Abstract
In migratory birds, early arrival on breeding sites is typically associated with greater breeding success, but the mechanisms driving these benefits are rarely known. One mechanism through which greater breeding success among early arrivers can potentially be achieved is the increased time available for replacement clutches following nest loss. However, the contribution of replacement clutches to breeding success will depend on seasonal variation in nest survival rates, and the consequences for juvenile recruitment of hatching at different times in the season. In particular, lower recruitment rates of late‐hatched chicks could offset the benefits to early arrivers of being able to lay replacement clutches, which would reduce the likelihood of replacement clutch opportunities influencing selection on migratory timings. Using a simulation model of time‐constrained capacity for replacement clutches, paramaterized with empirically‐derived estimates from avian migratory systems, we show that greater reproductive success among early‐arriving individuals can arise solely through the greater time capacity for replacement clutches among early arrivers, even when later renesting 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 replacement clutches can mitigate this effect. The time benefits of early arrival facilitating replacement clutches following nest loss may therefore be an important but overlooked source of selection on migratory timings. Empirical measures of seasonal variation in nest survival, renesting, and juvenile recruitment rates are therefore needed in order to identify the costs and benefits associated with individual migration phenology, the selection pressures influencing migratory timings, and the implications for ongoing shifts in migration and breeding phenology.
Highlights
For migratory species, the timing of migratory journeys can have important fitness consequences
If the fitness benefits of early arrival operate primarily through the time available for replacement clutches, the advances in spring migration that are currently occurring in many species (Knudsen et al, 2011; Rubolini, Møller, Rainio, & Lehikoinen, 2007) could have profound implications for both productivity and the phenology of successful nests
Our models show that replacement clutches are likely to be most beneficial at intermediate nest survival rates (Figure 2), and that early arrival and the capacity to lay replacement clutches can potentially increase the total number of offspring recruited, even if nest survival rates increase during the breeding season (Figure 3)
Summary
The timing of migratory journeys can have important fitness consequences. If the fitness benefits of early arrival operate primarily through the time available for replacement clutches, the advances in spring migration that are currently occurring in many species (Knudsen et al, 2011; Rubolini, Møller, Rainio, & Lehikoinen, 2007) could have profound implications for both productivity and the phenology of successful nests. The contribution of replacement clutches to the benefits of early arrival will depend on breeding phenology (including the length of incubation and the time between nest loss and replacement), nest survival rates and offspring recruitment probabilities, and how these rates vary seasonally. We explore how these relationships vary with seasonal variation in nest survival and the consequences for offspring recruitment
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