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The question as to the number of feathers on a bird seems a simple
one without complication. Dr. Wetmore, the well-known ornithologist
who was secretary of the Smithsonian Institute, has given us some
data. The number varies with the species, of course: the smallest
bird, a hummingbird from Cuba, had the fewest, 940 feathers; larger
birds had more, the robin 2587, and the mourning dove 2635 feathers. A
glaucous-winged gull had 6540; a mallard 11,903 feathers; a Plymouth
Rock chicken was said to have 8325 feathers; and a later investigator
reported 25,216 feathers on a swan.
But as one thinks of it, more questions arise, as in any
investigation. The answer to one question poses two more. The first
question is, do not the birds in winter need a wanner plumage to keep
out the cold than they do in summer, when it is warm? Do they have more
feathers then? This was definitely true in the case of the goldfinch:
a bird in summer dress had only 1439 feathers, while one in winter
plumage had 2368 feathers, obviously an adaptation for cold weather.
The next question is more abstruse, but eminently practical: the
smaller a body, the larger exposed surface for its weight it presents.
That is, for its weight a small bird has a proportionately much greater
surface from which heat is lost than does a larger one. With equal
heat-producing mechanism and metabolism, a small bird would need
more insulation than a large one. Reduced to its simplest: one would
expect small birds to have relatively more feathers than large ones:
more feathers per gram of weight. Is this true? Two members of the
Department of Poultry Husbandry at Cornell University, Dr. F. B. Hutt
and Lelah Ball, supplied the answer. Small birds do have more feathers
per gram of body weight than do larger ones. A hummingbird weighing 2.8
grams had 940 feathers or 335 feathers per gram; a nighthawk weighing
67.9 grams had 2034 feathers or 29 feathers per gram; while a swan
weighing 6123 grams had 25,216 feathers or 4 feathers per gram of body
weight.
Presumably there are still other relations: Do the birds that live in
the tropics where it is warm have fewer feathers than species of the
same size of arctic climates, as one would expect? Are certain types of
feathers such as those of aquatic birds better insulated than those of
land birds, so that the bird requires fewer of them to keep warm? Does
a dense coat of down reduce the number of feathers needed to keep warm?
Do the loose feathers of ostriches, lacking barbules, necessitate some
adjustment in numbers? The things we've learned point the way to other
questions to be investigated.