NESTING SITE 
73 
has also decreased the number of its natural foes, is no doubt respon- 
sible 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 
Butcher 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 fto 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; 
