vi A THEORY OF BIRDS’ NESTS 123 
while our thrushes, warblers, and finches, as well as the crow- 
shrikes, chatterers, and tanagers of the tropics, together with all 
raptorial birds and pigeons, and a vast number of others in 
every part of the world, all adopt the latter mode of building. 
It will be seen that this division of birds, according to 
their nidification, bears little relation to the character of the 
nest itself. It is a functional not a structural classification. 
The most rude and the most perfect specimens of bird- 
architecture are to be found in both sections. It has, how- 
ever, a certain relation to natural affinities, for large groups of 
birds, undoubtedly allied, fall into one or the other division 
exclusively. The species of a genus or of a family are rarely 
divided between the two primary classes, although they are 
frequently divided between the two very distinct modes of 
nidification that exist in the first of them. 
All the Scansorial or climbing, and most of the Fissirostral 
or wide-gaped birds, for example, build concealed nests ; and 
in the latter group the two families which build open nests, 
the swifts and the goatsuckers, are undoubtedly very widely 
separated from the other families with which they are asso- 
ciated in our classifications! The tits vary much in their 
mode of nesting, some making open nests concealed in a hole, 
while others build domed or even pendulous covered nests, 
but they all come under the same class. Starlings vary in a 
similar way. The talking mynahs, like our own starlings, 
build in holes, the glossy starlings of the East (of the genus 
Calornis) form a hanging covered nest, while the genus 
Sturnopastor builds in a hollow tree. One of the most 
striking cases in which one family of birds is divided between 
the two classes is that of the finches; for while most of the 
European species build exposed nests, many of the Australian 
finches make them dome-shaped. 
Sexual differences of Colour in Birds 
Turning now from the nests to the creatures who make 
them, let us consider birds themselves from a somewhat 
unusual point of view, and form them into separate groups, 
1 Recent research places the goatsuckers nearest to (though still far 
from) the owls, while swifts are again brought nearer to the swallows. 
Dr. BR. W. Shufeldt in Journ. of the Linn. Soc., vol. xx. Zoology, p. 383. 
