Winter World_ The Ingenuity of Animal Survival - Bernd Heinrich [96]
Crows are now federally protected birds, safe from the likes of state game commission dynamiters and others who took their lead to kill crows at every opportunity. Nevertheless, they also have their natural enemies, primarily great horned owls. Great horned owls nest in the winter, when they require much food to feed their fast-growing young. I have found two of their nests and heard them sing at night within two miles of downtown Burlington. The owls are perceived by crows to be their greatest enemies, and if they find one, the alarm goes out on the roost and dozens of crows converge quickly to harass the owl relentlessly, with the goal of driving it as far as possible away from where the crows will sleep at night. As my detailed observations of Bubo, a tame but free great horned owl, in One Man’s Owl revealed, crows easily outmaneuver these great but somewhat clumsy predators in the daytime. At night it’s a different story. It pays the crows to be in Hamilton’s selfish herd at night, even as it pays them to be in Zahavi’s information center at dawn. The two are not mutually exclusive. A communal roost can serve more than one function. The more benefits it confers, the more it is likely to evolve and to be maintained.
What I thought I was seeing in Burlington—crows trying to get into the city center—was no aberration. In one study of crow roosts in the Sacramento Valley city of Woodland, California, biologists Paul W. Gorenzel and Terrell P. Salmon document that the common crow’s winter communal roosts are preferentially located in commercial (rather than residential) areas of cities, characterized by high nighttime light levels, and paved parking lots and commercial areas that have high noise and disturbance levels from vehicles and people. There are no great horned owls prowling downtown Woodland or Burlington or most other well-lit, noisy city centers.
That finally brings us back to the inexplicable anecdote about the crows “fighting” and “hacking” one another to death in a roost. What we see is not necessarily what is. The story is almost always in the details. The observer had mentioned that the episode occurred in the woods, hence it might have been in a location where one or a pair of great horned owls had access to the roost. It also could have been on a heavily overcast night in which much confusion ensued when the predators struck. With perhaps thousands of birds fluttering around in confusion, the predators would have maimed many crows, which would have then fallen, injured, to the ground. I have myself once seen an owl-killed crow, from which only a small portion of flesh was removed. When the observer arrived on the morning after an owl attack, he might have heard a bedlam of alarm from the injured crows. With many crows all around in the dark, the owls would not need to hold back. The carnage of crows that was described at the crow roost reminded me of a scene related to me by ornithologist Jeremy Hatch where a great horned owl raided a tern colony: “A dry pond-bed was strewn with about forty corpses: most headless, wings are generally torn off, or at least broken. Occasionally legs are gone. No evisceration. No plucking.”
Crows in pain and in fear and not knowing what had hit them would blame and may strike out in frustration and anger at others near them. It is impossible to reconstruct what actually happened in the case of the warring crows, but there is one thing of which we can be absolutely sure. There are many advantages in different animals to aggregate in the winter besides keeping warm, but doing so in order to fight is not one of them.
A currently more public and more scary myth than that of the presumed warring crows is that the species might become extinct due to the much publicized West Nile virus. Crows occasionally die from this virus (and other causes), and there are cases of humans killed by it as well. After