Stable isotope evidence for suspended moult and age-related differences in moult location in the trans-Saharan migratory Alpine Swift

Stable isotope analysis has been used extensively in migratory bird studies to provide ecological insights that may otherwise be difficult to obtain. However, an understanding of moult is critical for appropriate feather sampling, and here we make the first assessment of its relevance for examining the non-breeding ecology of the Alpine Swift Tachymarptis melba, a long-distance Afro-Palaearctic migrant. We sampled three feather types from birds found dead after their return from migration and investigated variability in δ¹³C, δ¹⁵N and δ²H within and among feathers and between age classes. We found isotopic evidence supporting an interrupted primary moult in adults, with the innermost primary of adults (known to be moulted on the breeding grounds) significantly depleted in ¹³C and ²H compared with either the outermost primary or outermost tail feather, both of which were representative of sub-Saharan non-breeding areas. In contrast, the absence of significant differences in immature birds suggests the probable existence of a non-migratory strategy within this age class. These isotopic insights into moult and life history highlight the potential for stable isotope analysis as a tool for investigating non-breeding strategies in the Alpine Swift.