Other things being equal, small objects, in an environment that is colder than they are, cool down faster than large objects. If you increase the dimensions of an object, the volume and mass grow faster than the surface area does. The result is that small objects have more surface area per unit of mass than large ones, and hence more area through which heat can escape.
The difference is even more pronounced if you think about living birds rather than geometric objects. Larger birds have thicker layers of feathers than small ones, and are therefore better insulated. (Many arctic birds, such as the snowy owl, are actually surprisingly small birds wrapped in surprisingly thick layers of feathers.)
So what’s up with the golden-crowned kinglet? This ranks among the hardiest of the songbirds, braving the winter up into southern Canada and occurring as a regular, generally common wintering bird on the Vineyard. Yet kinglets are tiny, a good inch shorter than our black-capped chickadees and weighing only about half as much. By all rights, a kinglet should freeze solid at the first taste of winter weather.
If you watch a kinglet in action, part of the secret of its hardiness becomes clear. These birds are in constant motion, flitting from branch to branch, hovering in the air, walking sideways along twigs, nervously twitching their wings. All that work generates heat, and at least when a kinglet is actively foraging, I can easily imagine that its internal furnace keeps it quite toasty.
But this partial explanation raises two new problems. First, to keep the furnace running, you need fuel. The frenetic exploration of a kinglet is clearly aimed at gathering food, but what on earth do they find in the dead of winter? My own investigations into this question have not been satisfactory: examining twigs and bark where kinglets have fed, I’ve find nothing at all that looked edible, and I’ve had no better luck scrutinizing apparently untouched twigs in areas where kinglets were actively feeding. As far as I can tell, they eat nothing at all, relying on wishful thinking for the energy they need.
Happily, other researchers have had better luck, and the short version is that kinglets are utterly brilliant at finding tiny, often nearly microscopic insect eggs and larvae. These prey items can be well concealed in bark crevices, at the bases of conifer needles, or in the leaf buds of deciduous trees. But it is items like this that researchers have found using the effective if unfriendly method of shooting kinglets and dissecting their digestive tract to see what’s there. Sharp vision, a sharper bill, and instincts and experience give these birds the knack for piecing a living together from minute specks of insect material.
The other problem is surviving overnight, when darkness makes continuous stoking of the internal engine impossible. Here, too, the kinglet impresses. They’re said to spend coldest nights (and observers have reported these birds surviving nighttime lows as cold as -40o) amid the insulation offered by abandoned bird or squirrel nests. And they huddle up in small groups, effectively increasing their weight and volume so they lose heat much more slowly. If they behave like other birds I’ve seen snuggling up in the cold, the huddle percolates all night, with individuals taking turns on the colder fringes and the warmer interior of the huddle, so no single bird has to bear the brunt of the chill.
Golden-crowned kinglets are aptly named: dull green above, dusky white below, they have a strong yellow stripe along their crowns (males add a central orange stripe, for good measure). Good luck seeing this, though: these birds spend most of their time well up in the canopy, showing a strong preference in winter (and even more so in the breeding season) for the dense foliage of conifers. I detect them mainly by their call notes: talkative birds, golden-crowned kinglets give a nearly incessant, sibilant note near the upper pitch level that my ears can detect: “see, see, see.” If I stop and watch upon hearing the call, I can usually get a glimpse of the plump, drab, active, short-tailed birds that are producing it.
Golden-crowned kinglets are essentially boreal birds, though they nest sparingly as far south as Massachusetts (and apparently, on at least one occasion, even on the Vineyard). They’re closely related to ruby-crowned kinglets, similar birds that lack the yellow crown patch (male ruby-crowneds have a red crest, raised when the bird is aroused but otherwise rarely visible). Ruby-crowned kinglets are distinctly less cold-hardy than golden-crowned, and while a very common fall migrant here, they are scarce in deep winter. Researchers have found that ruby-crowned kinglets have a lower metabolic rate than their golden-crowned cousins, and apparently never developed the knack for roosting communally at night – more proof, if it is needed, of the remarkable adaptations that let golden-crowned kinglets survive where, in theory, it’s impossible.