74 ICTERID^E. 



with black, and the trees when they alight appear to have a black 

 foliage. At such times one wonders that many small species on which 

 they are parasites do not become extinct by means of their pernicious 

 habit. In Buenos Ayres, where they are most numerous, they have 

 a migration, which is only partial, however. It is noticeable chiefly in 

 the autumn, and varies greatly in different years. In some seasons it 

 is very marked, when for many days in February and March the birds 

 are seen travelling northwards, flock succeeding flock all day long, 

 passing by with a swift low undulating flight, their wings producing a 

 soft musical sound ; and this humming flight of the migrating Cow- 

 birds is as familiar to every one acquainted with nature in Buenos 

 Ayres as the whistling of the wind or the distant lowing of cattle. 



The procreant instinct of this Molothrus has always seemed so 

 important to me, for many reasons, that I have paid a great deal of 

 attention to it ; and the facts, or, at all events, the most salient of them, 

 which I have collected during several years of observation, I propose 

 to append here, classified under different headings so as to avoid 

 confusion and to make it easy for other observers to see at a glance 

 just how much I have learnt. 



Though I have been familiar with this species from childhood, when 

 I used to hunt every day for their wasted eggs on the broad, clean 

 walks of the plantation, and removed them in pity from the nests of 

 little birds where I found them, I have never ceased to wonder at their 

 strange instinct, which in its wasteful destructive character, so unlike 

 the parasitical habit in other species, seems to strike a discordant note 

 in the midst of the general harmony of nature. 



Mistakes and Imperfections of the Procreant Instinct of Molothrus 



bonariensis. 



1. The Cow-birds, as we have seen, frequently waste their eggs by 

 dropping them on the ground. 



2. They also occasionally lay in old forsaken nests. This I have 

 often observed, and to make very sure I took several old nests and 

 placed them in trees and bushes, and found that eggs were laid 

 in them. 



3. They also frequently lay in nests where incubation has actually 

 begun. When this happens the Cow-bird's egg is lost if incubation 

 is far advanced ; but if the eggs have been sat on three or four days 

 only, then it has a good chance of being hatched and the young bird 

 reared along with its foster-brothers. 



4. One female often lays several eggs in the same nest, instead of 

 laying only one, as does, according to Wilson, the Molothrus pecoris 



