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ASSTRACT.--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 (Chalkbrowed Mockingbird, Mimus saturninus) rejects immaculate eggs but accepts spotted ones. No species, via its rejection behavior, protects the Shiny Cowbird from competition with a potential competitor, the sympatric Screaming Cowbird (M. rufoaxillaris). Cross-fostering experiments and natural-history observations indicate that nestling cowbirds require a diet composed of animal protein. Because most passerines provide their nestlings with such food, host selection is little restricted by diet. Species-specific nest survivorship, adjusted to appropriate values of Shiny Cowbird life-history variables, varied by over an order of magnitude. Shiny Cowbirds peck host eggs. This density-dependent source of mortality lowers the survivorship of nests of preferred hosts and creates natural selection for greater generalization. Host quality is sensitive to the natural-history attributes of each host species and to the behavior of cowbirds at nests. Received 4 June 1984, accepted 26 June 1985. VARIATION in resource quality can have great ecological and evolutionary consequences. Obligate brood parasites never build nests but leave the care of their eggs and young to other species, their hosts. The parental behavior of hosts is a critical and quantifiable resource to brood parasites. The first task in understanding the use of resources is to ascertain the quality of each alternative. I surveyed the quality of various passerine species as hosts of the Shiny Cowbird (Molothrus bonariensis) in Buenos Aires Province, Argentina. Host quality is tractable to analysis because selection is spatially and temporally focused at nests. Dimensions of host quality examined include response to parasitic eggs, nestling diet, and characteristic survivorship of each species' nests. This is the only systematic attempt to characterize the quality of an array of species for any brood parasite. The Shiny Cowbird is widely distributed throughout South America (Friedmann 1929). It is an extreme host generalist, known to have

Brood Parasitism in a Host Generalist, the Shiny Cowbird: I. The Quality of Different Species as Hosts