Insights, Ants, and Old Growth: a Nuanced View of the Ivorybill’s Decline and Possible Survival

I’ve just finished reading Tanner’s dissertation and have gained some new insights into topics that have been discussed in a number of earlier posts.

Conventional wisdom, following Tanner, holds that the Ivory-billed Woodpecker’s decline and possible extinction were caused by habitat loss, specifically the logging of old growth forests during the 19th and early 20th centuries. Birdlife International’s fact sheet on the species suggests “that large contiguous tracts of mature woodland would be required to support a viable population”, referencing Jackson 2002. Snyder et al. have proposed an alternative hypothesis that “human depredation was the primary factor.”  (p.9).

Tanner’s model depends on the idea that food supply was the limiting factor on ivorybill populations, because the species is highly specialized, and that old growth conditions were optimal or essential. While Tanner was aware that ivorybills bred successfully in an area that was predominantly second growth, at Mack’s Bayou, he glossed over this fact in the monograph, and became more dogmatic about old growth as a requirement in later years.

Snyder and some others have contended that the ivorybill is a generalist. According to Snyder, “the data available on diet and foraging methods simply do not provide compelling evidence for strong feeding specialization.” Snyder goes on to suggest that “[i]ts apparent skill in exploiting recently dead timber, coupled with its ability to feed in a variety of other ways, may even have given it some significant foraging advantages over the pileated woodpecker, a species apparently much less capable of bark stripping. Indeed, the pileated woodpecker, like other Dryocopus woodpeckers, may well be more of a food specialist than any of the Campephilus woodpeckers.” (p. 37).

As I see it, there are elements of truth in both models, but neither is complete. In addition, I think that each model relies on at least one flawed premise.

The old growth/virgin forest component of Tanner’s model fails to account for the facts that the Singer Tract population was dwindling even before logging began in earnest and that birds appear to have remained in the Tract until well after it had been extensively logged. Tanner suggested another possibility, “perhaps the greatest factor reducing the rate of ivorybill reproduction is the failure of some birds to nest. One reason for their not breeding is immaturity, for it is probable that ivorybills do not nest until they are two years old. Another possibility is that the quantity of food available to the woodpeckers may determine whether they will nest or not.” (p. 83).

Tanner struggled to account for the fact that the ivorybill population at Singer was dwindling by the mid-1930s, even though overall habitat quality had, if anything, improved relative to what it had been a few decades earlier. He attributed the higher relative abundance in previous years to tree mortality due to fires that took place in 1917 and 1924. Tanner also recognized the probable importance of fire in the pre-contact era, although he seems to have been unaware of the ways pre-contact Native Americans used fire, both for agriculture and habitat management. (The impacts of Native American fire use were almost surely different from what occurred in the 20th century Singer Tract).

Neither Tanner (whose study predates the emergence of the discipline) nor Snyder, take environmental history sufficiently into account. There had been major ‘changes in the land’ long before large scale logging began in the southeast and before the reports of local abundance on which Snyder relies. These changes include: the post-contact collapse of Native American civilizations, the introduction of European plant and animal species, the clearing of log jams on major and secondary North American rivers, habitat fragmentation due to the plantation economy, and the near extirpation of the beaver.

All of these elements likely contributed to a major decline in ivorybill populations. Ivory-billed woodpeckers likely concentrated locally in response to major disturbances, regardless of whether forests were old-growth or advanced second-growth, and this type of specialization caused birds to congregate, making it easier for collectors to kill them in large numbers in short periods of time. Snyder likely misinterpreted this collection of large numbers of Ivory-bills in short periods of time as reflecting a greater regional abundance. In contrast, and more consistent with Tanner, this ecological response to disturbed areas led, in some places, to the collectors extirpating regional populations.

In the latter part of the 19th century, hunting probably sped the collapse of the remaining population, but Snyder’s claim that available data on diet and foraging methods do not provide compelling evidence of specialization fails to account for the anatomical and other evidence that suggests otherwise. It also fails to account for the Pileated Woodpecker’s far more extensive range and ability to thrive in a wider variety of habitats, including badly fragmented and degraded ones. I made some of the case for specialization in a series of recent posts, but there’s more to add, especially with regard to ants.

