Do life-history traits in the ancestor of Cowbirds (Molothrus spp.) predispose them to become brood parasites?

Hamilton & Orians (1965) stated that the non-parasitic ancestor of cowbirds accelerated its incubation period and nestling growth. Later, by comparing cowbirds with nesting birds, it was suggested that those characteristics, together with small eggs with thicker eggshell, were adaptations. When these hypotheses were tested considering the characteristics of the group in which cowbirds evolved, i.e. the "grackles and allies", it was found that only the thicker eggshell was a plausible adaptation. Moreover, nesting grackles and allies showed the same reproductive patterns as cowbirds (shorter incubation periods, faster nestling growth, and smaller eggs than those predicted by general allometric equations). In the present study, we tested whether small eggs with short incubation periods and nestlings with accelerated growth evolved in the ancestor of grackles and allies. For all the traits tested, we reconstructed ancestral states and built prediction intervals generated with independent variables that were allometrically related to them. Except for accelerated growth, we applied two evolutionary models incorporating outgroups in three steps. We found that the short incubation period and the faster nestling growth never evolved in the ancestor of grackles and allies. In most ancestor reconstructions, neither egg mass nor female body mass changed. However, when it changed, it increased both egg and female masses. The prediction intervals indicated that those increases did not imply an allometric change in size. Consequently, neither a decrease in egg mass nor an acceleration in nestling development and incubation period are considered characteristics that predisposed cowbirds to become brood parasites. © The Neotropical Ornithological Society.Fil:Mermoz, M.E. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina

Life history traits and breeding success of the scarlet-headed blackbird (Amblyramphus holosericeus) in the Argentinean Pampas

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.Fil:Fernández, G.J. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina.Fil:Mermoz, M.E. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina.Fil:Llambías, P.E. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina