204 
ZOOLOQT, 
the gulls, auks, etc., drop tlieir eggs on bare ground or 
rocks; as extremes in the series are the elaborate nests of 
the tailor-bird, and the lianging nest of the Baltimore ori- 
ole, while the woodpecker excavates holes in dead trees. 
As a rule, birds build their nests concealed from sight; in 
tropical forests they hang them, in some cases, out cf reach 
cf predatory monkeys and reptiles. Birds may change 
their nesting habits sufficiently to prove that they have 
enough reasoning powers to meet the exigencies of their 
life. Parasitic birds, like the cuckoo and cow-birds, lay 
their eggs by stealth in the nests of other birds. '^Some 
of the swifts secrete from their salivary glands a fluid which 
rapidly hardens, as it dries on exposure to the air, into a 
substance resembling isinglass, and thus furnish the ' edi- 
ble bird's nests ^ that are the delight of Chinese epicures. 
In the architecture of nearly all the Passerine birds, too, 
some salivary secretion seems to play an important part. 
By its aid they are enabled, to moisten and bend the other- 
wise refractory twigs and straws and ghic thorn to their 
place. Spiders' webs also are employed with great advantage 
for the purpose last mentioned, but perhaps chiefly to at- 
tach fragments of moss and lichen so as to render the whole 
structure less obvious to the eye of the spoiler. The tailor- 
bird deliberately spins a thread of cotton and therewith 
stitches together the edges of a pair of leaves to make a 
receptacle for its nest In South America we have a 
family of birds {FurnariidcB) which construct on the branch- 
ing roots of the mangrove globular ovens, so to speak, of 
mud, wherein the eggs are laid and the young hatched. 
.... The females of the hornbills, and perhaps of the 
hoopoes, submit to incarceration during this interesting 
period, the males immuring them by a barrier of mud, 
leaving only a small window to admit air and food, which 
latter is assiduously brought to the prisoners.'^ (Newton.) 
The duties of incubation are, as a rule, performed by the 
female, but in most Passerine birds and certain species of 
other groups, the males divide the work with the females^ 
