How to Study Birds 



THE NESTING SEASON 



BY FRANK M. CHAPMAN 

 SECOND PAPER 



Time of the year. — Why should a bird build its nest at a certain time 

 of the year? Some variation in nesting dates, it is true, is 

 Time of the shown by all species of birds, but they do not affect the truth 

 nesting season of the statement that most species have a definite nesting 

 season vi^hen, 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 15 the Bluebirds have returned to their favorite 

 box; about May 10 the Phcebe will have finished its mossy structure; 

 while the middle of June will have come before the Cedarbird and Gold- 

 finch are domiciled. 



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 only 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 itself. In the bird world, as in the 



plant world, there exist cycles of phj^siological development. The tree 



leaves, blossoms, fruits, loses its foliage and rests; then, 

 Birds and ,,.,.,. , ^ j • ^u • 



all m their due time, the same events are repeated in their 



proper order. Thus the bird migrates (if it be a migratory 



species), mates, builds its nests, lays its eggs, ncubates, 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 may be found among the sea-birds of the tropics, which, 

 as regards climate and food, live 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 true that in the tropics food is to be had the year 



(25) 



