Half a World Apart? Overlap in Nonbreeding Distributions of Atlantic and Indian Ocean Thin-Billed Prions
Résumé
Distant populations of animals may share their non-breeding grounds or migrate to distinct
areas, and this may have important consequences for population differentiation and dynamics.
Small burrow-nesting seabirds provide a suitable case study, as they are often restricted
to safe breeding sites on islands, resulting in a patchy breeding distribution. For
example, Thin-billed prions Pachyptila belcheri have two major breeding colonies more
than 8,000 km apart, on the Falkland Islands in the south-western Atlantic and in the Kerguelen
Archipelago in the Indian Ocean. We used geolocators and stable isotopes to compare
at-sea movements and trophic levels of these two populations during their nonbreeding
season, and applied ecological niche models to compare environmental conditions
in the habitat. Over three winters, birds breeding in the Atlantic showed a high consistency
in their migration routes. Most individuals migrated more than 3000 km eastwards,
while very few remained over the Patagonian Shelf. In contrast, all Indian Ocean birds migrated
westwards, resulting in an overlapping nonbreeding area in the eastern Atlantic sector
of the Southern Ocean. Geolocators and isotopic signature of feathers indicated that
prions from the Falklands moulted at slightly higher latitudes than those from Kerguelen Islands.
All birds fed on low trophic level prey, most probably crustaceans. The phenology differed
notably between the two populations. Falkland birds returned to the Patagonian Shelf
after 2-3 months, while Kerguelen birds remained in the nonbreeding area for seven
months, before returning to nesting grounds highly synchronously and at high speed. Habitat
models identified sea surface temperature and chlorophyll a concentration as important
environmental parameters. In summary, we show that even though the two very distant populations
migrate to roughly the same area to moult, they have distinct wintering strategies:
They had significantly different realized niches and timing which may contribute to spatial
niche partitioning.