230 ELEMENTS OF ORNITHOLOGY, 
altogether, by laying in the nests of other Birds, a practice 
facilitated by the small size of their eggs. 
One brood annually is the rule with Birds, though many hatch 
two or even three broods in the year. 
Parent birds sometimes assist their unhatched brood to break 
the shell when they hear the cry of the young one within it. 
The only Birds which neither incubate their eggs themselves 
nor provide them with foster mothers, are the mound-building 
birds of the Australian region, such as Megapodius. They raise, 
as before said *, large heaps of vegetable substances—refuse of 
all kind—and earth, and therein deposit their eggs, which are 
hatched by the heat produced through such an accumulation of 
decaying and slightly fermenting matter. Their eggs are large, 
and the young developed in them are so fully formed when 
hatcheu, that they can not only force their way to the surface of 
the mound, but having reached it can fly away at once for short 
distances. 
Some species lay their eggs in the loose hot sand of the 
beach (above high water-mark), where the rays of the sun 
suffice to hatch them. 
‘Birds differ much as to the state of development in which 
they are hatched. Many are born nearly naked and helpless, 
and require good shelter till they acquire feathers, as is the 
case, ¢.g.,in the Canary and the Sparrow. Others, like the 
Heron, are born nearly naked, and acquire a downy covering 
before they acquire feathers. Others again, like the Hawks, are 
born helpless but covered with down; while yet others, like the 
Chicken, are hatched covered with down, and can run about at 
once. Birds of the latter kind are said to be precocious. The 
most precocious of all are the Mound-builders above mentioned. 
Young birds are assiduously fed by their parents, and the 
crops of Pigeons secrete a nutritious fluid which the young 
partake of, extending their heads down the gullet of one or 
other parent for the purpose. 
The relations of the colours of the plumage of the young and 
the adults of both sexes, and the process of moulting, have been 
already noted f. 
Nidification—As country boys know, the shapes of, and the 
materials used in making, the nests of Birds are different in 
different species. Some make carefully made covered nests, 
* See ante, p. 9. t See ante, p. 140. 
