A DEAD BEAT. 233 



Luckily, accidents happen to the cow-bird's eggs. She lays 

 many on the ground ; she lays some in deserted nests of the 

 year before; others she puts in new nests that are scarcely 

 completed, and these are frequently deserted by the owners 

 or another story is built over the intruding egg. But too 

 many of them are received and tended. Once the foster 

 mother adopts the big egg, her own brood is doomed. 

 The new egg gets more than its share of warmth; it has 

 wonderful vitality; it hatches very quickly, and its thick 

 shell protects it from accident, for the cow-bird has a habit 

 of breaking the eggs in the nests she visits, even her own if 

 she finds there one laid previously. It is not known whether 

 she pricks them with her beak or with her claws, but each one 

 is punctured so that it will not hatch. How she does it, young 

 naturalists may attempt to discover. 



The instance of that South American cow-bird, which takes 

 care of its young in a nest built by other birds, indicates that 

 at some remote period the others probably did the same. It 

 has been suggested that several females might have been in 

 the habit of laying in the same nest to avoid the work of 

 building; and that thus they got into the habit of turning 

 over their eggs to each other's care, each expecting some other 

 bird to do her work for her, until at last in order to hatch any 

 young they were obliged to lay in the nests of unrelated 

 species that were better mothers. Whether laziness or in- 

 ability to build good nests be the cause of the parasitic habit 

 we cannot determine, but it is noteworthy that the cuckoos, 

 the only other North American birds that are much inclined 

 to similar habits, are poor builders. 



But there is another theory, more ingenious and perhaps 

 equally true. In South America many birds build large, 

 domed nests, and these prove so attractive that other species 



