Allen on an Inadequate " Theory of Birds' Nests.” 25 
most readily available for their construction being not everywhere 
the same ; they also vary in accordance with the climatic conditions 
of the locality, the same species building a quite different nest, as 
respects warmth and stability, in the colder portions of its habitat 
from that which it constructs in the warmer portions. 
But while these deviations under diverse circumstances readily 
explain variation in the situation and character of the nests of the 
same species, they fail to explain why closely allied species, living 
together under precisely the same conditions of environment, and 
sometimes so closely resembling each other in size, color, and all 
external characters as to require the eye of an expert to detect their 
specific diversity, should build totally unlike nests, and display 
almost the widest possible differences in respect to their situation. 
To cite, in illustration, a single example from the many that might 
be given, we may instance our common Pewees and Flycatchers. 
In this small group we find a wide range of diversity in breeding 
habits among species most intimately related in structure and gen- 
eral habits. The Least Pewee builds a small, compact, felted nest 
of fine soft materials, and its nearest allies, the Acadian and Traill’s, 
build far ruder and much more bulky structures of coarse grasses, 
strips of bark, and other similar materials. Another near relative of 
these species, the Wood Pewee, selects for its nesting-site a lichen- 
covered dead branch, on which to saddle its small, highly artistic, 
cup-shaped nest, covered externally with lichens glued to the surface 
in such a manner as to render the structure almost indistinguish- 
able from a natural protuberance of the branch itself. The Bridge 
Pewee, another allied species, builds a large bulky nest, formed 
outwardly of a heavy layer of mud, copiously lined with dry grass 
and feathers, and shelters it in the chinks of walls, under shelv- 
ing rocks, in sheds, outbuildings, and under bridges. The Great- 
crested Flycatcher chooses hollow trees or deserted Woodpeckers’ 
holes in which to form its nest and deposit its eggs, -while its allies, 
the Kingbirds (genus Tyrannies), build large open nests, which they 
make no attempt to conceal. 
Notwithstanding all this diversity of situation and structure 
among closely allied species, birds’ nests have been divided into 
two classes, according to “ whether the contents (eggs, young, or 
sitting bird) are hidden or exposed to view,” and the broad general- 
ization based thereon that the character of the nest is intimately 
related to the color of the female parent-bird. This, in fact, is Mr. 
