90 BIRDS OF LA 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, Le. t 

 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 Dendrocolaptidx. 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 



