MOLOTHRUS BONARIENSIS. 81 



In December, when the Cachila Pipit rears its second brood, the Mllvago 

 chimango also has young, and feeds them almost exclusively on the 

 young of various species of small birds. At this season the Chimango 

 destroys great numbers of the young of the Cachila and of Synallaxis 

 hudsoni. Yet these birds are beautifully adapted in structure, colora- 

 tion, and habits to their station. It thus happens that in districts 

 where the Molothrus is abundant, their eggs are found in a majority of 

 the Cachilas' nests : and yet to find a young Cow-bird out of the nest 

 is a rare thing here, for as soon as the young birds are able to quit the 

 nest and expose themselves they are all or nearly all carried off 

 by the Chi mangos. 



Conjectures as to the Origin of the Parasitic Instinct in M. bonariensis. 



Darwin's opinion that the " immediate and final cause of the Cuckoo's 

 instinct is that she lays her eggs not daily, but at intervals of two or 

 three days " (' Origin of Species '), carries no great appearance of pro- 

 bability with it ; for might it not just as reasonably be said that the 

 parasitic instinct is the immediate and final cause of her laying her eggs 

 at long intervals ? If it is favourable to a species with the instinct of 

 the Cuckoo (and it probably is favourable) to lay eggs at longer 

 intervals than other species, then natural selection would avail itself of 

 every modification in the reproductive organs that tended to produce 

 such a result, and make the improved structure permanent. It is said 

 (' Origin of Species/ chapter vii.) that the American Cuckoo lays 

 also at long intervals, and has eggs and young at the same time in its 

 nest, a circumstance manifestly disadvantageous. Of the Coccyzus 

 melanocoryphus, the only one of our three Coccyzi whose nesting-habits 

 I am acquainted with, I can say that it never begins to incubate till the 

 full complement of eggs are laid that its young are hatched simul- 

 taneously. But if it is sought to trace the origin of the European 

 Cuckoo's instinct in the nesting-habits of American Coccyzi, it might be 

 attributed not to the aberrant habit of perhaps a single species, but to 

 another and more disadvantageous habit common to the entire genus, 

 viz., their habit of building exceedingly frail platform-nests from which 

 the eggs and young very frequently fall. By occasionally dropping an 

 egg in the deep, secure nest of some other bird, an advantage would be 

 possessed by the birds hatched in them, and in them the habit would 

 perhaps become hereditary. Be this as it may (and the one guess is 

 perhaps as wide of the truth as the other), there are many genera inter- 

 mediate between Cuculus and Molothrus in which no trace of a parasitic 

 habit appears ; and it seems more than probable that the analogous 

 instincts originated in different ways in the two genera. As regards the 



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