A couple of weeks ago I saw a couple of golden-crowned kinglets in Prospect Park. The golden-crowned is one of the smallest species of bird in the northern hemisphere: they weigh in at five grams, about the weight of two pennies. They have a gorgeous golden-orange mohawk crest and are always a delight to see. Their cousins the ruby-crowned are a little larger and more commonly seen, and can be as bold as streetwalkers, getting quite close to the observer, but the golden-crowned are generally farther away. Anyway, it seemed awfully cold for these birds to be here in the winter, but it turns out they are found even further north. In fact, along with raven, they are cold weather specialists of the boreal. In Winter World, which I’m reading now, Bernd Heinrich tries to figure out how they do it. Feathers are a great insulator, and wing feathers can also act as what my English cousins would call a waterproof – there’s a notion that the proto-birds used feathers for insulation and waterproofing, not flight.
Kinglets, meanwhile, run a body temperature of about 111F, hotter than most birds, which means they need lots of fuel. It’s been a mystery what these insect-eaters find to eat when the weather drops to –30F at night (Heinrich’s studies are up in Maine). He found them full of inchworms, wee geometrid caterpillars. Say what, in January in Maine? The caterpillars freeze without ill-effects (um, except for being swallowed by kinglets). Heinrich tried to ID the bugs (there are many, many insects and their various stages that remain mysterious) over a couple of years by trying to get them to pupate in the spring; he finally got a moth and had it identified as a one spot variant, Hypagyrtis unipunctata. The over-wintering strategy of this species had not been known beforehand.
Now, check this out: a kinglet’s brain mass per body-weight is 6.8% (humans: 1.9%). That’s metabolically very expensive. To keep warm, and to keep that energy-sucking brain going, in harsh conditions that would kill us if we were naked (or just wearing our hand-knit wool socks), caterpillars are absolutely key. And of course, the caterpillars need to eat, too. It goes around and around.