Knowledge of migratory bird ecology has been limited by the difficulty of studying the same individuals during more than one stage of their annual cycle.
Researchers can study individually identifiable birds on their breeding, wintering, or migratory stopover grounds, but it is virtually impossible to track the same bird as it moves from one of these areas to another on its migratory journey. This makes it very difficult to assess how the conditions a bird experiences at one stage of the annual cycle influence its experience at another stage.
However, recent advances in measuring stable isotope ratios in biological samples afford an opportunity for indirect study of linkages between different phases of the annual cycle. Stable isotopes are naturally-occurring varieties of elements, such as hydrogen, whose relative proportions vary in predictable ways geographically or ecologically, and at scales that mirror the continental sojournings of migratory birds.
For example, the ratio of two varieties of hydrogen varies predictably with latitude in North America. Isotope ratios enter the local food chain through plants and are taken up by animals higher in the chain. The isotopes are then incorporated into these animals' growing tissues, such as a bird's feather.
Most migratory songbirds molt their feathers on or near their breeding site. However, recent work by scientists at the Migratory Bird Center has demonstrated that molt location is somewhat variable, with birds that breed later in the season being forced to intersperse molt with migration.
Such a molt-migration overlap might result in birds having a slower southward migration in fall and thus arriving on winter quarters late, after the best habitats are already occupied. Plus, feathers molted at migratory stopover sites appear to be of lower quality than those molted near breeding areas. Birds with poor quality feathers might have reduced chances of surviving migration or of securing a mate.
We are embarking on a new study to examine whether the location of molt is correlated with black-throated blue warblers (Dendroica caerulescens) breeding effort (number of young fledged and their date of fledging). Hydrogen isotope ratios from tail feathers of returning birds-birds that bred at the site the previous summer-will be used to determine the latitude at which the birds molted.
Scientists hope the results of this research will reveal some of the costs associated with extended breeding in black-throated blue warblers.