180 HANDBOOK OF BRITISH INLAND BIRDS 
the Cuckoo are, perhaps, those of the Hedge 
Sparrow, Pied Wagtail, Meadow Pipit, Sedge 
Warbler, and Reed Warbler. The egg is 
extremely small for the size of the bird, no 
doubt to agree better with the other eggs in the 
nests of the various small birds which act as foster- 
parents ; but all the same, it is almost always a 
little larger than the rest. A theory is held in 
some quarters that the Cuckoo deliberately chooses 
a nest with eggs which resemble her own, and 
adds her own to them, but this is probably an 
error. It does not agree well enough with the 
facts, for if a large series (not a picked selection) 
of eggs found with Cuckoos' eggs is fairly com- 
pared, it is seen that the Cuckoo's by no means 
always resembles the other eggs in its own nest 
any more closely than it does some other nestful 
altogether. There is another theory that Cuckoos 
are split up into a number of families or clans, 
each of which is attached to some particular 
species of small bird, and that the eggs laid by the 
Cuckoo specially resemble the eggs of the small 
bird in question. This, on the whole, seems very 
probable, though it is unlikely that the hen Cuckoo 
never drops her egg into a different sort of nest to the 
one in which she was herself brought up. Some- 
times the Cuckoo puts its egg into the other bird's 
nest while the rightful eggs are being laid, some- 
