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Caterpillar on beech twig.

As before, I again brought some of the caterpillars home. All had survived being frozen, and all looked healthy when I took them out of our freezer in May. As before, I offered the larvae maple and fir foliage (carefully checking for any stray spiders this time). But the caterpillars refused to eat, except one lone larva (a very thin dark animal) that fed on the fir. It pupated on June 22. Unfortunately, I made another mistake. I had left it on the windowsill in a small jar, and moisture that had condensed on the glass wall drowned the pupa. I was, of course, very disappointed to finally almost have a moth, and then lose it.

Getting close to an identification (I hoped), I tried again the next year. I reared a number of the larvae that I again collected on sugar maple but fed on balsam fir, to adulthood. Beautifully gray-mottled moths variously marked with subtle browns and cream emerged, and Ferguson identified them as the well-known, one-spotted variant, Hypagyrtis unipunctata. The food plant of this species is reported to be extremely variable, including alders, willows, birches, oaks, and balsam firs. Charles V. Covell’s book Eastern Moths describes the variant as “extremely variable sexually, geographically and seasonally.” It had not previously been known where the larvae overwinter.

The “variant” moth reared from winter caterpillars.

It had been difficult to identify the caterpillars. We were poor taxonomists, even of trees, as my exam of the students proved. But the kinglets had apparently learned what I’d been trying to teach my students, generally less successfully. Are they intelligent? Kinglets, being small, cannot have large absolute brain size. Nevertheless, on a per-body-weight basis, their brain mass is massive. It accounts for an incredible 6.8 percent of their whole body weight (as opposed to ours of about 1.9 percent). Thus although a kinglet’s total brain mass does not amount to much in absolute terms, it does represent an enormous commitment to neurons given the size of the bird.

A brain is metabolically expensive, and kinglets live on an energy edge in winter and have no energy to spare. In humans the 1.9 percent of body tissue that is devoted to brain mass reputedly accounts for 20 percent of our energy drain. There is much debate about what our energy base could have been in order for us to evolve to support such a large brain and why we should have it.

In kinglets, the energy drain of the brain could be triple ours, and we now have the answer to the energy source question: caterpillars. Turtles are successful because the brain drain has been reduced to a minimum—a barely enlarged bump on the nerve cord—helping them to survive up to a year without food. When all is said and done, chances are we’ll never know why the kinglets’ brain size scales large. However, we can be reasonably sure that if their brain is not part of the problem of energy balance, then it is likely to be part of the solution. We don’t know how kinglets decide what to eat in the winter, what else they eat, and hence what their flexibility is. But finding out they eat moth caterpillars in the winter is not only a satisfying accomplishment for all of us who took part, but it is also the discovery of a link in their survival. To care for the welfare of kinglets, it is necessary to care for moths.

10

HIBERNATING BIRDS

The idea that birds hibernate started with a belief that the swallows that skim over the ponds in the fall spend the winter in the mud under the ice. After we learned about intercontinental migration, a more remarkable phenomenon, the first idea seemed so ridiculous that any mention even of hibernation in birds was automatically deemed nutty. Nevertheless, torpid birds were eventually found, and by highly reputable observers, including Nobel laureate Konrad Lorenz. When W. L. McAtee found a torpid, seemingly almost dead chimney swift, a normally migratory species, in mid-October 1902 (in Indiana), which quickly recovered