68 NUMBER OF BROODS 
wings frequent affords concealment for their nests. The Woodcock, 
on the other hand, nests shortly after its arrival; possibly because a 
site is at once available. 
Consequently, in addition to those physiological factors which 
induce an annual nesting-season as one of the phenomena in the cycle 
of the bird’s year, the exact date of a bird’s nesting appears to be 
governed by (1) the nature of the food of its young; (2) whether it 
is resident or migratory, though this remains to be determined; and 
(3) the condition of its nesting haunts. To these will doubtless be 
added other causes, as we become more intimate with the facts involved. 
Number of Broods—Why do some birds raise only one brood and 
others two or even three? We should look for a partial answer to this 
question in the length of time required by a species to rear a brood. If 
the period from the beginning of the nest-building to the date when the 
resulting young are able to care for themselves is so short that the 
parents are still in the physiological condition incident to reproduction, 
a second family may be expected, and under similar circumstances a 
third may follow. The eggs of the English Sparrow hatch in about 
twelve days, the young remain in the nest only about a week, and the 
species is reported to have reared six broods in a season near New York 
City, but this number is doubtless exceptional. Robins’ eggs hatch in 
thirteen days; the young leave the nest when about two weeks old, 
and the species raises two or even three broods. But the eggs of Fish 
Hawks, for example, require four weeks’ incubation; the young do not 
fly until about six weeks old, and the species is one-brooded. These 
facts, however, fail to explain why many birds in which the periods of 
incubation and rearing of the young are quite as short as those of the 
Robin, should have only one brood. The time of a bird’s arrival on 
the nesting-ground doubtless has some bearing on the question, and we 
should also take into account the time of return to its winter haunts, 
without in the least being able to say why it should come and go at a 
certain time. Still, among permanent residents and migrants, which 
arrive and depart at about the same season, some are single-brooded 
while others raise two or even three broods. For instance, of the former, 
the Song Sparrow rears two and on occasions, three broods, while the 
Chickadee has but one. Here size of the brood may be a factor. Among 
migrants, the Robin is two- or rarely three-brooded, while the Purple 
Grackle, which comes just as early and remains nearly as long, is one- 
brooded. Possibly there are here temperamental differences not to be 
explained by observable influences. 
The question, not infrequently asked, whether any of our migrant 
birds nest in their winter homes, makes it necessary to add that a bird 
has only one nesting-season, and with those species which rear more 
than one brood there is no appreciable interval of rest between the first 
and succeeding broods. 
A table of dates showing when one may expect to find full sets of 
birds’ eggs of the first laying near New York City is appended: 
