Isotopic Characterization of Flight Feathers in Two Pelagic Seabirds: Sampling Strategies for Ecological Studies

Abstract

We wish to use stable-isotope analysis of flight feathers to understand the feeding behavior of pelagic seabirds, such as the Hawaiian Petrel (Pterodroma sandwichensis) and Newell's Shearwater (Puffinus auricularis newelli). Analysis of remiges is particularly informative because the sequence and timing of remex molt are often,known. The initial step, reported here, is to obtain accurate isotope values from whole remiges by means of a minimally invasive protocol appropriate for live birds or museum specimens. The high variability observed in delta C-13 and delta N-15 values within a feather precludes the use of a small section of vane. We found the average range within 42 Hawaiian Petrel remiges to be 1.3 parts per thousand for both delta N-13 and delta N-15 and that within 10 Newell's Shearwater remiges to be 1.3 parts per thousand and 0.7 parts per thousand for delta N-13 and delta N-15, respectively. The delta N-13 of all 52 feathers increased from tip to base, and the majority of Hawaiian Petrel feathers showed an analogous trend in delta N-15. Although the average range of delta D in 21 Hawaiian Petrel remiges was 11 parts per thousand, we found no longitudinal trend. We discuss influences of trophic level, foraging location, metabolism, and pigmentation on isotope values and compare three methods of obtaining isotope averages of whole feathers. Our novel barb-sampling protocol requires only 1.0 mg of feather and minimal preparation time. Because it leaves the feather nearly intact, this protocol will likely facilitate obtaining isotope values from remiges of live birds and museum specimens. As a consequence, it will help expand the understanding of historical trends in foraging behavior.