In one of those posts, I hypothesized that the inability to exploit ants as a food resource was a key component, perhaps the primary component, in explaining the decline of the ivorybill. A commenter asked whether there’s evidence to support the idea that ivorybills and other Campephilus woodpeckers don’t feed on ants and also whether there’s evidence to support the idea that Campephilus woodpeckers don’t regurgitate.

Adult Campephilus woodpeckers rarely feed on ants but do not feed them to their young. They make frequent trips to the nest with food items stored in the bill or at the back of the bill. (M. Lammertink, pers. comm.) Dryocopus woodpeckers and those in closely related genera (the “tribe” Malarpicini) feed their young by regurgitating, while other woodpeckers do not. (Manegold and Topfer, 2012). I think the capacity of Pileated Woodpeckers to consume ants in large quantities and to feed them to their young is a significant distinguishing factor and that Tanner was correct in suggesting that food supply was a major limiting factor on Ivory-billed Woodpecker populations.

Ants comprise up to 33% of the world’s terrestrial animal biomass. In Finland, they comprise as much as 10%. In tropical forests, the percentage is much higher, exceeding vertebrate biomass by 400%. Tanner’s comparative analysis of available ivorybill and pileated food did not include ants, so Tanner’s comparative estimate of available insect prey – suggesting that pileateds in the Singer Tract had access to approximately four times what ivorybills did – was in fact extremely low.

Tanner’s dissertation concludes with a discussion of Audubon’s ivorybill dissection, something that was omitted from the monograph. While I had a passing familiarity with the Audubon material, I had not looked at it carefully. Nor had I compared his ivorybill and pileated dissections.

Tanner wrote: “The proventriculus is both muscular and glandular. Audubon’s drawings and text indicate that the proventriculus of a Pileated is much larger in proportion to the stomach than is the case in the Ivory-bill.” Audubon described the ivorybill proventriculus as being only minimally wider than the esophagus. By contrast, the pileated proventriculus as “an immense sac, resembling a crop, 2 1/4 inches in length and 1 and 5 twelfths in width,” or nearly three times as wide as the esophagus.

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Audubon’s drawing of Ivory-billed Woodpecker digestive tract showing slightly widened proventriculus.

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Audubon’s drawing of Pileated Woodpecker digestive tract showing large, sac-like proventriculus.

The proventriculus and stomach of one of Audubon’s specimens contained “a vast mass of ants and other insects”. According to Bent, Beal found one pileated stomach that contained 2,600 ants. (Others contained fewer, 153 and 469, according to Sutton.) Thus, it’s clear that even if ivorybills sometimes ate ants, they lacked the capacity to consume them in large quantities, let alone feed them to their young.

This supports Tanner’s view that specialization was a limiting factor on ivorybill populations. I’ve previously suggested that this might apply only to breeding season, but it seems reasonable to infer that it’s a factor year-round, based on the differences in proventricular structure.

All of that said, I’d argue that this specialization should not necessarily be read to include dependence on large tracts of mature, contiguous forest. The data from the Singer Tract suggest that even under these ‘optimal’ conditions, breeding was limited. And the fact that the Mack’s Bayou birds bred successfully in an area of second growth suggests that birds could thrive under ‘suboptimal’ conditions. The extent to which survival might be possible in fragmented habitat is less clear, but Snyder (citing Jackson) refers to the Mississippi population of six pairs in a 19.2 square mile forest that Tanner missed; the tract is less than 1/6 the area of the Singer Tract and is smaller than many contemporary wildlife management areas.

The tract, known as Allen Gray Estate, was west of Skene, Mississippi in Bolivar County; some or all of it is now part of Dahomey National Wildlife Refuge; the US Fish and Wildlife Service Habitat Management Plan for the refuge (2013) states that the forested portion of the refuge comprises 8100 acres and provides this historical information, “Dahomey NWR is located on the grounds of the old Dahomey Plantation founded in 1833 by F.G. Ellis and named after the homeland of his slaves. Much of the land west of the refuge was probably cleared for cultivation around this time. The land went through several owners and was purchased by Allen Gray in 1936. The portion that became the refuge was known as the “Allen Gray Woods”. This was the only significant portion of the plantation still forested.”  This 8100 acre figure is 25% lower than the figure reported by Jackson and Snyder.

