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

The potential use of stable nitrogen (delta N-15), carbon (delta C-13) and hydrogen (6D) isotope ratios in feathers of marsh warblers Acrocephalus palustris, river warblers Locustella fluviatilis and whitethroats Syluia communis was evaluated as a means to help identify the location and isotopic composition Of autumn stopover sites in northeast Africa. Feather 6D values were compared with regional precipitation 6D maps averaged over autumn months. Compared with whitethroats, feather delta N-15, delta C-13, and 6D 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, delta C-13 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 8D values and regional 6D 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 delta N-15 and delta C-13 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.