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 



