90 BIRDS OF ly PLATA 
that all Doves that build simple platform-nests (like 
Columba livia and others that build on a flat surface) 
also lay white eggs (the rule being that eggs laid in 
dark holes are white, exposed eggs coloured), also 
that one species, C. livia, does lay in holes in rocks, 
would lead us to believe that the habit of this species 
was once common to the genus. We should conclude 
that an insufficiency of proper breeding-places, i.e., 
new external conditions, first induced Doves to build 
in trees. Thus C. livia also builds in trees where 
there are no rocks; but, when able, returns to its 
ancestral habits. In the other species we should 
believe the primitive habit to be totally lost from disuse, 
or only to manifest itself in a faint uncertain manner. 
Now in Molothrus bonariensis we see just such a 
vague, purposeless habit as the imaginary one I have 
described. Before and during the breeding-season 
the females, sometimes accompanied by the males, 
are seen continually haunting and examining the 
domed nests of some of the Dendrocolaptide. This 
does not seem like a mere freak of curiosity, but their 
persistence in their investigations is precisely like 
that of birds that habitually make choice of such 
breeding-places. It is surprising that they never do 
actually lay in such nests, except when the side or 
dome has been accidentally broken enough to admit 
the light into the interior. Whenever I set boxes up 
in my trees, the female Cow-birds were the first to 
visit them. Sometimes one will spend half a day 
loitering about and inspecting a box, repeatedly 
climbing round and over it, and always ending at 
