NESTING DATE 67 
relation between nesting time and season is obvious enough. But in the 
south, so far as climate is concerned, birds might rear their young any 
month in the year; nevertheless, even in the tropics most species 
appear to have a more or less well-defined nesting-season. 
So we look for a deeper reason why there should be this regular, 
annual, nesting period, and we apparently find it in the bird itself. In 
the bird-world as in the plant-world there exist cycles of physiological 
development. The tree buds, leaves, blossoms, fruits, loses its foliage 
and rests; then, all in due time, the same events are repeated in proper 
order. Thus the bird migrates (if it be migratory), mates, builds its 
nest, lays its eggs, incubates, rears its young, molts, and retreats to 
winter quarters. There are exceptions to this program, as where a 
bird raises more than one brood, or has more than one molt, but they 
are only the results of variations in the underlying physiological pro- 
cesses which, through a regular series of events, prepare the bird for 
the nesting season. 
Insect-, seed- or fruit-eating birds require an abundant supply of 
food during the nesting-season, when, within a comparatively limited 
area, they must find sustenance for their young as well as for themselves. 
Now while it is true that in the tropics food is to be had throughout 
the year, it is far more abundant and varied during the spring or early 
summer. There, with the coming of rains, the trees renew their foliage, 
then insects become more numerous, and coincidentally the instincts of 
the nesting-season become active in birds. 
Confining our attention to our own birds, we observe that some 
species nest early and some late in the nesting-season. Why is this? 
The character of the food of the young is the most obvious cause 
determining the exact date of a bird’s nesting. Hence those birds of 
prey which feed their offspring on mammals or birds are the first 
birds to nest, while those birds that rear their brood on insects or fruit 
nest later. 
But is not a bird’s nesting-time also dependent on whether it be 
migratory or resident? This is a difficult question to answer, since it is 
by no means easy to determine whether or not a species is resident, 
in the strict sense of the word. Among resident species of not dissimilar 
feeding habits, there is often much difference in nesting dates. The 
White-breasted Nuthatch near New York City, for instance, nests in 
the middle of April, while the Downy Woodpecker waits until a month 
later. The Bluebird nests in the first half of April, the Cedar Waxwing 
the latter half of June. Possibly a study of the food of their young may 
explain this difference in dates. 
Some migratory birds which arrive at about the same time also 
nest on widely different dates. Robins and Red-winged Blackbirds, 
for instance, reach New York City in late February or early March, 
but the Robin nests nearly a month earlier than the Redwing. Haunts 
may here exert some influence. The early nesting Robins find favorable 
sites in evergreens, long before the vegetation in the marshes the Red- 
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