Winter World_ The Ingenuity of Animal Survival - Bernd Heinrich [93]
18
AGGREGATING FOR WINTER
All kinds of creatures form tight-knit societies in winter, even those that don’t crash a cozy cabin and even those that don’t need to seek warmth. One November day some years ago I squatted down on the freshly fallen but already matted leaves in the woods. Within seconds I detected the unmistakable odor of stinkbug. Digging under the leaves, I found dozens of them massed together, presumably settling in for hibernation. I’d disturbed them in their bivouac, and they were giving off their foul-smelling defense secretions. I did not need to taste them—I knew they tasted as bad as monarchs (but not nearly as bad, according to this gourmet, as a mass of overwintering spider eggs!).
It is not only the stinkbug that smells or tastes foul. Almost any insect that is brightly colored except some mimics of them) is sure to do the same. The ladybird (or ladybug) beetles that aggregate by the thousands both in my Maine cabin and my Vermont home in the fall, when hoping to stay the winter and when unduly disturbed, put out a foul burnt-rubber smell that is overpowering. Some species of these beetles of the family Coccinellidae aggregate by the millions, and in California and other areas of the western United States where they mass up under rocks or at the base of trees up in the mountains, they are scooped up in buckets and sold to gardeners for aphid control. The reason for aggregating in both stinkbugs and ladybird beetles is likely for the purpose of massing stink power. That is, if you want to be associated with stink, make your own, look like a stinker, and better still go where others stink like you.
Aggregating in winter brings animals, at least some snakes, other advantages. One of the most amazing snake aggregations are those of the Manitoba red-sided garter snakes (Thamnophis sirtalis parietalis). Like stinkbugs, garter snakes give off a foul smell when you tread on or otherwise molest them. Each fall writhing masses of these snakes pack themselves like live spaghetti into specific crevices—ten thousand in a single depression of a few cubic feet—in the rocks of a barren region near Winnipeg. The snakes spending the winter at these spots avoid freezing and gain protection. While they’re all together, they perform another primal function just before dispersing in the spring.
Males emerge from the rocky crevices before females and then wait at the periphery to intercept the females. As soon as a female emerges from the den, she is enveloped in a ball of dozens of suitors. Curiously, some of these males mimic females, and are mistaken as such by other males (Shine and Mason 2001). Their behavior just doesn’t make sense (yet), but with more information I trust that it eventually will.
Aggregation behavior has its share of such mysteries. Some years ago I received a letter from a man in Alaska who wrote of seeing a communal crow roost in winter woods, where “the ground was littered with fighting crows who were murderously hacking one another to death.” He had never seen anything like it. Neither have I. Nor could I make any sense out it, no matter how hard I tried to twist the scenario into a logical possibility.
Communal bird roosts are not closed societies. In the Maine woods, my colleague John Marzluff (who worked with me on ravens for three years) routinely introduced long-captive ravens into established communal raven roosts, and these new birds showed no hesitation in joining and were immediately accepted by the group. The next morning they then followed the roost occupants to the crowd’s feeding place, such as a deer or cow carcass. There was never any exclusion of strangers. Communally roosting birds even tolerate other species. Crow roosts in the Old World sometimes contain