THE COWBIRDS 605 



or early iu November, rearing only one brood; so that tliese ten nests 

 found late in December were of birds that bad lost their first nests. 

 Probably three-fourtbs of the lost nests of Milvulus are abandoned in 

 consequence of the confusion caused in them by the Cowbirds. 



6. The Cowbirds, male and female, destroy many of the eggs in the 

 nests they visit, by pecking holes iu the shells, breaking, devouring, 

 and stealing them. This is the most destructive habit of the bird, and 

 is probably possessed by individuals in different degrees. I have often 

 carefully examined all the parasitical eggs iu a nest, and after three or 

 four days found that these eggs had disappeared, others, newly laid, 

 being in their places. I have seen the female Cowbird strike her beak 

 into an egg and fly away with it; and I have often watched the male 

 bird perched close by while the female was on the nest, and when she 

 quitted it seen him drop down and begin pecking holes in the eggs. In 

 some nests found full of parasitical eggs every egg has holes pecked in 

 the shell, for the bird destroys indiscriminately eggs of its own and of 

 other species. 



Advantages posskssed by M. boxariensis over its dupes. 



After reading the preceding notes one might ask, if there is so much 

 that is defective and irregular in the reproductive instinct of 21. 

 honariensis, how does the species maintain its existence, and even 

 increase to such an amazing extent, for it certainly is very much more 

 numerous, over an equal area, than other parasitical species. For its 

 greater abundance there may be many reasons unknown to us. The 

 rarer sjiecies may be less hardy, have more enemies, be exposed to 

 more perils in their long migrations, etc. That it is able to maintain 

 its existence in spite of irregularities in its instinct is no doubt due to 

 the fact that its eggs and young possess many advantages over the 

 eggs and young of the species upon which it is parasitical. Some of 

 these advantages are due to those very habits of the parent bird which 

 at first sight appear most defective; others to the character of the egg 

 and embryo, time of evolution, etc. 



1. The egg of the Cowbird is usually larger, and almost invariably 

 harder shelled than are the eggs it is placed with ; those of the Yellow- 

 breast, Psendoleistes virescens, being the one exception I am acquainted 

 with. The harder shell of its own egg, considered in relation to the 

 destructive egg-breaking habit of the bird, gives it the best chance of 

 being preserved; for though the Cowbird never distinguishes its own 

 eggs, of which indeed it destroys a great many, a larger projiortiou 

 escape in a nest Avhere many eggs are indiscriminately broken. 



2. The vitality or tenacity of life appears greater in the embryo 

 Cowbird 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 incubation had begun, and have been 

 surprised at finding those of tbe Scissortail addled, even when placed 



