Antiparasitic defences in Chalk-browed Mockingbirds

RABUFFETTI FL; FIORINI VD; REBOREDA JC

Parque Nacional de Puyehue, Chile

Congreso; VII Neotropical Ornithological Congress; 2003

Neotropical Ornithological Society y Unión de Ornitólogos de Chile

Brood parasites lay eggs in nests of other birds, the hosts, which perform all the parental care. Parasitism reduces the reproductive success of the hosts, which may result in the evolution of antiparasitic defences. Most common behaviours of the host that could reduce the impact of parasitism include high levels of nest attention during laying, aggression towards parasites that approach the nest, and rejection of parasitic eggs. Chalk-browed Mockingbirds (Mimus saturninus) are common hosts of Shiny Cowbird (Molothrus bonariensis) and parasitized nests fledge considerable fewer chicks than non-parasitized ones. In this work we evaluated if this host has developed antiparasitic defences and the effectiveness of these defences. We measured: 1) the percentage of time that at least one member of the pair was close to the nest during laying and if host nest attentiveness was associated with probability of parasitism, 2) the frequency and intensity of mockingbird agonistic responses towards models of the parasite or a control species, and 3) the proportion of experimentally added parasitic eggs of the white or spotted morphs that were rejected. We found that: 1) non-parasitized nests were attended a higher proportion of time than parasitized ones, 2) mockingbirds responded more aggressively towards models of the parasite than to models of the control species, and 3) they rejected parasitic eggs of the white but not the spotted morph. Our results show that this host has developed some antiparasitic defences. However, these behaviours were not very effective reducing the frequency (82%, n = 113 nests) and intensity (2.04 ± 0.13 eggs, n = 93 nests) of parasitism. We suggest that this low efficiency may be the result of a trade-off between nest defence and other behaviours like female foraging and male mate guarding.