Isotopic ornithology: a perspective

The use of measurements of naturally occurring stable isotopes of the light elements C, N, H, O, S in avian tissues and their associated foodwebs has grown tremendously during the last 30 years. Three primary applications of this approach have been (1) the delineation of diets or trophic relationships and sources of nutrients to individuals or populations, (2) the assessment of the relative contributions of endogenously and exogenously derived nutrients to reproduction in birds that travel to breed, and, more recently, (3) the assignment to origin of migratory individuals. This paper will briefly summarize the roots of these theme areas and highlight the benefits of an isotope perspective. However, the primary focus of this paper is on key assumptions, misunderstandings, information gaps and the path ahead. Specific recommendations for future research are outlined for all of these theme areas. Finally, a plea is made for more constructive criticism in the field of isotopic applications to animal ecological studies, especially those involving birds.