Abstract
Sixty Screaming Cowbirds (Molothrus rufoaxillaris) and 150 Shiny Cowbirds (M. bonariensis) were trapped and banded in Buenos Aires Province, Argentina. Analysis of recaptures provided statistical evidence for pair formation in Screaming but not Shiny cowbirds, a result supported by observations of free-living birds. Pairs of Screaming Cowbirds were stable throughout the breeding season. They are probably monogamous. Shiny Cowbirds showed no pattern of association between the sexes. This is consistent with a promiscuous mating system, although this conclusion is tentative. I also present ancillary data on sexual size dimorphism, sex ratio, and related aspects of behavior. Both species are dimorphic in size to a similar extent, although the Screaming Cowbird is slightly larger. Neither species departs from an adult (quaternary) sex ratio of unity. Pairs of Screaming Cowbirds are conspicuous near host nests. Female Shiny Cowbirds are not accompanied by males at nests, but may be accompanied by other females. Shiny Cowbird females were surreptitious near nests. Differences in host selection behavior may have profound effects on other aspects of the species breeding biology.
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