More Bachman

One of the warblers that has been seen recently in Central and Prospect Park is the chestnut-sided warbler. Even I've seen this bird. As you can see from the link, the bird has a much different appearance in the fall (typically as a juvenile) than it does in the spring.

These differences between breeding and non-breeding and adult and juvenile plumage created real problems for early ornithologists who operated without the benefit of binoculars and the ability to observe birds on their breeding grounds. Audubon describes the chestnut-sided warbler in Birds of America but notes:

Where this species goes to breed I am unable to say, for to my inquiries on this subject I never received any answers which might have led me to the districts resorted to by it. I can only suppose, that if it is at all plentiful in any portion of the United States, it must be far to the northward, as I ransacked the borders of Lake Ontario, and those of Lakes Erie and Michigan, without meeting with it.

In fact, according to the Cornell Ornithology Lab, this bird now breeds as far south as northern Georgia. Its range may have expanded in the 19th century because it favors successional or second growth forests, which likely proliferated in the aftermath of logging and farm abandonment. The illustration which Audubon made of the bird is only of the adult in breeding plumage which suggests he may not have recognized the juvenile as belonging to the same species.

A celebrated example of Audubon's confusion of an immature bird with a separate species is the so-called "Bird of Washington" - an eagle which Audubon believed to constitute a separate species but which most ornithologists today believe was an immature Bald Eagle. In his book Natural History in America, Wayne Hanley criticizes Bachman for defending Audubon on this point. Hanley states:

"Washington's Eagle (Falco washingtonii) proved to be nothing more than an immature bald eagle. Audubon, however, defended Falco washingtonii as a valid species for the remainder of his life. "

Well, not so fast. In his excellent new book Under a Wild Sky, William Souder defends Audubon. He notes that Audubon observed a breeding pair of the bird and that "[b]ald eagles tending a brood would have had the characteristic white heads [of an adult bird]." He also notes that Audubon painted the Washington's Eagle and that the painting was done to life size. Souder measured the painting and found that the depicted bird's length, toes and folded wings all substantially exceeded in size those of a juvenile bald eagle. As Souder says:

"What about the chance that Audubon was right - that the bird he shot and drew actually was a separate species, possibly a rare individual from a remnant population that was in the process of going extinct."

The next Ivory Billed Woodpecker?