While I have been unable to find a detailed logging history of Bolivar County, it is in the heart of the Mississippi Delta, which was known for its plantations. Between 1900 and 1940, Bolivar County was more densely populated than Madison Parish: 39.1 people per square mile as opposed to 18.9 in Madison Parish in 1900, 78.92 as opposed to 22.78 in 1930, and 74.57 as opposed to 28.33 in 1940. Based on population density and the number of towns, it seems self-evident that the habitat in Bolivar County was considerably more fragmented than was the Singer Tract.

Thus, there is good reason to question Tanner’s old growth model as well as the idea that large contiguous tracts of mature forest are required. Similarly, there’s good reason to question Snyder’s argument that hunting rather than specialization was the primary cause of the ivorybill’s collapse.

Efforts to reintroduce the beaver in the southeast began in the 1930s, and the population has been growing ever since. Beavers injure trees by partially or fully girdling them and by altering hydrology, which weakens or kills trees at the edges of the ponds they create. Beaver damage renders trees more vulnerable to infestation by ivorybill prey species, something we’ve observed repeatedly in our search area. In Tanner’s day and in the late 19th century, the beaver was barely a part of the southeastern ecosystem, but by the 1950s, beavers again were playing a role in altering southern forests, whether mature or successional.

If the ivorybill was able to survive the logging of the last large tracts of old growth forest, as I think it was, the reintroduction of the beaver may have been central to its persistence. If this hypothesis is valid, there is considerably more potential habitat today than there was in Tanner’s era; much of this potential habitat has been overlooked or dismissed in organized search efforts; and the dismissals of post-Tanner reports based on his habitat model rely, at least in part, on a false premise.





More on Scaling and Squirrels, Part 1

This is will be the first in a series of 3-4 posts. The subject is multifaceted and subtle. Nuances can be hard to convey in words and accompanying illustrations; it’s easier to do in talks, with bark chips in hand to provide a more visceral sense of what’s being described. Still, it seems important to make the effort.

I realize now while my initial approach to evaluating feeding sign was rigorous, I grew somewhat lackadasical and overconfident. I also got distracted by the abundant scaling on downed sweet gums we started finding a year or so ago. I’m now confident that squirrels did much of this scaling, but the same does not apply to most of the other work we’ve found over the years.

Even before we discovered that squirrels were scaling bark on downed sweet gums (and quite possibly on standing trees as well), I was contemplating a post that broke down the bark scaling we’re finding into several categories. I was aware of having gotten away from the criteria I had laid out in the past and was feeling a desire to be more specific. That seems like a good place to begin, before delving too deeply into the nuances of distinguishing between squirrel and putative ivorybill work.

The following are the different types of interesting feeding sign we’re finding. Bear in mind that this pertains only to hardwoods that appear to be alive or recently dead and are known or suspected to have tight bark, except in cases where work appears to be old but still has characteristics that suggest it was done when bark was tight. The types of sign are ranked in the order of what I think is the likelihood that most or all of it is being left by Ivory-billed Woodpeckers, although the gap among categories 1-4 is small. (Frank would reverse categories 1 and 2.)

  1. Scaling on standing boles, low enough on the trunk to be examined up close. This includes both standing trees and ones with tops broken off. The sapwood of trees in this category has multiple large exit tunnels. The most prevalent species in this category is bitternut hickory, which has very thick, dense, tight bark, but we’ve also found it on sweet gums and oaks. This scaling is extensive and has a distinctive pattern that’s immediately recognizable in the field, an almost jagged appearance, although the actual edges are curved. The tree on the homepage is one example, and the image below illustrates how even when the scaling is not recent, this distinctive appearance remains. Bark chips are easiest to find for this type of work.

    Old scaling on hickory, 2015

    Old scaling on hickory, 2015. Note the exit tunnels and the absence of other woodpecker work, save for a small dig near the top of the scaled area.

  2. Scaling on standing boles, low enough on the trunk to be examined up close. Few or no large exit tunnels but signs of insect infestation under the bark. Superficial bill marks may be evident in the remaining cambium or on the surface of the sapwood. Tree species in this category include sweetgums and oaks. Chips are similarly easy to find.IMG_0144Lateral strike marks in the cambium
  3. Scaling that has the appearance of the work in category 1 but that cannot be examined up close. No possibility of examining bark chips.Hickory3
  4. Scaling higher on boles and lower branches of standing trees where exit tunnels may be visible, but close examination is not possible. In some cases, these trees are seen at a distance, across water bodies, so there’s no opportunity to look for chips. As is the case in categories 1 and 2, older scaling may go untouched by woodpeckers for extended periods. The first example below is recent; the second is probably more than two years old.

