BIRDS OF THE UNITED STATES. 
225 
this peculiar deviation, as we are led to infer of the existence of one 
somewhat remotely situated from what that observer says. Where there is 
no danger to be apprehended from enemies, a jDOsition like the one de- 
scribed, affords the requisite advantages. In buildings that are freely used 
by man, ]3ersistenee in this habit might be attended with results, 'which 
would be escaped by nidification in chimneys. In Southeastern Illinois 
Mr. Ridgway discovered a nest in a hollow sycamore-tree, and also another 
securely fastened to a plank of an out-building. Mr. Allen met it in 
Kansas, breeding in hollow trees, for which it seemed to manifest a decided 
preference. In long-settled sections the species has been forced by circum- 
stances to resort to chimneys, or else abandon the country. In but a single 
instance have we known from actual experience the nest to he built against 
the rafter of a deserted out-building. 
Nesting usually commences within a week after the birds have reached 
the accustomed haunts. Whether mating is accomplished preparatory to 
leaving their southern homes, or subsequent thereto, we are not prepared 
to assert, but incline to the belief that they arrive already paired, since 
we have never been able to observe the sexes engaged in their amours. 
Perhaps these are performed within the sheltering walls of the flues which 
they choose for their homes. The labor of nest-building requires the 
united efforts of the builders for three days. The structure is a rather 
curious and remarkable affair. Its composition consists of small twigs, 
nearly uniform in dimensions, which are severed from the living branches 
of trees, by the birds while upon wing, with considerable dexterity and 
adroitness. They sweep upon the coveted twig much after the fashion of 
a hawk in rushing iq^on its prey. These twigs are fastened to each other 
by the saliva of the builders, and by the same cement the entire fabric is 
made to adhere to the side of the chimney in which it is placed. This 
saliva, after a slight exposure, hardens into a glue-like material, which 
becomes as firm as the branches it joins together. In separating these 
nests from their reposing surfaces we have even known fragments of the 
bricks to give way, rather than the cementing substance. "When rendered 
