190 THE STORY OF THE BIRDS. 
number for various species to cover and hatch surely, 
and that she deposited her own or threw out the 
other’s eggs accordingly. But that she has some 
ideas of her own may be inferred from the foregoing 
case, and from the fact that where there are already 
many eggs in the nest the cowbird deposits only one 
| or few. But if she gets 
to a nest early she may 
lay many in it, as seen 
in the following cases of 
record : 
A vireo, while build- 
ing, had deposited in her 
nest a sufficient number 
of cowbird eggs to come 
within one of her usual complement. She deposited 
only the one egg needed, and immediately began to 
incubate. Another vireo was found where her clutch 
was completely filled by the time her nest was finished, 
and she went to sitting without laying any eggs at all. 
Here the vireo certainly knew her nest number, 
and it may be just possible that the cowbird did also, 
though this is not very probable. 
It is not easy to see how the sight of a proper 
number of eggs in her nest should so affect the bird’s 
physiology as to render her able at once to cease lay- 
ing. Dissection often shows many small eggs yet un- 
formed. Laying does not always, however, appear to 
be a matter wholly voluntary, for it is well known that 
at the beginning of the nesting season a bird may have 
to deposit an egg anywhere before she can build ; or 

Red-eyed vireo. (Natural size.) 
