HOST RELATIONS OF PARASITIC COWBIRDS 193 



first to visit them, . . . It is amusing to see how pertinaciously they 

 hang about the ovens of the Ovenbirds, apparently determined to take 

 possession of them, flying back after a hundred repulses, and yet not 

 entering them when they have the opportunity. Sometimes one is 

 seen following a wren to its nest beneath the eaves, and then clinging 

 to the wall beneath the hole into which it disappeared. I could fill 

 many pages with instances of this habit of M. bonariensis, which use- 

 less though it be, is as strong an aft'ection as the bird possesses. That 

 it is a recurrence to a long disused habit I can scarcely doubt ... it 

 seems to me that if M. bonariensis when once a nest builder, had 

 acquired the semiparasitical habit of breeding in doomed nests of other 

 birds, such a habit might conduce to the formation of the instinct 

 which it now possesses . . . . " 



If we recall that the bay-winged cowbird still prefers the old nests 

 of ovenbirds and woodhewers for its breeding and that the species will 

 build its own nest mainly if no others are available, the interest shown 

 by the shiny cowbird seems to be a relict habit from the self -breeding 

 mode of life of its remote ancestral stock. This is borne out by the 

 fact that the male shows this interest as well as the female; in the 

 bay-wing, both sexes may struggle with the builders for the possession 

 of their nest or share in the construction of a new one. Related to 

 this is the observation of Young (1930, pp. 256-257) to the effect that, 

 in his experience in British Guiana, the male shiny cowbird seemed to 

 do most of the work of prospecting for nests. 



Other suggestive evidence pertinent to the above is the tendency of 

 year-old shiny cowbu-ds breeding in flocks late in the season to lay 

 very large numbers of eggs in some of these domed nests, especially 

 the mud nests of the ovenbird. Individual nests of this bird have 

 been found which contained 15, 17, 20, 25, 26, and even, in one case, 

 37 eggs of the shiny cowbird. In all such cases the nests were de- 

 serted, often before most of the parasitic eggs were deposited. AU of 

 these instances were noted late in the breeding season — in January 

 (the season extends from September to February). No such multiple 

 depositions of eggs in single nests have been recorded early in the 

 season although, as in the North American brown-headed cowbii'd, 

 cases of 8 eggs in a nest occasionally have been noted. In Salta, in 

 northwestern Argentina, Leo Miller (1917, pp. 584) noted that, in 

 many of these instances of excessive parasitism, flocks of shiny cow- 

 birds were seen in the same tree as the overburdened nest. In my 

 original appraisal of this situation (1929, p. 97) I pointed out that 

 "in migration and in the establishment of breeding areas the adult 

 Cowbirds come first and the year-old birds follow a good deal later. 

 . . . most of the adults would be through breeding before the year-old 

 birds and from this it follows that the latest eggs of the season would 



