Winter World_ The Ingenuity of Animal Survival - Bernd Heinrich [48]
Down feathers (enlarged).
Intermediate—contour feather.
Water-shedding, shading, and flight feather.
Fundamentally, it’s a problem of finding several likely functions of feathers that could have been a bridge between two opposite operations. A prerequisite for any such crossover of function in the bird’s evolution of flight is the original fluffy downlike insulating feathers of an endothermic dinosaurlike creature must have become even more useful as they became less insulative (by becoming longer and flattened), long before their utility in supporting flight became possible. Several ideas for the function of stiffened, flattened, and elongated feathers on forelimbs have been proposed. These include using such feathers on the wings as enlarged fingers to help capture prey, to enhance sexual signaling, to boost running speed or maneuverability, and to aid tree-climbing capacity to escape predators.
Shielding downy young under mother’s wing from rain, cold, and sun.
I’m not claiming that adding another hypothesis—that birds evolved flat feathers on their forelimbs as parasols to reduce wetting of the insulation—adds clarification. It’s hard to know now in retrospect, after a hundred million years. On the other hand, it is perhaps almost too obvious that when feather venules are hooked together into a sheetlike structure (as they are in flight feathers and body contour feathers), then they do provide a barrier to both air and water simultaneously. When the forelimbs with such strategically placed feathers are held dorsally over the back, then almost any elaboration of feathers in this direction provides some parasol effect that would help shed water and reduce the wetting of insulative down feathers underneath. If elaborated on further, then eventually such a parasol would need relatively little additional development before becoming useful in several others of the aforementioned functions, including flight itself. It is analogous to a flying squirrel’s fluffy, insulating tail having the hairs arranged into a flat aerofoil. As I thought about the kinglets perched somewhere in the cold with rainwater flowing off their long wing feathers to protect their down underneath, it seemed to me that regardless of whether the parasol theory could resolve a long-standing evolutionary puzzle, it might at least help explain how kinglets survive a stormy night.
Newly hatched chicken.
Ten days old—now less under hen’s wings.
09
THE KINGLET’S WINTER FUEL
I’m searching for kinglets in a stand of red spruces and balsam firs on the north side of Alder Stream on a morning in mid-January. It is dark in these woods even in the summertime because the dense tree crowns shut out the sunlight. So little light gets through that only moss grows like a green velvet carpet on the brown springy humus. Yellow and purple mushrooms erupt there after summer rains. In spring the blackburnian, yellow-rumped, and magnolia warblers sing and build their nests here.
The warblers are gone now, and it is a different world. The snow packed onto the dark-needled branches above me excludes even more light than it did before, while the snow covering the mossy ground reveals tracks. Deer have recently been crossing the brook and their worn path through this patch of woods continues on to their food in more open hardwoods beyond. A porcupine has worn a tunnel-like groove through the snow from its shelter under a pile of rocks to a lone hemlock where it feeds at night. Few snowshoe hares cross this area, because there