A negative trade-off between current reproductive effort and reproductive success: an experiment with clutch-size in a tropical bird

Abstract

Clutch-size is thought to be the result of a trade-off between current reproductive effort, which may be constrained by current food supply, and current and future reproductive success. Studying the selective pressures that determine clutch-size is crucial for understanding life-history strategies. In order to examine how the costs associated with increasing energy demands during reproduction affect reproductive success, we experimentally manipulated clutch-sizes of Lesser Elaenia (Elaenia chiriquensis) in the Brazilian Cerrado. This species has a clutch-size of two, the most common clutch-size for tropical birds. We manipulated nests to create artificial clutches of one and three eggs. We tested if nestling survival and growth, risk of nest predation and incubation efficiency in larger clutches influenced reproductive success, and how parents responded to these demands in food provisioning and incubation behaviour. Neither clutch-size nor nestling age affected nest predation or incubation efficiency, but parental activity did increase with increased clutch-size and increasing age of nestlings. However, growth of nestlings was negatively influenced by increasing clutch-size, which is likely to affect both the quantity and quality of young produced. Our data show that parental investment during the nestling stage may be a key factor in the evolution of small clutch-sizes in tropical birds.