    Top of scaled hickory.


    Older scaling on dead top with exit tunnels.


  5. Scaling on higher branches of standing trees. Since these are often seen at some distance and in poor lighting conditions, it can also be more difficult to assess the freshness of the work, and the nature of infestation. Bark chips are usually much harder to find under these circumstances. Squirrels typically girdle limbs and often scale on the undersides of large, higher branches. Thus, when larger branches are at less than approximately a 70 degree angle, work on the underside may indicate a squirrel as the source, while the presence of extensive scaling limited to the upper side may be strongly suggestive of or diagnostic for woodpecker.BigSGLimb
  6. Scaling on downed trees or limbs that are at least in part more than 4’ from the ground.Detail of scaling on downed sweet gum.
  7. Scaling on downed trees and limbs that are mostly or all horizontal and less than 4’ from the ground.
    Work now suspected to squirrel on downed sweet gum.

    Work now suspected to be squirrel on downed sweet gum.


    Suspected Pileated Woodpecker work on same downed sweet gum

There’s an additional category that is somewhat different from the others. This involves work we’ve found on freshly dead, small sweet gums (>1’ dbh) with evidence of ambrosia beetle infestation. These trees have been stripped of bark, with some accompanying signs of excavation, ranging from targeted digging that resembles the work of Magellanic Woodpeckers (as on the left branch below) to the appearance of having been attacked with a hatchet. We think this work has strong potential for ivorybills, since we’ve found only three examples of it, in close proximity, and in an area with an abundance of other suggestive sign.HackedUpGum

It’s important to point out that when I use the word “scaling”, I am referring only to the clean removal of bark with little or no damage to underlying sapwood. While I have been quite adamant about this as a characteristic, some elaboration is probably in order, as my statements were made in reaction to woodpecker work that was often described as “scaling” in the early search years but was really bark removal in conjunction with excavation, something that’s typical of Pileated Woodpeckers. There still seems to be a good deal of misunderstanding on this subject, and the distinction is not always easy to communicate.

On close examination of some scaled areas, especially in category 1 but also in others, there are signs of targeted digging (but not deep or extensive excavation). This can range from a very slight expansion of an exit tunnel, apparently by probing with the tip of the bill, to what may be a harder strike or two, to a somewhat deeper but still targeted dig into the sapwood. Since many other species of woodpecker are capable of doing such targeted digging, I only consider this aspect when it’s in association with extensive, contiguous removal of bark. This will be explored in more depth in the next post in the series.

In category 1, known ivorybill prey species have been found under the bark or on the scaled surface of two trees. When exit tunnels are found on these trees in this category, they are consistent with infestation by large Cerambycid beetle larvae. I hypothesize that these trees are being scaled when the gregarious larvae have dug their exit tunnels but have not yet sealed their pupation chambers. This would be the stage at which they are most nutritious and most easily accessible for a species of woodpecker adapted to bark scaling, but the opportunity exists only within a very narrow time frame.

For several trees in categories 1 and 2, camera deployments of 2-4 months duration produced no return visits or evidence of what was doing the scaling; in a couple of cases Pileated Woodpeckers were photographed on the target trees for fairly protracted periods. In one, the pileated removed a few small pieces of bark, and in the other it appeared to do a little pecking and gleaning but did not remove any bark. We have revisited several of the other trees over periods ranging from months to two and a half years. One tree in category 2 (no tunnels) had a return visit approximately four months after the first one, when the bark was still tight. Several others, both with and without tunnels and including one first found in June of 2013 and re-examined during my last trip, had no obvious new scaling and little or no excavation of any kind, despite being in a more advanced state of decay

It’s also important to note that we have reason to believe that at least some of the work in all categories is being done by woodpeckers. For example, on the downed sweet gum shown above to illustrate category 7, found in November, there is obvious woodpecker work (likely Pileated) on the bole and apparent squirrel work on the upper limbs. Similarly we suspect woodpeckers did the scaling on the larger downed sweet gum (category 5) – mostly scaled higher but with some work within 4’ of the ground. I found this tree in May 2014, approximately 50 yards from the site of where we captured the squirrel stripping bark; while I do not recall looking for or examining bark chips, the edges of the scaled areas appear chiseled rather than gnawed, and the scaling on some of the higher limbs is on the upper side only.

