Nighttime Niches—Part 2

Perching birds have evolved a neat trick that keeps them from tumbling off their roost, after they fall asleep. As a kid, I sometimes fell out of bed, but the floor was a lot closer for me than the ground is to a chickadee perched high in a tree. So evolution has given them an automatic clenching grip; a tendon in their leg that involuntarily locks onto the perch and won't let go until they awaken and straighten their leg. I guess they can't afford to have twitchy legs in the night, lest they come tumbling down.

(I jokingly envision a perching bird settling on an ice-coated twig on some winter night, locking its grip around the twig and promptly falling asleep. Soon its feet begin to melt the ice a little, the twig becomes slippery, and the bird flops over, head down, remaining asleep, with its claws still clamped to the twig. That's an unlikely scenario, but songbirds have been observed sleeping upside down—feet still locked onto the branch, even without ice.)

Once it finds a niche that seems to be safe from predators, a small bird's top priority becomes staying warm. Birds maintain a high body temperature (105^o F or more) and their tiny body loses heat quickly. Hence, their metabolism screams along at high speed—so high that they must spend many hours each day trying to fill their tiny tummies with enough food to fuel themselves. It can be a real threat for a bird to snooze through the night without eating—especially when the overnight temperatures drop precipitously.

So how do they keep from freezing? (When sudden cold snaps occur, in fact, many songbirds do perish.) First of all, evolution has given them feathers—a superb insulating coat. A bird will fluff its down feathers out beneath its outer flight feathers, and turn its head around, burying its face in its warm fuzzies. In fact, birds' ancestors—some species of dinosaurs—were the first to evolve feathers, partly to keep them warm. Feathers are a better insulator than mammal fur. Humans still strive to synthesize materials that do as well as goose down.

Second, some birds enter a state of torpor while they sleep—significantly lowering their body temperature to conserve energy. Tiny hummingbirds take this route. (Smaller birds lose body heat far faster than big birds.) But I wonder if a hummingbird could go so deep that it ignores the morning alarm clocks (other birds singing loudly), and thus miss filling its gustatory needs for the day.

Third, some birds seek out a nighttime perch that is not only safe from predators, but is much cozier than staying out in the cold: they tuck themselves into cavities or sneak into man-made shelters. Finally, the flocking technique; where many birds huddle together for safety. It helps to share body heat. But I wonder if they jostle through the night for interior sleeping positions that are warmer, as penguins do in southern polar climes.

I guess that most of my bird-watching times will remain during daylight hours. Even if I could find a way to observe them at night (get me a pair of infrared binoculars?), it'd be rather boring—sort of like the proverbial exciting sport of watching grass grow.