Teste de fatores que afetam o tamanho da ninhada de Elaenia chiriquensis (Tyrannidae) do Cerrado do Brasil Central

AUTOR(ES)
DATA DE PUBLICAÇÃO

2008

RESUMO

The study of the selective pressures that determine clutch size is essential to understand lifehistory strategies. Brood manipulation studies are the standard tools for testing the influence of food availability, nest predation risk and increasing parental effort on limitation of clutch size. In order to assess the relative importance of food limitation to the nestlings, nest predation risk and energetic demands of incubation as selective pressures influencing clutch size, we manipulated clutch sizes of Elaenia chiriquensis from 2006 to 2007 in Estação Ecológica de Águas Emendadas, DF. This species represents the typical tropical species with a small clutch size of two eggs (control). We created two experimental treatments: (1) 40 enlarged clutches (three eggs), and (2) 35 reduced clutches (one egg). We assessed food availability to the nestlings by monitoring food delivery rates to the nest, nestling growth, nestling period, and number of fledglings produced by successful nests. We calculated nest predation probabilities with the program MARK, in which predictive variables included clutch size and age of the nest/nestlings. We assessed the females ability to incubate an enlarged clutch through nest attendance, hatching success and incubation period. Food availability to nestlings was lower in enlarged than control clutches because nestling growth was related to increasing clutch size, despite the fact that food delivery rates per nestling were the same between enlarged and control clutches. However, reduced nestling growth appeared to be unrelated to food limitation. The number of fledglings produced in successful nests increased marginally with clutch size, which indicates that deaths due to starvation were not more frequent in enlarged broods than control ones. We suggest that parents were unwilling to increase parental effort proportionally to nestling energetic demands. Nest predation risk was not affected by clutch size or nestling age, although enlarged clutches and nests with older chicks experienced more parental activity to feed the nestlings. Energetic costs with incubation were not evident during the incubation stage since clutch enlargement did not affect nest attendance, hatching success and incubation period. Our results do not support Skutchs nest predation hypothesis (1949) and indicate that E. chiriquensis is able to incubate an extra egg. However, it is not possible to determine the consequences of increased incubation effort upon parental fitness. Instead, our results indicate that clutch size is in part limited by parental availability to feed young, which can cause an increase in fledgling mortality rates.

ASSUNTO(S)

manipulação de ninhadas food availability disponibilidade de alimento esforço de incubação ciencias biologicas clutch manipulation incubation effort food delivery rate parental care nest predation risk risco de predação taxa de entrega de alimentos

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