560 



THE QUARTERLY REVIEW OF BIOLOGY 



daily notes of everything concerned with 

 it. On January 8, 19x4 I noted that a pair 

 of Screaming Cowbirds flew into the tree 

 in which the nest was and stayed around 

 in the nearby branches but were kept away 

 from the nest by the Bay-wings. Sud- 

 denly another pair of Screaming Cowbirds 

 flew into the tree and joined the first pair. 

 A minute later the second pair, (the newly 

 arrived birds), flew over to the nest and 

 were chased back by the Bay-wings . They 

 flew back to the first pair and a little while 

 later both pairs flew off together, scream- 

 ing as they flew. It seemed that in this 

 case one of the two pairs of Screaming 

 Cowbirds was encroaching on a nest in the 

 territory of the other. The birds being 

 strictly monogamous, only one pair would 

 occur in any one territory, and the other 

 pair must have just recently come in. 

 Screaming Cowbirds were not very plenti- 

 ful in that district, and there was plenty 

 of land available for the other pair to use. 

 There were also plenty of Bay-wings 

 scattered over the country. This looked 

 as though the second pair had not yet 

 established themselves in a breeding terri- 

 tory although it was very late in the 

 season. It is hardly possible that this 

 pair had not attempted to do so before; it 

 seems likely that they were birds that had 

 deserted a territory and were not yet 

 settled in a new one. 



POPULATION PRESSURE AS A MODIFIER OF 

 BREEDING HABITS 



In the case of the Shiny Cowbird, the 

 numerical abundance of the species usually 

 modifies or hides the true state of affairs. 

 The sexual and territorial relations of this 

 species are easily over-ridden by the pres- 

 sure of population, resulting in undue 

 competition for breeding areas. In this 

 species the factors influencing the extent of 

 the individual territories are not associated 

 directly with the food supply, but with 



the abundance of nests in which to deposit 

 the eggs. The denser the small bird popu- 

 lation, the smaller the territory of each 

 Cowbird. Where the Cowbirds are very 

 abundant the territories as such become 

 almost impossible of definition and 

 demarkation. The results of a protracted 

 study of this species indicate the following 

 facts. In areas where the birds are not 

 extremely abundant, they pair off regu- 

 larly and each pair has its own territory. 

 In places where the Cowbird population is 

 great, the birds still pair off, but inasmuch 

 as they make no pretense of protecting the 

 territory other individuals filter in, remain 

 there a day or so and then pass on. Conse- 

 quently it is more usual to see several of 

 these birds together (with the males 

 predominating in number) than to see 

 them in groups of two. The following 

 observations, taken from my book, will 

 illustrate this point. I watched a certain 

 pair of Shiny Cowbirds, whose territory I 

 knew, every day for several weeks. The 

 female laid the first egg in a nest of a 

 Chingolo Song Sparrow (Brachyspi%a 

 capensis) on October x5t.l1. 



I was surprised, however, to find that another 

 female Cowbird also laid in this nest on the same 

 date. It looked as though the male was constant in 

 its territorial relations but that females came and went 

 promiscuously. However, in the next few days I 

 found that the same female had laid an egg in each of 

 four chingolo nests in this territory, including nest 

 no. 1. The eggs were laid at intervals of one day, 

 but at the same time, each day I kept finding eggs of 

 other female Cowbirds in nests where they certainly 

 were not the day before. Thus in nest 1 no less than 

 four different female Cowbirds deposited one egg each, 

 two of them removing (or apparently removing) one 

 of the eggs already in the nest. In nest no. z, eggs 

 were deposited by two different Cowbirds; nest no. 3 

 contained eggs of two different Cowbirds; nest no. 4 

 contained only 1 Cowbird egg. All in all I judged, 

 by the size, color, marking and texture of the eggs as 

 well as by the date of deposition that no less than six 

 different females had deposited eggs in nests within 

 the limits of this particular territory. However, the 

 important point is that one bird, which I shall call 



