Why fledge early in the day? Examining the role of predation risk in explaining fledging behavior

Predation represents the primary cause of mortality for both nestling and fledgling birds and is often greatest in the days immediately before and after nest departure. Due to the selective pressures of such high mortality rates, behaviors likely evolved to increase the survival of young. Among altricial species, fledging often occurs in the morning with most nestlings leaving within 6h of sunrise. However, why nestlings tend to fledge in the morning and whether this strategy is a response to predation risk is unknown. We investigated how the time of day when fledging began and how rapidly broodmates fledged were influenced by nest predation rates and nest site features that affect nest predation risk. We video recorded 477 fledging events at 202 nests of 17 species. Nestlings occupying nests with greater predation risk initiated fledging earlier in the day than those at safer nests. Similarly, broodmates in riskier nests fledged over a shorter period of time than broodmates in safer nests. Our findings support the hypothesis that predation risk influences the time of day when fledging occurs. By fledging earlier and more quickly, young in high risk nests presumably decrease their chances of being depredated in the nest, whereas those occupying safer nests are likely under reduced pressure to fledge as early and quickly as possible. These results indicate that nestlings preparing to fledge likely face more complex situations than currently understood, and the timing of nest departure is an important decision made in an effort to maximize fledgling fitness.