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 formedi 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- pl/bid’ S” nite side of thelsubject, and. ask which bird ohthe pair Sela” [be chooses the nesting-site. With some species it '15 known Site? to be the male, with some the female, and With others
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- cult to prove a case of this kind, though there is abundant 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 approximately the same limb.
SI.” 0!, H” When second or third brood is reared a new nest-is usually built, when it is of interest to compare its site Laftr Nrrtr , . . with that chosen for the earlier nest to ascertain how much variation the same individuals may exhibit.
Return to 2/}: Same Sit?
(TO BE CilN‘l'lNl'HH
