SOCIAL PARASITISM IN BIRDS 



561 



the real mate of the male whose territory is under 

 discussion, laid an egg in each of the four nests, or 4 

 eggs in all, while of the other five females using 

 these nests, four laid but one egg apiece and the other 

 laid two. Furthermore the two eggs laid by the last 

 bird were laid four days apart. This means either 

 that this particular female wandered about from one 

 territory to another or else that it laid during the 

 interval in nests in this territory which I never found. 



[In this connection I should advise any reader 

 interested particularly in this subject to study the 

 observations of Mr. E. P. Chance on the European 

 Cuckoo, Cuculus canorus, and acquaint himself with 

 his theory of a "dominant" female in each territory, 

 an explanation with which my observations on 

 Molothrus are in complete agreement, keeping in mind 

 the sole difference that the Cowbirds lack specificity 

 in their parasitism.] 



Nevertheless, in spite of all this confusing data 

 the total evidence leads me to believe that the Shiny 

 Cowbird is chiefly monogamous and each mated 

 female sticks to one territory but that both the 

 sexual and territorial relations arc so weak as to be 

 very easily modified or sometimes even destroyed by 

 conditions, particularly by the unnatural, increased 

 density of cowbird population per given area around 

 cultivated districts. Of course this frequently 

 results in what seems to be sexual promiscuity and 

 docs destroy, in great measure, the "territory," in the 

 sense that that particular area is no longer the domain 

 of only one female but has become the happy hunting 

 grounds of all that may care to make use of it. The 

 same is largely true for the North American species, 

 M. ater. 



In fact Barrows (9), than whom no more reliable 

 observer ever wrote on Argentine birds, said that he 

 was inclined to think that Molothrus bonariensis 

 differed less from M. ater in its habits than was 

 generally supposed. "Its great abundance and the 

 comparative openness of the country will in great 

 measure account for the large number of eggs found as 

 well as for the numbers sometimes observed in 

 single nests .... Of course this overdoes the 

 matter so as to compel the rightful owner to desert 

 the nest, but I suspect that our own Cowbird would 

 be no wiser under similar pressure." 



One more point needs to be discussed here. The 

 males outnumber the females to the same extent as 

 they do in M. ater of North America, — about 3 males 

 to every x females. Assuming that every breeding 

 female has a mate and but one mate, there would be 

 still one-third of the males without mates and conse- 

 quently without any means of satisfying their sexual 

 desires. If several males having no "territories" or 



"spheres of influence" joined in the pursuit of the 

 same female, disaster to the race would undoubtedly 

 ensue. But each male (except in the case of the 

 yearling birds that begin breeding very late) has his 

 own territory and there he awaits the coming of a 

 mate. The greater the number of Cowbirds to a 

 given area, the greater is the competition for terri- 

 tories with the result that the territories are smaller 

 than elsewhere where the Cowbirds are fewer. The 

 smaller each territory the less assurance a female has of 

 a requisite number of nests in which to lay, and so 

 what probably happens is this : After utilizing all the 

 available nests in the territory of any one male she 

 passes on to that of the next. If that territory is 

 already occupied by a female, the newcomer finds the 

 available nest supply inadequate and passes on still 

 further afield. Often she may leave an egg or even 

 two in a certain territory before passing on. Inas- 

 much as there are at least fifty per cent more males 

 than females, it means that after each female has 

 exhausted the "territory" of her particular male she 

 still has half as much more coming to her in other 

 places. This arrangement not only gives all the 

 females a fairly equal chance to lay the maximum 

 number of eggs but it also brings about a state of 

 affairs wherein each male can find satisfaction in the 

 appeasement of its sexual desire. However, this 

 state of affairs can hardly be called polyandry for, 

 although in the course of a season each female may 

 have several mates, she has only one at a time and 

 only one in a territory. Most monogamous birds 

 change mates with each brood and yet no one would 

 call the females polyandrous, the males polygamous 

 or the species promiscuous. In the Cowbirds, if the 

 birds were originally more than one brooded, the 

 broods have been merged in adaptation to the para- 

 sitic habit. One would be more justified in calling 

 the males polygamous as they have intercourse with 

 some of the wandering females while still mated 

 themselves. Yet the male does not leave his territory 

 to collect a harem but takes what comes his way, and 

 not having any concept of parental instinct can hardly 

 be accused of polygamy. There is a great difference 

 between this and a nonparasitic species wherein the 

 male has a paternal interest in two nests simul- 

 taneously. 



In fact if the females did not wander further afield 

 after exhausting the possibilities of the territories of 

 their respective mates, at least one-third of the males 

 would not be able to appease their sexual desires 

 without forsaking their territories and intruding into 

 those of other males. The loyalty of each male to his 

 territory is not to be thought of as "virtuous" in any 

 way [For the biology of "virtue" see any of the 



