NESTING SITE 73 



has also decreased the number of its natural foes, is no doubt respon- 

 sib)^ for much of this variation; few of our native birds have so pros- 

 pered through the change from forest to farm; though even more 

 marked departure from feral nesting habit has been shown by the 

 Chimney Swift, Barn and Cliff Swallows, Martin, House Wren and 

 Bluebird; not to mention the irrepressible English Sparrow. Herring 

 Gulls, on the same islet, build in trees as well as on the ground, and as 

 Dutcher and Baily have shown, the tree nests have a solid foundation 

 of sticks and twigs which is lacking in those nests built on the ground. 

 (The Auk, XX, 1903, p. 419.) 



On Gardiner's Island, where there are no predatory mammals, and, 

 with the exception of Crows, practically no enemies of nesting birds, 

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

 with apparently equal chances of rearing their young. Here too 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, their eggs and young are as safe as those of the tree-nesting in- 

 dividuals of their kind. 



As I have elsewhere said ("Camps and Cruises," p. 37, also pp. 38-61 ; 

 and Bird-Lore, V, 1903, p. 59), it is not probable that in instances of 

 this kind certain birds have with deliberate intent abandoned the 

 customs of their species, but the tendency to vary, being unchecked, 

 finds tangible 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 wide departure from the standard 

 may be attended by disastrous results. Environment is thus the mold 

 in which habit is cast. 



Through these generalizations we come to the more practical,* 

 definite side of the question of nesting-site, and ask which sex selects 

 it. With some species it is known to be the female, with others the 

 male, and with others still the situation must evidently be satisfactory to 

 them both; but exact observations on this subject are few. 



More difficult it will be to learn whether the same individual occu- 

 pies the same site and even the same nest season after season. Fish 

 Hawks and birds which return [to the same islet year after year 

 are known to do this, and the habit is probably common to many 

 species. Doubtless the present-day practice of banding birds with 

 numbered metal tags will, in time, yield much valuable information in 

 this field. 



With birds which have more than one brood in a season, a new nest 

 is usually built. It is then of interest to compare its site with that chosen 

 for the earlier nest, to ascertain how much variation in site-selection 

 the same individuals may exhibit. 



Material. The material of which a bird builds its nest depends 

 in most cases upon the nature of the bird's haunts. The nests of marsh- 

 haunting birds are usually made of reeds or woven of wet marsh grasses; 



