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THE QUARTERLY REVIEW OF BIOLOGY 



bay-wings succeed in ousting the builders 

 and usurp the nests, throwing out any 

 eggs or young that may happen to be 

 present. The birds then breed in these 

 nests, taking care of their eggs and young 

 as do ordinary birds. If no old or occu- 

 pied nests are available the birds build for 

 themselves and construct very creditable 

 nests, indicating that they still possess 

 the nest-building instinct but bring it into 

 action only as a last resource, when all 

 other means fail them. However, even 

 when (as in most cases) they take over old 

 nests, they do a certain amount of nest- 

 building: — repairing or adding to the 

 lining, enlarging the entrance in the case 

 of domed nests, rearranging small twigs on 

 the outside, etc. Then, after the nests are 

 renovated or completed, as the case may 

 be, the females lay their eggs, usually five 

 in number, and begin incubating, and rear 

 their young as do most normal nesting 

 kinds of birds. 



The Screaming Cowbird is apparently a 

 direct evolutionary offspring of the bay- 

 winged stock. In the adult stage the 

 plumages of the two species are very 

 different, but the young (juvenal) birds 

 are exactly alike, both having the colora- 

 tion of the Bay- winged Cowbird. Like 

 the Bay-wing, the Screaming Cowbird is 

 non-migratory. It is, if anything, even 

 more strictly monogamous, as it is found in 

 pairs all year round, even in the middle of 

 the Argentine winter. So closely are the 

 members of each pair united to each other 

 that it is quite the exception to see a 

 single individual at any time. On a few 

 occasions I have found single birds or 

 groups of three, but in an overwhelming 

 majority of the cases the birds were in 

 twos. Like the Bay-winged Cowbird, it is 

 a very late breeder — the season for eggs 

 being from December to the end of Febru- 

 ary for these two species, whereas most 

 small birds in Argentina breed from Sep- 



tember to January — and the geographical 

 and ecological ranges of the two species 

 are entirely coincident. The eggs and 

 young of the two are practically identical, 

 but the eggs can be told apart with con- 

 siderable certainty by one who has studied 

 them intently. The Screaming Cowbird 

 is, however, parasitic in its breeding 

 habits and confines its parasitic attentions 

 to one species — the Bay-winged Cowbird. 



It is interesting to note, in passing, the 

 ironical aspect of this situation. As far 

 as the available evidence indicates, the 

 main factor coincident with the late 

 breeding of the Bay-winged Cowbird is the 

 abundance and availability of nests built 

 and since deserted by other species. In 

 other words, there seems to be some 

 correlation between the apparent dislike 

 for nest-building on the part of the Bay- 

 wing, and its late breeding season. By 

 breeding late it avoids the task of build- 

 ing. The Screaming Cowbird also is a late 

 breeder but is parasitic, and inasmuch as 

 no other birds are breeding so late in the 

 season, the Bay-wing automatically be- 

 comes the chief, if not the only, victim. 

 In this way additional work devolves 

 upon the species that originally adopted a 

 late laying season to avoid work. 



The Shiny Cowbird is a far more widely 

 ranging species than either of the two 

 already mentioned, and occurs from Pata- 

 gonia to southern Darien (Panama), while 

 the Bay-wing and the Screaming Cowbirds 

 are found only in the northern part of 

 Argentina, Uruguay, southern Brazil, 

 Paraguay, and Bolivia. It is migratory in 

 the southern part of its range and is less 

 monogamous than either of the other two 

 species found in South America. In 

 general it tends toward monogamy, but in 

 localities where it is very numerous its 

 sexual relations seem unable to maintain 

 themselves unchanged in the face of the 

 pressure of population numbers and the 



