How to Study Birds 59 



and, with the exception of Crows, practically no bird enemies, Robins 

 build their nests in almost any situation, even on the ground, with equal 

 chances of rearing their young. Here, too, the Fish Hawks nest, not 

 only in trees, but also in the most exposed situations on the beach; and 

 because of the protection afforded by an insular home where the foes of 

 birds are happily absent, their eggs and young are as safe as those of 

 tree-nesting birds. 



It is not probable that in instances of this kind certain birds have 

 deliberately or intelligently abandoned the customs of their species; but 

 the tendency to vary, being unchecked, finds expression under conditions 

 where new habits may be successfully formed. Doubtless the same 

 tendency exists in the Fish Hawks nesting on the mainland ; but there 

 the struggle for existence is so much more intense that any departure 

 from habit may be attended by disastrous results. Environment is thus 

 the mould in which habit is cast. 



Through these generalizations we come to the most practical, defi- 



„., . , „ nite side of the subject, and ask which bird of the pair 



yy hich hex , , . . .,,. , .... 



„ chooses the nestmg site. With some species it is known 



„. ^ to be the male, with some the female, and with others 



Sitef 



doubtless the situation must be approved by both sexes. Very 



few exact data on this subject exist, however, and there is here abun- 

 dant opportunity for original investigation. 



The return, year after year, of the same birds to the same nest is a 

 well-established fact, particularly among the birds of prey, — the Fish Hawks 



„ being good illustrations. With smaller birds it is more diffi- 



Keturn to , r , • , • , , , , • 1 1 



, r. o- cult to prove a case or this kind, though there is abundant 



the bame oite . , 111 1 1 i- , 



evidence to show that they return to the same locality and 



select the same, or nearly the same, nesting site. A pair of Wood- 



thrushes that nest on my lawn select each year a certain maple, and 



ipproximately the same limb. 



o. ^ , When a second or third brood is reared a new nest is 



Ciite of the n l -i l • • ^ • ■ • 



J. -_ usually built, when it is of interest to compare its site 



i^dter jN ests 



with that chosen for the earlier nest to ascertain how much 



variation the same individuals may exhibit. 



(TO BE CONTINUED) 



