Why do earlier-arriving migratory birds have better breeding success?

In migratory birds, early arrival on breeding sites has been widely associated with greater breeding success, but the mechanisms driving these benefits are rarely known. Acquisition of higher quality territories or carry-over effects of better non-breeding season conditions are frequently cited as possible mechanisms through which early-arrivers can achieve greater reproductive output. However, benefits of early arrival could also result from increased time available for breeding, independent of variation in territory quality and associated fitness. Increased time available for breeding among early arrivers could directly influence reproductive success through the time available for replacement clutches following nest loss. However, the benefits of replacement clutches may also depend on seasonal variation in nest survival, and the consequences for juvenile recruitment of hatching at different times in the season. Here we construct a simulation model to explore the potential for time-constrained capacity for replacement clutches to drive relationships between timing of arrival and reproductive success in avian migratory systems, and the influence of seasonal variation in both nest survival and subsequent offspring recruitment probability on these relationships. We show that positive relationships between arrival timing and reproductive success can arise solely through the greater time capacity for replacement clutches among early-arrivers, even when juvenile recruitment declines seasonally and thus later re-nesting attempts contribute fewer recruits to the population. However, these relationships vary depending on the seasonal pattern of nest survival. The benefits of early arrival are greatest when nest survival rates are constant or decline seasonally, and early arrival is least beneficial when nest success rates increase over the breeding season, although re-nesting capacity can mitigate this effect. The time benefits of early arrival facilitating replacement clutches following nest loss may therefore be an important source of selection on migratory timings, and empirical measures of seasonal variation in nest survival, re-nesting and juvenile recruitment rates are needed in order to identify the costs and benefits associated with individual migration phenology and the selection pressures influencing migratory timings.