Low frequency of Shiny Cowbird parasitism on Scarlet-headed Blackbirds: anti-parasite adaptations or nonspecific host life-history traits?

Interspecific brood parasitism is a strong selective force that has favoure d the evolution of anti-parasite strategies. The Shiny Cowbird Molothrus bo nariensis is a generalist parasite that uses more than 200 species as hosts . Scarlet-headed Blackbirds Amblyramphus holosericeus are infrequently para sitized by Shiny Cowbirds in contrast to other sympatric marshbird species that are heavily parasitized. In this paper, we attempted to identify speci fic anti-parasite defences of the Scarlet-headed Blackbird. Parasitism by S hiny Cowbirds of Scarlet-headed Blackbirds averaged 14.2%, whereas in Brown -and-yellow Marshbirds Pseudoleistes virescens, a sympatric marsh-nesting s pecies, it was 52%. Both host species reared Shiny Cowbird chicks. Scarlet- headed Blackbird did not reject artificially added parasite eggs and did no t respond more aggressively toward a female Shiny Cowbrid dummy model than to control species, the Yellow-winged Blackbird Agelaius thilius or the Bro wn-and-yellow Marshbird. However, high levels of nest attentiveness were re corded during nest-building, egg-laying and incubation stages. About 95-98% of the time at least one parent remained less than 20 m from the nest. Our results show that the low frequency of parasitism on the Scarlet-headed Bl ackbird may be better explained by life-history traits such as territorial defence, rather than by evolved anti-parasite strategies.