WHY TWO KINDS OF NESTLINGS? 17 
to which a bird is exposed, especially noticeable in 
many ground haunters. But other correlations may 
compensate where the eggs are few, such as fre- 
quency of broods, safety of perch or haunts, or other 
peculiar habits, such as in the nocturnal, swamp- 
dwelling habit of the Apterye. A bird may also, by 
size or peculiar weapons, be able to defend itself or 
its nest or it may have special means of escape. 
A bird might acquire the habit of laying many eggs 
while it was yet a ground haunter, which would not be 
kept up when its progeny became tree haunters ; for 
it can be shown that the laying bird has some physio- 
logical control over the number of eggs she deposits 
for one brood, as we shall see (in Chapter XX VIII). 
At present most altricial birds are either tree builders 
or occupy holes or burrows in the earth or under 
rocks, or elise nest on high cliffs or islands inaccessible 
to their enemies below man. But this start for pre- 
serving the race by a large number of eggs was doubt- 
less set up before birds acquired such safe nesting 
habits. While the number of nestlings is now de- 
creased because safe position does not require so 
many, the nestling itself is still held helpless; for it 
can be readily seen that where the nest is safely lo- 
cated, a helpless nestling remaining in it till it can fly 
perfectly is the better condition. 
The fact that all birds now after laying the usual 
nest number have in them yet other little dormant 
egos which can be developed to order when needed 
for replacing broken ones or for second or third 
broods, is a strong hint that they (or their ancestors 
