ARGENTINE COW-BIRD 81 
new conditions introduced by land-cultivation and 
their effect on the species. The altered conditions 
have, in various ways, served to remove many 
extraneous checks on the parasitical instinct, and the 
more the birds multiply, the more irregular and 
disordered does the instinct necessarily become. 
In wild districts where it was formed, and where 
birds building accessible nests are proportionately 
fewer, the instinct seems different from what it does 
in cultivated districts. Parasitical eggs are not 
common in the desert, and even the most exposed 
nests there are probably never overburdened with 
them. But in cultivated places, where their food 
abounds, the birds congregate in the orchards and 
plantations in great numbers, and avail themselves 
of all the nests, ill-concealed as they must always 
be in the clean, open-foliaged shade and fruit trees 
planted by man. 
DIVERSITY IN COLOUR OF EGGS 
There is an extraordinary diversity in the colour, 
form, and disposition of markings, etc., of the eggs 
of M. bonariensis; and I doubt whether any other 
species exists laying eggs so varied. About half the 
eggs one finds, or nearly half, are pure unspotted 
white, like the eggs of birds that breed in dark holes. 
Others are sparsely sprinkled with such exceedingly 
minute specks of pale pink or grey, as to appear quite 
spotless until closely examined. After the pure white, 
the most common variety is an egg with a white 
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