Nest-building, Incubation, and Migration. 245 



offspring are either smothered or crowded out of the nest 

 by their stronger foster brother ; or, failing this, starved, 

 when he absorbs the entire attention of the foster 

 parents. It is ludicrous, says Major Bendire, to see a fat, 

 fully fledged young cowbird following a pair of chipping 

 sparrows, or some small warbler, clamouring incessantly 

 for food and uttering its begging call of seerr-seerr most 

 persistently, only keeping quiet while its gaping bill is 

 filled with some suitable morsel ; and stranger still to note 

 how devoted the diminutive nurses are to their foster child. 



A smaller species or variety (Moluthrus ater, var. 

 obscurus) has similar habits ; as has also the bronzed or 

 red-eyed cowbird of Mexico and central America (Callothrus 

 robustus). 



The observations of Mr. W. H. Hudson on the 

 Argentine cowbirds (Moluthrus bonariensis), show that 

 they frequently waste their eggs by dropping them on the 

 ground, and occasionally lay in old forsaken nests. They 

 also destroy many of the eggs in the nests they visit by 

 pecking holes in the shells, parasitical eggs and others 

 being indiscriminately treated in this way. The harder 

 shell of their own eggs gives them, however, a better 

 chance of being preserved ; for, although this cowbird 

 never distinguishes its own eggs, of which it destroys a 

 great many, from those among which it is laid, yet a larger 

 proportion of them escape. The hard shell may therefore 

 fairly be regarded as a result of the process of natural 

 selection. Furthermore, the short period of incubation, 

 about eleven days as compared with the fourteen to sixteen 

 of the small birds on which it is parasitic, gives it an 

 advantage which may also be ascribed to natural selection. 

 After a few days, as in the case of its North American 

 congeners, the foster child is found to be the sole survivor 

 of the nestlings ; and, as is commonly found in the eggs of 



