ARGENTINE COW-BIRD 77 



considered in relation to the destructive egg-breaking 

 habit of the bird, gives it the best chance of being 

 preserved ; for though the Cow-bird never distin- 

 guishes its own egg, of which indeed it destroys a 

 great many, a larger proportion escape in a nest 

 where many eggs are indiscriminately broken, 



2, The vitality or tenacity of life appears greater 

 in the embryo Cow-bird than in other species ; this 

 circumstance also, in relation to the egg-breaking 

 habit and to the habit of laying many eggs in a nest, 

 gives it a further advantage, I have examined nests 

 of the Scissor-tail, containing many eggs, after incu- 

 bation had begun, and have been surprised at finding 

 those of the Scissor-tail addled, even when placed 

 most advantageously in the nest for receiving heat 

 from the parent bird, while those of the Cow-bird 

 contained living embryos, even when under all the 

 other eggs, and, as frequently happens, glued immov- 

 ably to the nest by the matter from broken eggs spilt 

 over them. 



The following instance of extraordinary vitality in 

 an embryo Molothrus seems to show incidentally that 

 in some species protective habits, which will act as 

 a check on the parasitical instinct, may be in the 

 course of formation. 



Though birds do not, as a rule, seem able to 

 distinguish parasitical eggs from their own, however 

 different in size and colour they may be, they often 

 do seem to know that eggs dropped in their nest 

 before they themselves have begun to lay ought not 

 to be there ; and the nest, even after its completion, 