The next post on this topic will examined the targeted expansion of exit tunnels and will revisit the similarities between what we’re finding and the work of other Campephilus woodpeckers. The following one will focus on bark chips and distinguishing between signs of gnawing and signs of scaling.

Feeding Sign, Foraging Preferences, and Prey Species: Some Observations and Speculation

Frank and a visiting ornithologist spent this past weekend in our search area. I’m eager to read and will be posting Frank’s report before long. For now, suffice it to say they set up three trail cams, one on the snag where we captured the image discussed here and here and one on this downed sweet gum top found in April:Big Limb

It most likely fell on April 19th. When I found it a couple of days later, it had fresh green leaves attached and no sign of insect infestation. Since then it has been partially scaled. This is an important data point, as we know the scaling took place within five and a half months of death, and Tanner documented the IBWO’s preference for freshly dead wood. We hope there will be a return visit soon.

They also placed a camera on an even more recently fallen water oak, something that started me thinking about possible patterns in the feeding sign we’re finding.

I’ve counted the examples of feeding sign from our current search area I’ve posted on the blog (which is by no means all the suggestive work we’ve found but is generally the most impressive), and the results for sweet gums are interesting, especially in light of Tanner’s observations suggesting an IBWO preference for sweet gums. Our results also suggest a preference for hickory. (Hickories were scarce in the Singer Tract, and apparently the species present in our area were not present there.) In both cases, the frequency with which we’re finding scaling seems to exceed the relative abundance of either type of tree, although we have not made formal counts. This sign was found between the spring of 2012 and the Spring of 2015, except for the downed top pictured above, which was scaled a little later.

The tally includes a couple of examples of work that falls short of what we consider to be diagnostic for IBWO. It also includes the small sweet gum snag that looks like it was attacked with a hatchet.


While there seemed to be a preference for sweet gums prior to the 2014-2015 season, the preference was considerably more pronounced this year when the abundance of fresh scaling on sweet gums in a relatively small area was astonishing. Here’s the multi-year breakdown:

Sweet gum:                       25

Hickory:                             10

Presumed sweet gum:        6 (One example possibly PIWO)

Oak species:                       3

Willow oak:                          2

Unknown:                            1

Maple:                                  1 (Possibly PIWO)

Ivorybills fed on sweet gums in 42.6% of Tanner’s observations, scaling in 40 instances and digging in 3. Sweet gums made up 20.8% of the forest composition in Tanner’s study area. Next on Tanner’s list of preferred foraging trees were Nuttall’s oaks. By contrast, Pileateds “appeared to have no preference for any species of tree.” Tanner observed PIWOs feeding on sweet gums on fourteen occasions; nine involved digging and five involved scaling. He further noted, “What scaling Pileateds were observed to do was mostly on loose bark and was never as extensive or cleanly done as the work of the Ivory-bills.”

On a more speculative note, I think I’ve been able to identify one species of beetle that’s infesting the sweet gums, including the small one shown above. They’re an invasive, the granulate (formerly Asian) ambrosia beetle Xylosandrus crassiuculus (or another closely related invasive). Ambrosia beetles are tiny, but they are gregarious, with adult females creating chambers and tending broods of larvae in the sapwood. They can kill small trees but also infest larger ones. They have a relatively short life-cycle, and one source suggests they can produce 3 or 4 broods a season in the deep south. It’s worth repeating that I’ve seen signs of ambrosia beetle infestation elsewhere in Louisiana (near our old search area and in upland hardwood forest adjacent to our current one) but did not find work suggestive of ivorybills in either place.

We’ve found known IBWO prey species in our search area, on trees that we suspect were fed on by ivorybills. We also suspect that, contrary to Tanner, they may feed on darkling beetles. Could they also be feeding on an invasive species? We can see no reason to suspect otherwise and will continue our investigations with this in mind. I plan to return to Louisiana Thanksgiving week.

