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 MM. 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? 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. 
1. 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, 
