Stable isotope analyses of feathers help identify autumn stopover sites of three long-distance migrants in northeastern Africa

The potential use of stable nitrogen (δ¹⁵N), carbon (δ¹³C) and hydrogen (δD) isotope ratios in feathers of marsh warblers Acrocephalus palustris , river warblers Locustella fluviatilis and whitethroats Sylvia communis was evaluated as a means to help identify the location and isotopic composition of autumn stopover sites in northeast Africa. Feather δD values were compared with regional precipitation δD maps averaged over autumn months. Compared with whitethroats, feather δ¹⁵N, δ¹³C, and δD values of marsh warblers and river warblers suggest the two warblers occupy and grow their feathers in geographic locations with relatively mesic environments, and with higher proportions of C3 (vs. C4) plants. However, δ¹³C values of marsh and river warblers were distinct enough to indicate use of different foodwebs. From previous studies, it is evident that during autumn stopover, river warblers moult their primaries in Ethiopia. It is likely that marsh warblers, like river warblers, stay in Ethiopia and/or in neighbouring regions. Based on feather δD values and regional δD precipitation maps, this region should lie between southeast Sudan and southwest Ethiopia. However, without additional regional isotopic maps in Africa, more precise locations of the stopover sites remain unclear. The relatively enriched δ¹⁵N and δ¹³C values of whitethroat feathers compared with the two other species, reflect the fact that whitethroats moult in relatively drier environments and/or with a lower proportion of C3 vs. C4 plants.