Some Thoughts on The Ivorybill’s Decline And A Change of Plans

Jamie Hill, who has worked with the Cornell and Auburn teams, recently posted a Facebook link to a very interesting article from the September 2014 issue of Smithsonian. Ivory-billed Woodpecker aside, the piece is well worth reading, but for the purposes of this blog, the article got me thinking about reasons for the ivorybill’s decline and the possible role of the longleaf pine. These ideas are not entirely new or original with me; Lester Short went even further, suggesting that pine might have been the ivorybill’s primary habitat; Jerome Jackson devoted several pages of In Search of the Ivory-billed Woodpecker to pines, and Fangsheath of the ivorybill researchers forum has hinted at this too.

I was struck by just how congruent the historic range of the ivorybill is with the range of the longleaf pine (Pinus pilastrus). The overlap is not exact, and the pre-Columbian range of the ivorybill extended as far north as Ohio. Nonetheless, conditions in the Singer Tract were objectively quite different from what they were in many other parts of the historic range.

A recent blog post on the Tallahassee Democrat site reiterates the conventional wisdom about the species and the reasons for its decline. Author Budd Titlow writes: “Before the Civil War, when much of the southeastern U.S. was covered with vast tracts of primeval hardwood swampland, ivory-billed woodpeckers ranged from North Carolina south to Florida, west to Arkansas and Texas, and north into Oklahoma and Missouri. Then, after the Civil War, extensive logging of these old-growth swamps wiped out most of the ivory-billed’s habitat in one fell swoop.”

While there’s some truth to this history, it’s also a stereotype that’s based in large part on an imperfect reading of Tanner’s monograph and even more on Tanner’s dedication to protecting the Tract as the last remaining extensive old-growth stand in the southeast (although the Tract contained considerably less old growth than Tanner believed). Tanner’s efforts were admirable; the loss of countless acres of magnificent old-growth swamp forest was devastating environmentally and is unquestionably something to be mourned, but it seems unlikely that the destruction of these forests was the primary cause for the ivorybill’s decline.

The species was known to be disappearing by 1890 or even earlier, and Chester Reed’s 1906 Bird Guide to Land Birds East of the Rockies stated that the birds were restricted to isolated parts of Florida and possibly to “Indian Country” (Oklahoma). In The Travails of Two Woodpeckers, Noel Snyder, who attributes the decline primarily to hunting, points out that intensive logging of bottomland hardwoods began between 1890 and 1900. Logging of pine forests began considerably earlier, and these forests were severely fragmented, even before the Civil War. Snyder reads the early record (I think selectively) as indicating that ivorybills strongly preferred bottomland hardwoods and seldom used pines, in contrast to the Cuban ivorybill and the Imperial.

Jackson takes a different view, citing multiple references to the use of pines for feeding and nesting. Where Snyder reads Alexander Wilson’s early account as reflecting a preference for “swamps and bottomlands”, Jackson reads him as describing the preferred Carolina habitat as “a mosaic of baldcypress swamp and pine uplands, similar to the habitat in Florida”. Jackson goes on to suggest that, “It appears . . . that ivory-billed woodpeckers will inhabit both hardwood forests of river bottoms and pine forests of higher elevations, particularly old growth forests supporting healthy populations of beetles. They seemed to do best at the interface of these forest types, taking advantage of the resources of each.” (Emphasis added).

This meshes well with what Allen and Kellogg observed in Florida in 1924; the birds nested and roosted in cypress and were observed and photographed foraging in open pine forest. The Lambs’ limited observations in Cuba suggest something similar, a preference for roosting in pines but an equal division between pines and hardwoods for foraging.

Thus, it seems possible that the Singer Tract was actually suboptimal habitat for the ivorybill, since it contained no pine and little cypress. I’m also led to suspect that habitat fragmentation, rather than habitat loss may have been central to the decline of the ivorybill, with hunting as one of several other contributing factors. This fragmentation actually began well before the Civil War, but it accelerated with the post-war destruction of the longleaf pine forests, followed by the logging of the bottomlands. I’m personally convinced that the species beat the odds and survived, using one or both of the strategies discussed in this post. I wonder whether some of the modern search efforts have focused excessively on the bottomland hardwood model and not enough on areas where there’s an interface between forest types.

On a different note, I had planned to make my final trip to our search area for the season during this week and next. Water levels are very high right now, so I’ve decided to postpone until late July. Better to endure the heat and humidity than to be unable to move around in the woods.

