Dale, CA & ML Leonard. 2011. Reproductive consequences of migration decisions by Ipswich Sparrows (Passerculus sandwichensis princeps). Canadian J of Zoology, Vol 89, pages 100-108.
Summary (AGH): Songbirds migrate to escape the winter, and also to be in habitats where they can shore up their reserves for the following breeding season. We know very little about how songbirds choose where to winter: should they go farther where weather might be warmer and resources are more abundant, or should they trade those benefits off against the costs and risks of travel, and stay relatively near their breeding areas? Ipswich Sparrows are ideal for asking such questions, because they all breed on one island, where breeding conditions are similar, yet they winter along a long strip of coastline from Georgia to Nova Scotia, across which winter conditions likely vary tremendously. Thus, any differences in how well an individual breeds might especially vary with where it winters, rather than factors on the breeding grounds. To test this idea, this study used stable isotopes, which are naturally occurring variations in chemical elements that differ in their radioactivity and vary geographically, to identify where individual sparrows wintered. The isotopes were extracted from feathers that the birds grew on the wintering grounds, and thus offered a signature of where they wintered. Those locations could then be related to several measures of how well the birds performed during the breeding season. Interestingly, the results differed between males and females: males that wintered farther north were in better condition, bred sooner, and sired heavier nestlings, whereas females that wintered farther south started nesting sooner (which tends to correlate with higher reproductive output across the breeding season). Thus, the trade-off between wintering conditions and travel costs is different between the sexes: males may have to arrive sooner to better compete for the best territories, whereas females need good conditions to build up their reproductive resources.