Harm the Cow-Birds cause. 251 



to some other bird, occasionally throwing out the 

 nestlings of the strangers. They either lay their 

 eggs in the nest thus appropriated, or, oddly enough, 

 build one for themselves on the top of it. Here the 

 nest-building habit is assertive and almost invariable. 

 Then in Molothrus bonariensis we have parasitic 

 habits much more highly developed, with a very 

 much weakened tendency to nest-building — trials 

 made and a beginning accomplished ; but nothing 

 further, and, finally, the indiscriminate dropping of 

 eggs into the nests of other birds, but in such num- 

 bers that slight chance is left of many or any being 

 hatched — since the whole habit Of the victimized 

 birds would be overturned by numbers were incuba- 

 tion persisted in, whereas most of the nests are de- 

 serted ; while again, the M. pecoris of North America 

 has acquired instincts as perfect as those of the 

 cuckoo, for it never lays more than one egg in a 

 foster-nest, so that the young bird is securely reared. 



The vast harm caused by these cow-birds can be 

 but guessed at. Here are the words of a close ob- 

 server and good authority : 



" It can readily be seen what an amount of harm 

 the cow-bird causes in the economy of Nature, grant- 

 ing that only a single one of its eggs is hatched in a 

 season. To accomphsh this, a brood of insectivorous 

 and useful birds is almost invariably sacrificed for 

 every cow-bird ; and certainly they are not diminish- 

 ing in numbers." * 



And Major Bendire tells that he follows practically 

 the same good habit as Mr. John Burroughs : when 



* Bendire, Life History of Birds. 