George and Nancy Lamb’s 1957 Cuban Ivorybill Study

I have been re-reading George Lamb’s 1957 report on the Cuban Ivory-billed Woodpecker. A number of items struck me as potentially significant for North American searchers, some for how they diverge from Tanner and others for their level of detail. Since this report is likely unfamiliar to many, I thought I’d do a quick post listing some of the more interesting observations

Lamb references a number of local sightings of “groups” of ivorybills, with one report to John Dennis that involved six birds. Notwithstanding, Lamb estimated the population density in Cuba to be much thinner than in the Singer Tract, at one pair per 12-25 square miles. He also pointed out that “ . . . the Cuban Ivory-bills are living for the most part in a cut-over pine forest where only small and deformed trees remain.”

The Cuban ivorybills fed on pines and hardwoods more or less equally, although most of the feeding sign was found on pines, due to the difficulty of searching for sign in the denser hardwood habitat. Roosts and nests were found exclusively in pines (one unused cavity was found in a hardwood), which is interesting in light of the fact that hardwoods were also available. Cavities were found at heights ranging from under 20 feet to nearly 60 feet. Cavities were higher in mature forest; Lamb suggested but did not conclude that the preference was for higher cavities and that the lower ones reflected an adaptation to cut-over conditions.

Lamb describes a female scaling bark: “At this point she was only about 25 feet away while she was feeding around the base of a small pine. She began ‘barking’ this tree about 30 inches from the ground and slowly worked up to the top.” Dennis too had observed birds scaling small pines. They found more scaling than excavation.

This apparent preference for pines, including small ones, may be significant, particularly since the hardwood areas were “relatively untouched”.

An estimated 17 birds were killed by humans over a ten year period, a huge number for such a small population. And it seems an open question whether the thinner population density noted by the Lambs was due to habitat quality, hunting pressure, or a combination of the two.

Regarding flight style: “. . .the flight of the Cuban Ivory-billed Woodpecker was always level and purposeful. They are strong fliers, capable of covering considerable distance in little time, as indeed they must to live successfully in cut-over woodlands. Although the Ivory-bill did not seem to undulate in its flight, the wing beats were not steady, having an almost imperceptible 2-3-2-3 rhythm.”

There’s no mention of double knocks, but calls are discussed. Lamb describes the sound as like the “note of a penny tin trumpet . . . short and usually repeated in a series of single-double-single beats, or it may begin with a double call: that is a high nasal “pent, pent-pent, pent”, or just “pent-pent”. On several occasions the female Ivory-bill most frequently observed made a few long and very loud calls, soon after leaving here roost tree in the early morning. The notes were of greater duration than normal and were repeated in a series of sixteen to twenty-two kients.”

Food for thought . . .

Black Woodpecker (Dryocopus martius) Foraging Behavior: More Support for an IBWO Diagnostic?

I’ll be returning to Louisiana in late February and hope to make a couple of more trips during peak search season. Frank has retrieved the cards from our game cams and is in the process of going through several weeks of images. In contrast to the last set, there have been no intriguing hits thus far.

Last month, I came across a very interesting post on the Woodpeckers of the World Facebook group. The link took me to a French website that features some videos of the Black Woodpecker (Dryocopus martius), a Eurasian species, and one of the largest woodpeckers in the world, surpassing the Magellanic in size. In another analogy to the Magellanic, the Black Woodpecker appears to have certain features that are more Campephilus-like than others in genus Dryocopus. This includes flight style; acording to Gerard Gorman’s monograph on the species, Black Woodpeckers don’t generally undulate in flight. More importantly, the size and appearance of the bill certainly evoke the IBWO – generally larger, thicker, and heavier than a Pileated’s. Bill length can reach over 70 mm (although the average is 53-56 mm). According to Tanner, the mean length for ivorybills ranged from 67.8 mm to 74.1 mm depending on sex and region.

For my purposes, this is the most interesting clip. It’s an outstanding sequence that shows a Black Woodpecker scaling bark from a medium-sized hardwood branch. I see the following aspects as being significant and supportive of the hypothesis on feeding sign I’ve discussed in several posts, including here, here, here, here, and here.

