18/4.] MR. HUDSON ON THE MOLOTHRI OF BUENOS AYRES. 153 



1. Notes on the Procreant Instincts of the three Species 

 of Molothrus found in Buenos Ayres. By W. H. Hud- 

 son, C.M.Z.S. 



[Eeceived January 9, 1874.] 



About three years ago I wrote two letters to the Secretary on the 

 habits of the various species of the genus Molothrus found in this 

 country. Since that time I have continued my observations on these 

 interesting birds, and have now great pleasure in submitting to the 

 Society the following notes regarding their procreant instincts. 



I. " Mistakes and imperfections " of the instinct of Molothrus 

 bonariensis. — 1. The M. bonariensis frequently wastes its eggs by 

 dropping them upon the ground. 2. They also occasionally lay 

 eggs in old forsaken nests : this I had often observed ; and, to make 

 assurance doubly sure, last summer I fixed several old nests up 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 egg of the M. bonariensis is lost if incuba- 

 tion is very far advanced ; but if the eggs have been sat on three 

 or four days only, then the parasitical egg has a good chance of 

 being hatched, and the young bird reared along with its foster- 

 brothers. I have often found nests of the Yellowbreast (Pseudo- 

 leistes virescens) and of the Scissor-tail (Milvulus violentus) contain- 

 ing fledged young of both species. 



4. One female will often lay several eggs in the same nest, instead 

 of laying but one, as does, according to Wilson, the Molothrus pecoris 

 of North America. I conclude that this is the case from the fact 

 that in cases where the eggs of a species vary considerably in form, 

 size, and markings, each individual of the species lays eggs precisely 

 or nearly alike. So when I find two, three, or four eggs of the 

 M. bonariensis peculiar in form and size, also alike in coloration and 

 disposition of spots, in one nest, and yet, in half a hundred eggs out 

 of other nests, cannot find one to match with them, it is impossible 

 not to believe that the eggs found together, and possessing so strong 

 a family likeness, were laid by the same bird. 



5. Several females often lay in one nest ; so that the number of 

 eggs in it frequently makes incubation impossible. It occurred to 

 me this summer (December 16/2) to count the eggs of M. bonari- 

 ensis in several nests, in order to ascertain the average number depo- 

 sited in each nest — thence the wasted eggs ; for more than one bird 

 is seldom reared. I obtained ten nests of Milvulus violentus and 

 give the result (see page 154). 



It is worthy of remark that the Milvulus lays in October or early 

 in November, and rears but one brood in the season ; consequently 

 these ten nests, obtained late in December, are of birds whose first 

 nests had been lost. Probably three fourths of the lost nests of the 

 Milvulus are abandoned in consequence of the confusion caused in 

 them by the Molothrus bonariensis. 



Proc. Zool. Soc— 1874, No. XI. 11 



