodpecker—
the Magellanic woodpecker (Campephilus magellanicus)—a species of
Argentina and Chile that has often been proposed as an important com-
parison species for the ivory-bill because of its near equivalence in size,
close taxonomic relationship, and similar dependency on temperate for-
ests (see Short 1970, 1982; Shortand Horne1990). Among all Campephilus
species, the Magellanic is evidently the closest in size to the ivory-bill.
In linear measurements it is just slightly smaller, although how these
two species might compare in body weight is unclear, because only two
specific weight records exist for the ivory-bill, and these are of unknown
accuracy and differ substantially from each other (see Tanner 1942; Short
1982; Jackson 2002). Both of these records suggest that the ivory-bill may
have been the heavier species, but how consistently and by how much are
highly speculative.
In any event, Ojeda (2004) documented twelve pairs or family groups
of Magellanics with extensively overlapping home ranges in a 12 square
kilometer old-growth study area in Argentina. These data yield an average
density of one pair or family group per square kilometer (or 2.6 pairs or
family groups per square mile)—a density very similar to that evidently
indicated by Wayne for Wacissa ivory-bills but roughly fifteen times as
great as Tanner’s proposed maximum density for the ivory-bill. Further,
Ojeda emphasized that her density estimate was a minimum estimate,
LAST OF THE IVORY-BILLS [ 31 |
as she could have missed some pairs. An even higher apparent density of
Magellanics was reported in the study of Short (1970)—thirteen pairs in
a strip of forest approximately 0.2 square kilometers in area—a density
so great (sixty-five pairs per square kilometer), one wonders if it could
have resulted from highly abnormal circumstances or some error in
recording of data.
Yes, itis important to note that the Magellanic does not share its range
with other large woodpecker species and that an absence of close com-
petitors could allow relatively dense populations in comparison to the
ivory-bill, but it simply is not known if ivory-bill densities were ever sig-
nificantly affected by sympatric populations of other large woodpeckers.
Early reports of ivory-bills being more abundant than pileateds and the
apparent general absence of aggression between ivory-bills and pileateds
raise substantial uncertainties about such matters (see Tanner 1942: 54;
discussion in Kilham 1972: 40-42).
The abundance of other smaller species of Campephilus (e.g.,
C. rubricollis and C. melanoleucos) has also been relatively high in some
comprehensive studies of undisturbed tropical rainforests (see Terborgh
et al. 1990; Thiollay 1994; and Robinson, Brawn, and Robinson 2000).
These studies, conducted in Peru, French Guiana, and Panama, respec-
tively, documented densities of Campephilus woodpeckers ranging from
about one pair per square kilometer to as many as four pairs per square
kilometer (roughly three to ten pairs per square mile). Such densities are
consistent with my casual personal experiences in various Neotropical
countries. For example, in a recent trip to the Iwokrama Reserve in cen-
tral Guyana, I encountered the red-necked woodpecker (C. rubricollis)
frequently along trails, with adjacent pairs often spaced on the order
of a kilometer or less apart (plate 7).In view of the limited distances ©
from which these birds could be detected in dense forested habitat, such
observations suggested densities substantially greater than one pair per
square kilometer.
It is also intriguing to note that in all of the above published studies in
the Neotropics, Campephilus woodpeckers occurred in higher densities
than sympatric Dryocopus woodpeckers (see also Slud 1964; Skutch 1969;
Kilham 1972). There does not appear to be any general case to be made
that Campephilus woodpeckers are intrinsically rare or less abundant than
Dryocopus woodpeckers (the genus including the pileated woodpecker)
where both genera occur together. The frequent numerical superiority
[32] CHAPTER ONE
of Campephilus woodpeckers raises the question of whether members of
this genus might generally be less specialized in foraging than Dryocopus
woodpeckers, something suggested as well by the apparent emphasis on
ants as food by Dryocopus species (see Kilham 1972: 40-42).
Tanner had no personal experience with the ivory-bill as a truly com-
mon bird and seemed to have difficulty imagining the species in such
a context. He was able to envision the ivory-bill as once having been
somewhat more common than the density of one pair per 17 square
miles of forest that he documented for the Singer Tract in 1934, but he
apparently found it improbable to believe that the species could ever
have rivaled the pileated woodpecker in abundance. In effect, he seemed
to view the sparse Singer Tract ivory-bill population of the mid-1930s
as close to the maximum possible under local forest conditions, rather
than as a massively depleted remnant well below carrying capacity of
forest resources. And because forest habitats were still nearly virgin in
the Singer Tract and the one pair he studied closely ranged quite widely,
he apparently found it compelling to assume that individuals needed
huge amounts of mature forest to meet their needs. Yet as already dis-
cussed, wide-ranging behavior is often very deceptive in its overall den-
sity implications, and early ivory-bill populations may have been much
denser than estimated by Tanner. The Singer Tract population may well
have been far below carrying capacity of food resources due to a long
decline produced by factors other than food scarcity. We will have a
close look at one of these other factors—human depredations—later in
the chapter.
The Ivory-Bill as a Potential Food Specialist
Tanner’s belief that the ivory-billed woodpecker was an extreme fora
