76 BIRDS OF LA PLATA 



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. bonariensis, how does 

 the species maintain its existence, and even increase 

 to such an amazing extent t for it certainly is very 

 much more numerous, over an equal area, than any 

 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, 

 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. 



i. The egg of the Cow-bird is usually larger, and 

 almost invariably harder-shelled than are the eggs it 

 is placed with ; those of the Yellowbreast (Pseu- 

 doleistes virescens) being the one exception I am 

 acquainted with. The harder shell of its own egg, 



