234 SOME COMMON LAND-BIRDS. 



seek them to nest in just as our martins and sparrows and wrens 

 hunt for martin houses. In time, some of them have lost the 

 art of building nests for themselves, or else rarely practise it. 

 Among these birds is the honest cow-bird. She can build her 

 own nest and sometimes does do it, but she prefers to fight 

 for one of these fine, domed dwellings. She usually gets it, 

 and no sooner has she taken possession than she makes a 

 window in the side to let the light in. Here is a bird that 

 can make an open nest, but that prefers to live in a covered 

 nest. 



Now, it is observed that the other species of cow-birds of 

 South America, which are never known to build for themselves, 

 are greatly attracted to .these domed nests. They examine 

 them, linger about them, seem inclined to enter, but are afraid 

 to do so, and after a half day's debating between their desire 

 to go in and their fear of the dark, they back away reluctantly. 

 If the inside is light, they will lay in it, but they will not 

 make a hole to let in the light as the honest cow-birds do. A 

 lost instinct seems to prompt them to enter holes, indicating 

 that they once bred in such places, or else built a partially 

 covered nest. 



Among these South American cow-birds we observe three 

 stages in acquiring parasitic habits. The bay-winged cow-bird 

 often makes its own nest and brings up its own young, though 

 it more commonly uses the empty nests of other birds ; the 

 screaming cow-bird is parasitic on the bay-winged, and more 

 rarely on other birds ; the Argentine cow-bird is parasitic on 

 many other birds but not on other cow-birds. One takes an 

 empty nest to avoid the work of building ; one lays her eggs 

 in her cousin's nest to escape the care of her young ; one goes 

 entirely out of the family and imposes upon birds that are not 

 related. 



