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

MERMOZ, M. E.; FERNÁNDEZ, G. J..

Journal of Avian Biology

Año: 1999 vol. 30 p. 15 - 22

Interspecific brood parasitism is a strong selective force that has favoured the evolution of anti-parasite strategies. The Shiny Cowbird Molothrus bonariensis is a generalist parasite that uses more than 200 species as hosts. Scarlet-headed Black- birds Amblyramphus holosericeus are infrequently parasitized by Shiny Cowbirds in contrast to other sympatric marshbird species that are heavily parasitized. In this paper, we attempted to identify specific anti-parasite defences of the Scarlet-headed Blackbird. Parasitism by Shiny Cowbirds of Scarlet-headed Blackbirds averaged 14.2%, whereas in Brown-and-yellow Marshbirds Pseudoleistes virescens, a sympatric marsh-nesting species, it was 52%. Both host species reared Shiny Cowbird chicks. Scarlet-headed Blackbird did not reject artificially added parasite eggs and did not respond more aggressively toward a female Shiny Cowbrid dummy model than to control species, the Yellow-winged Blackbird Agelaius thilius or the Brown-and-yel- low Marshbird. However, high levels of nest attentiveness were recorded 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 Blackbird may be better explained by life-history traits such as territorial defence, rather than by evolved anti-parasite strategies.