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.