COSTS OF BROOD PARASITISM AND THE LACK OF DEFENSES ON THE YELLOW-WINGED BLACKBIRD SHINY COWBIRD SYSTEM

The shiny cowbird (Molothrus bonariensis) is a generalist brood parasi te that lays either white-immaculate or spotted egg morphs in eastern Argentina and Uruguay. Some hosts accept both morphs, others accept sp otted eggs and reject the white morph, but no host has been found to a ccept white eggs and reject spotted ones. It has been suggested that t he yellow-winged blackbird (Agelaius thilius) may be that type of host . The finding of a white acceptor-spotted rejector species would help to explain the occurrence and maintenance of the parasite egg polymorp hism. We studied the incidence of shiny cowbird parasitism on this hos t, its costs for their reproductive success and the presence of antipa rasitic defenses in the yellow-winged blackbird - shiny cowbird system . The parasite affected the reproductive success of the host in two wa ys. Cowbirds punctured host eggs causing a reduction in clutch size, a nd yellow-winged blackbirds deserted their nests whenever they suffere d high egg loss. In addition, parasitized nests suffered higher predat ion during the nestling stage, but not during egg stages, indicating t hat the difference found was related to the presence of the cowbird ch ick, and not to higher exposure of parasitized nests to both parasites and predators. Despite the costs imposed by the parasite, yellow-wing ed blackbirds have not evolved antiparasitic defenses. This host did n ot reject any egg morph of the shiny cowbird nor desert parasitized ne sts unless it had suffered high egg loss. Current explanations for the host lack of defenses, the ''time lag'' and the ''equilibrium'' hypot hesis, are discussed.