Winter World_ The Ingenuity of Animal Survival - Bernd Heinrich [51]
I shot the first kinglet at dusk when the bird’s stomach would presumably be full. Wanting to maximize the information that it might yield, I took its body temperature as soon as it hit the ground. As mentioned previously, it was an astonishing 44°C (111°F), which is about 2° to 3°C higher than that of most birds. My fingers were rapidly cooling and in danger of numbing, and I then opened the kinglet quickly to see golden yellow fat among the intestines and around its bean-sized gizzard. As I pried open the tiny gizzard to put the contents into a vial of alcohol, I found it filled to capacity. But this gizzard did not contain springtails, as the literature had led me to believe. Not a one. Instead, it contained a total surprise: the partially digested remains (mostly skins) of thirty-nine geometrid (“inchworm”) caterpillars. They were a species that neither I nor an entomologist, Ross T. Bell, who made a minute analysis of the stomach contents, could identify. Nobody would ever have predicted caterpillars on trees in the depth of winter, much less in a kinglet’s stomach. I would not have been more surprised if I’d have found earthworms.
Geometrid caterpillars of most species are well known to over-winter as pupae, safe from frost in the subnivian zone or deep underground. Nobody had ever reported finding caterpillars on trees in the northern winter before. But this bird and subsequent birds, were proof of something new and unexpected.
If birds could find caterpillars, so could we. So, later on in January 1995, after temperatures had again been near -30°F for a few nights, I went out into the woods with four students (Jeremy Cohen, Kristian Omland, Lauri Freedman, and Mike Tatro) on a caterpillar hunt. Having wielded a gun earlier, I now carried a club—a six-foot-long thick trunk from a freshly felled maple tree. The club was heavy, and when I banged trees (up to six inches diameter) as hard as I could, they vibrated from the shock and released a shower of bark and other debris and the tree’s crop of overwintering insects onto the snow.
The operation was a big success. I hit fifteen each red spruce, balsam fir, beech, and red and sugar maples and the yield from the seventy-five trees was thirteen tiny geometrid caterpillars (and two small spiders). No springtails. The caterpillars were visible on the snow, but hard to distinguish from needles and debris. They were gray and brown and very small, matching the remains of those I had found in the kinglet’s gizzard.
That caterpillars apparently exist on open branches in winter, and that they serve as winter food for kinglets was not known before. But that meant little unless they could be identified. After some checking around I determined that possibly only one man in the world could do it: Douglas Ferguson, at the Systematic Entomology Laboratory of the National Museum of Natural History of the Smithsonian Institution in Washington, D.C. I sent Ferguson some of the larvae I had collected. He examined them and speculated on what they might be, but said that they didn’t seem to be a precise match with any he knew. “The only way of identifying such larvae beyond doubt is to rear out adults, which are easy to identify,” he said. He had indeed tried to rear the larvae I had sent him but they had all succumbed, probably because they had not fed. There was no way of feeding them until one knew what their food plant is, and there is usually no way of knowing what the food plant is until one knows what the insect’s species is!
On another survey—January 1—I banged a total of 224 trees (102 conifers and 122 deciduous trees, 10 species in all). This yielded only eleven caterpillars, all geometrids, and all but four of them were from sugar maples. I decided to try to rear some of these larvae myself. After collecting them at my winter retreat in the Maine woods, I put the frozen caterpillars into glass vials and left them outside the cabin