For the most part, the bird is removing bark with direct strikes, not the lateral blows of a Campephilus woodpecker. This is possible because the limb is relatively thin, and the bird is able to position herself so that direct strikes will have the same effect that a more lateral blow would have on the bole; she generally engages in lateral movements to flick away bark after it has been loosened. The clip also seems significant insofar as it reveals the amount of effort involved to remove large but very thin strips of bark. In addition, even though the bark is thin, it seems the bird is still removing it in layers, at least some of the time. I think the video tends to support my idea that the work we think is diagnostic – on boles with thick tight bark – is beyond what PIWOs can do physically. At the same time, the footage suggests that the high branch work that Tanner emphasized is likely within the capacity of a Pileated Woodpecker and is indeterminate as we suspect.

In looking at images of Black Woodpecker foraging sign online, it appears that – bill structure notwithstanding – they typically remove bark in layers, just as Pileated Woodpeckers do, and this is true on both hardwoods and softwoods. I have been unable to find any examples of Black Woodpecker work that closely resemble what we think is diagnostic for ivorybill, but examples of the layered scaling are easy to find, for example:






And here.

Additional Thoughts on Behavior and Rapid Evolution

I’d like to address an interesting post from “Sidewinder” on the Ivory-billed Woodpecker Researchers’ Forum on the rapid evolution question. His key points:

“Cyberthrush and others have suggested that natural selection has favored high levels of wariness and human avoidance in the IBWO. This position assumes that the change has a genetic rather than experiential (learned) basis. I have questioned this possibility based on the simple fact that behavior is usually one of the fastest traits to evolve. I have no problem with intense human predation on the IBWO resulting in increased vigilance among the surviving remnant, but if the IBWO persists, human predation has been absent now for dozens of generations. While many studies demonstrate that predation pressure can select for increased wariness in animals, what about the inverse? Can multiple generations of relaxed selection over a relatively short term (<100 years) result in relaxed vigilance?”

He concludes that the evidence is mixed and that the, “ . . . findings do not really support or refute Cyberthrush’s hypothesis. Clearly, we need more study–particularly of birds–to learn whether avoidance of humans might persist for many generations after selective pressure (predation) no longer exists to maintain vigilance. In the meanwhile, let’s acknowledge the highly tentative nature of this hypothesis.”

I hadn’t considered relaxed vigilance as a possibility, and it’s an interesting idea. With regard to the general evolved vigilance hypothesis, it’s certainly possible; I just don’t see it as being necessary to explain the difficulty of detection. I think normal wariness, difficult habitat, and extremely low densities suffice.

The hypothesis that the IBWO would dramatically change its foraging behavior, which is to a large degree morphologically determined, is considerably more extreme than the idea that the species became more wary. I have taken issue with the notion (or simplistic reading of Tanner), that the species is (or was) an extreme specialist, but its anatomy and historic range point to some degree of specialization – considerably more than exists in the PIWO.

I suspect that Tanner significantly overstated matters when writing about the canopy and high branch work, but even Tanner made it clear that IBWOs foraged at all levels. Some of the known prey species primarily feed and develop in the boles and in some cases quite near the ground (H. polita, for example). I suspect the high branch foraging Tanner observed was for larvae that he dismissed as being unsuitable because they feed on longer dead wood – Tenebrionidae in particular, although there’s no evidence from stomach contents to support this idea. The larvae we found under bark of this downed sweet gum have been id’d as belonging to that genus, and the tree was not very long dead.

One of the Singer Tract (in a pin oak stub, Mack’s Bayou) was in a clearing.

Mack's Bayou Ivorybill nest tree. Courtesy of the Division of Rare and Manuscript Collections, Cornell University Library

Mack’s Bayou nest tree. Courtesy of the Division of Rare and Manuscript Collections, Cornell University Library

John Dennis’s photos of Cuban IBWOs at a nest also appear to be from a very open area (and even if the Cuban IBWO is a different species, it’s a very close relative, and the hunting pressure there was almost certainly equal, if not more intense.) These seem odd nest locations for a bird that has rapidly evolved to hide in the canopy.

It’s also pretty clear to me that the John’s Bayou birds learned to tolerate human presence, while other IBWOs in the Singer Tract did not. As I’ve pointed out in several posts, Tanner and Kuhn (to a lesser extent) had a difficult time finding ivorybills in other parts of the Tract. This also suggests a behavioral rather than a genetic basis for the wariness or at the very least a substantial behavioral component.