BROOD PARASITISM IN A HOST GENERALIST, THE SHINY COWBIRD: I. THE QUALITY OF DIFFERENT SPECIES AS HOSTS

The Shiny Cowbird (Molothrus bonariensis) of South America, Panama, and the West Indies is an obligate brood parasite known to have used 176 species of birds as hosts. This study documents wide variability in the quality of real and potential hosts in terms of response to eggs, nestling diet, and nest survivorship. The eggs of the parasite are either spotted or immaculate in eastern Argentina and neighboring parts of Uruguay and Brazil. Most species accept both morphs of cowbird eggs, two reject both morphs, and one (ChalkbrowedMockingbird, Mimus saturninus) rejects immaculate eggs but acceptss potted ones. No species, via its rejection behavior, protects the Shiny Cowbird from competition with apotential competitor, the sympatric Screaming Cowbird (M. rufoaxillaris).

PAUL MASON, The Auk 103: 52-60