Some Thoughts on The Ivorybill’s Decline

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.

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 . . .

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.