• Choose a category
• All
• Classic-Fiction

By Root 1316 0

common good at the expense of the individual was termed “group selection.” Animals can act cooperatively, but the specific example of Wynne-Edwards’s theory sounded so ridiculous that his baby was soon summarily tossed out. But other ideas were spawned. William D. Hamilton came along and proposed the “selfish herd” hypothesis, which posited that animals form groups for their individual safety, using one another as shields. By being in a communal roost, the individual birds also profit from many eyes with which to see approaching danger. Hamilton’s hypothesis made sense, and it also fit empirical observations.

The Israeli biologist Amotz Zahavi then vigorously promoted a third hypothesis that seemed plausible to account for other group behavior as well. Zahavi proposed that when birds roost communally they gain information on where to find food. His “information center,” or IC, hypothesis provoked a good deal of controversy and generated hundreds of papers in the learned journals, perhaps because it looked suspiciously like a form of “group selection.” Our data on raven communal roosts in Maine, principally the demonstration of naive birds being led to cattle carcasses after joining a roost (Marzluff, Heinrich, and Marzluff 1996), gave perhaps the first empirically tested proof of Zahavi’s IC hypothesis. (Of course, there are still those who, quibbling about mechanism, claim that the raven’s behavior is “group selection,” unless it can be defined in terms of tit-for-tat reciprocation among individual birds, not exploitation of individuals for information held within the group.) Neither hypothesis would explain the golden-crowned kinglets’ tendency to congregate at night, because being in hiding and possibly also being in a torpid state, they could not instantly respond and escape a predator, even if one of the kinglets gave an alarm. Nor would they benefit from the group vis-à-vis finding food, as their food is highly scattered. What they do group for is warmth; unlike corvids, they seek body contact.

As I watched the crows in the city and wondered which if any of the existing aggregation hypotheses might apply, I was reminded of seeing other huge crow roosting aggregations elsewhere in both North America and Europe. Crows used to be thought of as strictly rural birds, but in the last fifty years they have started roosting in cities all over the world. There was clearly something significant in the way that the birds avoided the woods to be in the town lights and the bustle. It was not just a stray observation. The birds were free to roost in forests available within a half mile of the city, yet they had flown miles just to come here where they had to search long for a suitable landing site, eventually choosing the few available trees downtown.

Crows have greatly increased in numbers all over the North American continent over the last century, and they have been moving ever increasingly into cities. As one indication of both of these suppositions, I refer to a 1946 report in the Oklahoma Game and Fish News (Vol. 2: 4–7, 18), written by H. Gordon Hanson, a biologist of the Oklahoma Fish and Game Commission. His report gives the map locations of 47 “major” winter crow roosts in Oklahoma, those with 200, 000 or more crows per roost. “It all started back in 1933,” Hanson writes, “when it was first brought to the Oklahoma Game and Fish Commission’s attention that the numbers of crows wintering in the state had begun to increase alarmingly and to spread over a much larger area.” The crows fed on crops and came from the northern nesting areas in the prairie provinces of Canada where they “molest the nests of waterfowl in the duck factories.” Although the winter crow roosts in Oklahoma had increased considerably in size and number, the Oklahoma Commission perfected metal cylinder bombs filled with steel shot and dynamite and then carried out an annual crow-bombing campaign. In eleven years government bombers bragged up a tally of approximately 3, 763, 000 crows killed. Given these data—and the probability that people living in