158 NATURAL HISTORY OF SELBORNE. 
at first perhaps by this little bird), raise but a moderate layer at a 
time, and then desist, lest the work should become top-heavy, and 
so be ruined by its own weight. By this method in about ten or 
twelve days is formed an hemispheric nest with a small aperture 
towards the top, strong, compact, and warm ; and perfectly fitted 
for all the purposes for which it was intended. But then nothing 
is more common than for the house-sparrow, as soon as the shell 
is finished, to seize on it as its own, to eject the owner, and to line 
it after its own manner. 
After so much labour is bestowed in erecting a mansion, as 
Nature seldom works in vain, martins will breed on for several 
years together in the same nest, where it happens to be well- 
sheltered and secure from the injuries of weather. The shell or 
crust of the nest is a sort of rustic-work full of knobs and 
protuberances on the outside; nor is the inside of those that I 
have examined smoothed with any exactness at all; but is rendered 
soft and warm, and fit for incubation, by a lining of small straws, 
grasses, and feathers, and sometimes by a bed of moss interwoven 
with wool. In this nest they tread, or engender, frequently during 
the time of building; and the hen lays from three to five white 
eggs.* 
At first when the young are hatched, and are in a naked and 
helpless condition, the parent birds, with tender assiduity, carry 
out what comes away from their young. Was it not for this 
affectionate cleanliness the nestlings would soon be burnt up, and 
destroyed in so deep and hollow a nest, by their own caustic 
excrement. In the quadruped creation the same neat precaution 
is made use of ; particularly among dogs and cats, where the dams 
lick away what proceeds from their young. But in birds there 
seems to be a particular provision, that the dung of nestlings is 
enveloped in a tough kind of jelly, and therefore is the easier 
conveyed off without soiling or daubing. Yet, as nature is cleanly 
in all her ways, the young perform this office for themselves in a 
little time by thrusting their tails out at the aperture of their nest. 
As the young of small birds presently arrive at their 7Acxia, or full 
growth, they soon become impatient of confinement, and sit all 
* Martins return to the same spot, or some corner of a window; this has been ascer- 
tained by direct experiment; but the nest, the structure of clay, is generally, if not 
always, rebuilt; and the clay, or sometimes almost sand, is rendered adhesive by the 
saliva, or a secretion for the purpose. In their natural habitats the nests are placed 
together frequently in contact, generally on the surface of some over-hanging cliff. We 
have seen from fifty to one hundred nests thus placed. 
