Winter World_ The Ingenuity of Animal Survival - Bernd Heinrich [94]
Nothing in the scientific literature about roost behavior would allow for group fighting, and birds that fight and are territorial during the breeding season relinquish their antagonisms in order to be in roosts. What could there be to fight over, since the occupants join up because they ultimately need each other? Also, when social birds do fight, they don’t hack themselves to death. (So how do I explain that letter from the man in Alaska? Bear with me a bit and I will try.) Crows sometimes act aggressively, but from what I have seen it has always been a group of them attacking another individual; I have never seen more than one intended victim at a time. It seemed bizarre that crows would wage the equivalent of a war at a communal roost, and that they would stay and fight there, without escaping, until bloody mutual destruction had set in. In short, I did not believe what I read because it did not fit into my previous knowledge, preconceptions, or experience. It just seemed to be another one of those bizarre reports that I hear all the time that are almost invariably the result of false identification or faulty observations. I would therefore have banished the report of the murderous crows from my mind had it not been for some observations of crows I made one evening in the city of Burlington, Vermont.
I have watched crows every winter for the past twenty years as they come to the Burlington area each winter. At dusk they start to form their communal sleeping roosts, which number in the thousands. They come flying into their roosts in endless long strings or queues flying high, forming diffuse, gray cloudlike aggregations against the snowcap of distant Mount Mansfield. Just when I think the last of them have arrived, I see many more behind, in what seems like an endless stream. All of them converge on one darkening spot near or in the city.
As spectacular as these flights to the evening roost are, to me the most notable thing about them is where the birds settle for the night. The roosts are never in the forests where crows commonly nest in the summer, nor are they in the pines along the fields that crows like to inhabit. Curiously, the city center itself now seems to be the preferred winter roost site, as the birds settle not far from the hustle, the bustle, the traffic, and the lights. Once I saw them roost in a patch of pines flush up against the I-89 Interstate highway, where they were surrounded by the busy Burlington exit ramp.
One evening I watched them again as they came into the heart of the commercial district on Church Street. Round and round they flew in swirling clouds above the evening town crowd going to restaurants and theaters. It seemed as though they were looking for a place to land. I watched them fly over patches of trees at the edge of town that looked to me like ideal roosting sites, yet the birds still kept coming back into the center of town. Eventually they settled in several young cottonwoods next to Bove’s restaurant. The birds were soon closely packed upon the branches, as ever more continued to stream in. That is when it suddenly occurred to me why the roost was here rather than elsewhere.
For decades there has been a heated debate about why birds join flocks. In the 1950s the British biologist V.C. Wynne-Edwards speculated that birds form communal roosts in order to assess their population size, so that they could then decide whether it was appropriate to reproduce, in order to maintain a stable population. To biologist’s ears now this idea sounds about as plausible as that of the sun orbiting the earth to an astronomer. Wynne-Edwards’s theory on bird flocking for the