MOLOTIIRUS BONARJENSIS. 75 



of North America. I conclude that this is so from the fact that in 

 cases where the eggs of a species vary considerably in form, size, and 

 markings, each individual of the species lays eggs precisely or nearly 

 alike. So when I find two. three, or four eggs of the Cow-bird in 

 one nest all alike in colour and other particulars, and yet in half a 

 hundred eggs from other nests cannot find one to match with them, it 

 is impossible not to believe that the eggs found together, and possessing 

 a family likeness, were laid by the same bird. 



5. Several females often lay in one nest, so that the number of eggs 

 in it frequently makes incubation impossible. One December I 

 collected ten nests of the Scissor-tail (Milvulus tyrannus) from my 

 trees; they contained a total of 47 eggs, 12 of the Scissor-tails and 

 35 of the Cow-birds. It is worthy of remark that the Milvulus breeds 

 in October or early in November, rearing only one brood; so that 

 these ten nests found late in December were of birds that had lost 

 their first nests. Probably three fourths of the lost nests of Milvulus 

 are abandoned in consequence of the confusion caused in them by the 

 Cow-birds. 



6. The Cow-birds, male and female, destroy many of the eggs in the 

 nests they visit, by pecking holes in 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 in 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 Cow-bird 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 possessed by M. bonariensis 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 M. bona- 

 riensis, 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 

 species may be less hardy, have more enemies, be exposed to more 

 perils in their long migrations, &c. 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 



