How to Study Birds
THE NESTING SEASON
BY FRANK M. CHAPMAN SECOND PAPER
Time of the yeah—Why should a bird build its nest at a certain time
of the year? Some variation in nesting dates, it is true, is
Time a] the shown by all species of birds, but they do not affect the truth
nesting team” of the statement that most species have a definite nesting
season when, year after year, they may be found occupied
with household cares. In March, near New York city, we look for the
Barred Owl; by April r5 the Bluebirds have returned to their favorite
box; about May 10 the Phtebe will have finished its mossy structure;
while the middle of June will have come before the Cedarhird and Gold- tinch are domiciledi
In a general way, it may be answered that the nesting period, as a whole, is determined by those seasonal changes which, independent of latitude, divide the year into winter, spring, summer and autumn. In the extreme North it is possible for birds to nest during onli a small portion of the year; here the relation between the nesting period and the season is obvious enough. But in the South the same reason does not hold good, for, as far as climate is concerned, birds might rear their young any month in the year.
So we look for a deeper reason why there should be this regular. annual nesting season, and we find it in the bird itselfi 1n the bird world, as in the plant world, there exist cycles of physiological development. The tree
. leaves. blossoms, fruits, loses its foliage and rests: then, Bird: am] . . . . .
tree: all in their due time, the same events are repeated in their proper orderi [bus the bird migrates (if it he a migratory species), mates, builds its nests, lays its eggs, ncubatcs, rears its young. molts, acquires a new plumage and migrates to iwinter quarters.
There are, of course, exceptions to this program, as where a bird raises more than one brood or has more than one molt; but these are only varia- tions in the underlying physiological laws which, through a regular series of phenomena, prepare the bird for the nesting season. Probably their sim- plest manifestation maybe found among the sea—birds of the tropics, which, as regards climate and food, livc all the year under practically the same conditions, and still have their annual nesting season, going to their breeding grounds with the utmost regularity.
The insect-, seed- and fruit-eating species, however, require an abun— dant supply of food during the nesting season, when, within a comparatively limited area, they must find sustenance for their young as well as for them? selves, Now, while it is [I'th that in the tropics food is to be had the year
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