Winter World_ The Ingenuity of Animal Survival - Bernd Heinrich [30]
I’ve never seen a kinglet carrying moss or any other nesting material in winter, and I think it is unlikely that these birds build winter nests for shelter. But if they don’t build winter nests, then why not? Do they not have the time because they are living too close to the edge? I have never seen a kinglet take even a one-second break from winter foraging. Maybe when their priority is to survive the day, then making an investment that doesn’t pay off until a later date makes no sense. Additionally, if the bird has to keep moving fast and continuously all day in an environment where food is patchy, then at the end of the day (when it may have traveled far), it may not have either the time or energy to relocate its nest. Having found an area of good food, it may be the better bet for a kinglet to stay put and forage until the last moment of daylight, and in fact that is what kinglets do. Finally, a winter nest would have to be designed differently than the open cup nests for rearing young. Although modifications of nest structure are possible, a new nest design might be too large an evolutionary hurdle for them to surmount.
The verdin’s and the woodpecker’s winter shelter nests are only slight modifications of their breeding nests. Their nests evolved first as receptacles for rearing young, and that same design then fortuitously made them preadapted for use as winter shelter later on. The nest of a finch or kinglet, in contrast, is cup-shaped. It is not preadapted for shelter in winter. Shelter nests require a cover. Cup nests in winter would quickly fill with snow.
There is no law that says a priori that a bird can’t switch from making one type of nest in one season to a different type of nest in another season. It is just one of those things that is unlikely, because the more specialized the programming is in one direction the more it precludes a new specialization in another direction.
Some effective shelters that birds make in the winter are much easier to construct than nests. The grouse that dives down into the deep fluffy insulating snow to escape the cold also modifies the snow by creating a cave at the end of its tunnel where it comes to rest. This shelter is for one-time use only. The bird may spend a night or a day or more in its shelter (depending on the weather), but it never returns to it. The next night it makes another shelter. However, if snow is absent, the grouse roosts in the cover of a dense coniferous tree instead.
As I was searching in the bog for the discarded summer nests of birds, I almost took for granted the most obvious winter shelter of all, the lodges of the resident muskrats and beavers. Beavers (Chapter 12) build lodges at the edge or directly in the pond, by piling up logs and sticks, mud, and sod into a steeply conical heap of five to seven feet above water level, and then excavating from underneath. A suitable lodge can be built in the fall, but old lodges are also refurbished and may be used by generations of beavers. I measured a lodge built the previous fall, and it reached seven feet above water (or ice) level and it was fifty-two feet in circumference. On this particular lodge almost all the sticks were light yellow, because the beavers had only recently eaten off the bark. Mud had been heaped onto the sides (but not the tip top) of the mound compacting and cementing the lodge together. In the winter when the mud freezes solid, the communal beaver lodge becomes practically impregnable to wolves and other potential predators.
The entrance to a beaver’s lodge is through a water lock at the bottom so that the beavers have a safe den site even in the summer. This was not lost on John Colter, the famous Yellowstone explorer who in 1809 escaped a band of pursuing Blackfoot warriors by diving into a beaver pond and hiding in the animals’ lodge. One parched summer a year or two ago, when the local