Katy and I just returned from Panama. I’ll write more about the trip later. This note is a short Panama addendum to the Turkey Vulture post of a few weeks ago.
In reading about Panama before we went down, I came across some of Frank M. Chapman’s observations on Turkey Vultures in his 1938 book Life in an Air Castle. The observations were made at Barro Colorado Island in the Canal Zone where Chapman, Curator of Birds at the American Museum, spent a good many dry seasons beginning in 1925. He was one of the ornithologists who performed some field experiments that established that Turkey Vultures could readily find carrion without reliance on vision. One of his trials was “The Empty-House Test.” In it, he and E. Thomas Gilliard placed the body of a coati on a shelf hidden from the outside in a vacant house. This was done the afternoon of 24 December 1935. No Turkey Vultures had been seen in the vicinity for at least a month.
Chapman wrote that he did not expect the presence of the dead coati would be detected by the vultures within a day, “while my associate in this experiment freely expressed his belief that they would never discover it. With unconcealed surprise, therefore, he announced at 9:45 on Christmas morning that a buzzard had just flown from a tree growing next to the house containing the coati.”
Within the next three hours, five more Turkey Vultures and one Black Vulture (presumably attracted by the sight of the Turkey Vultures descending) made appearances.
A similar “Box on the Hill Test” plus many trials where carcasses securely wrapped in three layers of gunny sacking–burlap–were laid on the ground under forest cover gave similar results. The wrapped carcasses were monitored with flash cameras triggered by a wire attached to the bait. The resulting photos showed that the hidden carrion was quickly found, and always by Turkey Vultures.
The unanimity of these trials pretty much eliminates the idea that the Turkey Vultures must see the dead animal to find their food. The trials do not by themselves show that the sense of smell is involved, and in fact, two naturalists within a couple of years of one another suggested that vultures might be finding dead animals by noticing carrion-feeding insects (or even small mammals) attracted to corpses.
William B. Taber, Jr. was the first, writing in the Wilson Bulletin for 1928. He pointed out that Turkey Vultures can be attracted to carrion by seeing a cluster of crows on the ground (around a dead animal) and suggested that they might very well be similarly attracted to a bunch of brightly colored carrion beetles.
While collecting insects in tropical America, P. J. Darlington, Jr., twice had bait put out to attract carrion-feeding beetles stolen by Turkey Vultures. The first case was dead fish hidden under fairly large stones, the second dead iguanas put out in scrubby woods. In an article in The Auk for 1930 Darlington (later on a well-known biogeographer at Harvard) admitted that the birds could have smelled the dead animals, but thought that the possibility that the birds had been attracted by seeing a congregation of insects or hearing their buzzing ought to be considered. He concluded that Turkey and Black Vultures “are highly organized animals which presumably react to a complex environment in a very complex manner, and which must be experimented with accordingly.”
Chapman saw nothing in his trials to make him think that the sight or sound of insects was attracting the vultures but made a similar point, one that he had made in connection with his earlier studies: The Turkey Vulture “is not a mere stupid gorger of carrion but a bird dependent for its existence on its power of flight and discriminating use of its senses of sight and smell.”
Maybe even hearing. Perhaps it’s time for someone to do a well-designed test of whether the buzzing of blowflies or the clicking or squeaking sounds made by a gathering of carrion-eating beetles would pull vultures down from the sky.