Multidimensional natal isotopic niches reflect migratory patterns in birds.

Naturally occurring stable isotope ratios in animal tissues allow estimation of species trophic position and ecological niche. Measuring multiple isotopes of migratory species along flyway bottlenecks offers the opportunity to sample multiple populations and species whose tissues carry information at continental scales. We measured δH, δO, δC, δN in juvenile feathers of 21 bird species captured at a migratory bottleneck in the Italian Alps. We examined if trends in individual isotopes reflected known migratory strategies and whether dietary (δC-δN) and spatially-explicit breeding origin (δH-δO) niche breadth (NB) differed among long-distance trans-Saharan (TS), short-distance (IP) and irruptive (IR) intra-Palearctic migrants, and whether they correlated with reported populations long-term trends. In both TS and IP groups, species δH declined with capture date, indicating that northern populations reached the stopover site later in the season, following a Type-I migration strategy. Values of δH indicated that breeding range of TS migrants extended farther north than IP and IR migrants. The breeding season was longer for IP migrants whose δC and δN values declined and increased, respectively, with time of capture. Average species dietary NB did not differ among migratory groups, but TS migrants displayed wider breeding origin niches, suggesting that long-distant migration is linked to broader ecological niches. Isotope origin NB well reflected species geographic range extent, while dietary NB did not correlate with literature accounts of species' diet. We found no relationship between species breeding NB and population trends in Europe, suggesting that conditions in the breeding grounds, as inferred by stable isotopes, are not the only determinant of species' long-term persistence. We demonstrate that ringing activities and isotopic measurements of passerines migrating through a bottleneck represents a unique opportunity to investigate large-scale life-history phenomena relevant to conservation.