Short- and long-term costs of reproduction in a migratory songbird

Costs of reproduction represent a common life-history trade-off. Critical to understanding these costs in migratory species is the ability to track individuals across successive stages of the annual cycle. We assessed the effects of total number of offspring fledged and date of breeding completion on pre-migratory body condition, the schedule of moult and annual survival in a migratory songbird, the Savannah Sparrow Passerculus sandwichensis. Between 2008 and 2010, moult was delayed for individuals that finished breeding later in the breeding period and resulted in reduced lean tissue mass during the premigratory period, suggesting an indirect trade-off between the timing of breeding completion and condition just prior to migration. Lean tissue mass decreased as the number of offspring fledged increased in 2009, a particularly cool and wet year, illustrating a direct trade-off between reproductive effort and condition just prior to migration in years when weather is poor. However, using a 17-year dataset from the same population, we found that parents that fledged young late in the breeding period had the highest survival and that number of offspring fledged did not affect survival, suggesting that individuals do not experience long-term trade-offs between reproduction and survival. Taken together, our results suggest that adult Savannah Sparrows pay short-term costs of reproduction, but that longer-term costs are mitigated by individual quality, perhaps through individual variation in resource acquisition.