610 REPORT OF NATIONAL MUSEUM, 1893. 



feed it, because parent birds invariably perch above their young to feed 

 tliem, and the young Cowbird prevented this by always sitting ou the 

 summit of the stalk it perched on. The habit is most fatal on the opeu 

 and closely cropped pampas inhabited by the Cachila, Anthtis corren- 

 dera. In December, when the Cachila Pipit rears its second brood, 

 the Milvago chiniango also has young, and feeds them almost exclu- 

 sively on the yonng of various species of small birds. At this season 

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

 of SynaUa.ris hudsoni. Yet these birds are beautifully adapted, in 

 structure, coloration, 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's nests; and yet to hnd a young Cowbird 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 Ghimangos. 



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



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 proba- 

 bility with it; for might it not just as reasonably be said that the para- 

 sitic instiiKit is the immediate and final cause of her laying her eggs at 

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

 Cuckoo (and it probably is favorable) 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, ami make the improved structure permanent. It is said (Origin 

 of Species, Chap. Vii) that the American Cuckoo lays also at long inter- 

 vals, and has eggs and young at the same time in its nest, a circum- 

 stance manifestly disadvantageous. Of the Coccyzus melanocoryphus, 

 the only one of our three Coccyzi whose nesting habits I am acquainted 

 with, 1 can say that it never begins to incubate till the full complement 

 of egfts are laid — that its young are hatched simultaneously. But if it 

 is sought to trace the origin of the Euroi)ean Cuckoo's instinct in the 

 nesting habits of American Coccyzi^ it might be attributed not to the 

 aberrant habit of ])erhaps a single species, but to another and more dis- 

 advantagecms 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 dro])ping 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 wi<le of 

 the truth as the other), there are many genera intermediate between 

 CucnlnH and Molothrus in wliich no trace of a i)arasiti(! habit ai)pears; 

 and it seems more than i)robable that the aiudogous instincts originated 

 iu different ways in the two genera. As regards the origin of the 



