Descripción: Despite the costs to avian parents of rearing brood parasitic offspring, many species do not reject foreign eggs from their nests.We show that where multiple parasitism occurs, rejection itself can be costly, by increasing the risk of host egg loss during subsequent parasite attacks. Chalk-browed mockingbirds (Mimus saturninus) are heavily parasitized by shiny cowbirds (Molothrus bonariensis), which also puncture eggs in host nests. Mockingbirds struggle to prevent cowbirds puncturing and laying, but seldom remove cowbird eggs once laid. We filmed cowbird visits to nests with manipulated clutch compositions and found that mockingbird eggs were more likely to escape puncture the more cowbird eggs accompanied them in the clutch. A Monte Carlo simulation of this 'dilution effect', comparing virtual hosts that systematically either reject or accept parasite eggs, shows that acceptors enjoy higher egg survivorship than rejecters in host populations where multiple parasitism occurs. For mockingbirds or other hosts in which host nestlings fare well in parasitized broods, this benefit might be sufficient to offset the fitness cost of rearing parasite chicks, making egg acceptance evolutionarily stable. Thus, counterintuitively, high intensities of parasitism might decrease or even reverse selection pressure for host defence via egg rejection. © 2012 The Royal Society.

Descripción: In this paper we present data on the nesting success and life history traits of the Scarlet-headed Blackbird (Amblyramphus holosericeus) nesting in the Argentinean pampas. During the 1995-1997 and 2001-2002 breeding seasons (September-December), we found 60 Scarlet-headed Blackbird nests. About 40% of these nests produced fledglings, and daily nest mortality did not differ among nesting stages (laying, incubation, nestling). Clutch size was 3 eggs, and the mean number of fledglings was 1.33 ± 0.78. Brood reduction was a common cause of nestling loss as 39% of nest with more than 1 nestling lost the last hatched one. Results obtained from experimental nests indicate that parent nest attention increases the probability of nest success as natural nests showed a lower predation rate. We suggest that the high nest attention detected in this species could be responsible for the higher nesting success compared to other sympatric marsh nesting species. However, high nest attendance could also imply an additional cost as it may constrain the time available to parents for foraging, thus limiting the food delivered to nestlings. © The Neotropical Ornithological Society.