Determining the sources of calcium for migratory songbirds using stable strontium isotopes

We investigated natural variations in the stable isotopic composition of strontium (a surrogate for calcium) in the bones of a single species of breeding migratory songbird, as well as in their eggshells, egg contents, and food sources. We use this information to determine the sources of calcium to these migratory songbirds and their offspring. Samples were collected from two locations in the northeastern USA (Hubbard Brook, NH, and Downer Forest, VT.) that differed in soil geochemistry. The mean Sr-87/Sr-86 ratios of food items (caterpillars and snails), eggshells, and egg contents were indistinguishable within each site, but significantly different between the two sites. Mean Sr-87/Sr-86 ratios for the bones of adult females were significantly different between the two sites, but values were significantly lower than those of food items and eggshells at each site. Two of four adult individuals studied at each site had Sr-87/Sr-86 ratios lower than the entire range of values for local food sources. Mixing calculations indicate that up to 60% of skeletal strontium and calcium was derived from foods consumed in the winter grounds where lower Sr-87/Sr-86 ratios predominate. At each study site, the Sr-87/Sr-86 ratio of eggshells differed significantly between clutches, but the mean clutch Sr-87/Sr-86 ratios were unrelated to the skeletal Sr-87/Sr-86 ratio of the laying adult. These findings suggest that strontium (and hence calcium) for eggshell production in this species is derived predominantly from local food sources in breeding areas. Thus, reductions in available calcium in northern temperate ecosystems due to the influences of acid deposition could be potentially harmful to this and other species of migratory bird.