SOCIAL PARASITISM IN BIRDS 



559 



never does. The late breeding of the 

 Goldfinch is doubtless an adaptation to 

 seasonal food supply but in the Screaming 

 Cowbird it seems an old habit phylo- 

 genetically derived from the ancestral 

 stock, of which the Bay-winged species is 

 the surviving member. Food supply can- 

 not explain why the Bay-wing and the 

 present species breed so late in the season, 

 as the food of the young of both these birds 

 is the same as that of the young Shiny 

 Cowbird, which is an early breeding 

 bird. However, the Bay-wing breeds 

 late because it habitually breeds in old 

 nests of woodhewers QAnumbius and 

 Synallaxis) and ovenbirds (JFurnarius). 

 Late in the season there are plenty of these 

 nests available, while earlier they are 

 fewer in number, most are occupied by the 

 builders and the Cowbirds would have to 

 fight for them. The greater ease and 

 certainty with which these nests could be 

 obtained later in the season probably was 

 largely responsible for the postponement 

 of the breeding season in the Bay-winged 

 Cowbird. The present bird, the Scream- 

 ing Cowbird, shows in several ways that 

 it is an offshoot from the Bay-wing stock, 

 and its habit of breeding very late is 

 doubtless due to a similar tardiness of 

 reproductive season in the stock from 

 which it evolved. 



DESERTION OF BREEDING TERRITORIES 



The late breeding coupled with fairly 

 early establishment of "territories" which 

 remain unused for a considerable length of 

 time has brought about a very interesting 

 condition in the Screaming Cowbird: 

 namely, not infrequent desertion of terri- 

 tories before egg-laying commences, with 

 the subsequent establishment of new 

 territories later as the reproductive urge 

 becomes more imminent. In this connec- 

 tion the following observations, taken 

 from my book, are of interest. 



In Concepcion district, Tucuman, Argen- 

 tina, I watched several pairs of Screaming 

 Cowbirds whose territories were more or 

 less adjacent. As the season wore on I 

 found that one pair forsook its territory 

 and disappeared. The male of this pair 

 was identifiable by an extreme harshness 

 in his notes. He and his mate were well 

 nigh inseparable. I never saw either bird 

 alone or more than a couple of feet away 

 from the other, even when feeding on 

 neutral ground. The desertion of this 

 territory took place between the twenty- 

 second and twenty-fourth of November. 

 On December second I was surprised to 

 find the same male securely established in a 

 new territory about a mile away. With 

 him constantly was a female, just as 

 before. Whether or not it was the same 

 female I could not say, but of the identity 

 of the male I had no doubt. The old 

 territory of this male was occupied by a 

 new pair of birds four days after it was 

 deserted by the first pair. From this it 

 would hardly seem possible that the 

 "fitness" of this territory had in some 

 way been lessened to the extent of causing 

 the original pair of tenants to desert it. 

 It was possible that the female of the 

 original pair had died and the male had 

 deserted on that account. In order to 

 test this I shot the females of three pairs on 

 three nearby, and, to me, well known 

 territories. In none of these cases did the 

 males desert; they remained and soon 

 found other mates. I can attribute this 

 desertion to no cause other than the 

 diminishing potency of the territorial 

 instinct with the passing of time between 

 the establishment and the utilization of 

 the territory in question. Another bit of 

 evidence in this connection was gathered 

 at the height of the breeding season of the 

 species (January) at Santa Elena in Entre 

 Rios, eastern Argentina. I was studying 

 a Bay-winged Cowbird 's nest, making 



