Within-season increase in parental investment in a long-lived bird species: investment shifts to maximize successful reproduction?

In nest-building species predation of nest contents is a main cause of reproductive failure and parents have to trade off reproductive investment against antipredatory behaviours. While this trade-off is modified by lifespan (short-lived species prioritize current reproduction; long-lived species prioritize future reproduction), it may vary within a breeding season, but this idea has only been tested in short-lived species. Yet, life history theory does not make any prediction how long-lived species should trade off current against future reproductive investment within a season. Here, we investigated this trade-off through predator-exposure experiments in a long-lived bird species, the brown thornbill. We exposed breeding pairs that had no prior within-season reproductive success to the models of a nest predator and a predator of adults during their first or second breeding attempt. Overall, parents reduced their feeding rate in the presence of a predator, but parents feeding second broods were more risk sensitive and almost ceased feeding when exposed to both types of predators. However, during second breeding attempts, parents had larger clutches and a higher feeding rate in the absence of predators than during first breeding attempts and approached both types of predators closer when mobbing. Our results suggest that the trade-off between reproductive investment and risk-taking can change in a long-lived species within a breeding season depending on both prior nest predation and renesting opportunities. These patterns correspond to those in short-lived species, raising the question of whether a within-season shift in reproductive investment trade-offs is independent of lifespan.