Data from: Migratory birds advance spring arrival and egg-laying in the Arctic, mostly by travelling faster

In the current, warming climate many organisms in seasonal environments
advance their timing of reproduction to benefit from resource peaks
earlier in spring. For migrants, the potential to advance reproduction may
be constrained by their migration strategies, notably their ability to
advance arrival on the breeding grounds. Recent studies show various
changes in migration strategies, including wintering closer to the
breeding grounds, earlier departure from the wintering grounds or faster
travels by spending less time on stopover sites. However, whether such
changes lead to earlier arrival or earlier breeding remain open questions.
We studied changes in migration and reproduction timing in 12 populations
of nine migratory birds, including seabirds, shorebirds, birds of prey and
waterfowl breeding on Arctic sites bordering the Greenland and Barents
Sea, a region undergoing rapid climate warming. Timing of migration and
reproduction was derived from tracking and field data, and analysed to
study (1) how timing has changed in response to changing moment of
snowmelt at the breeding grounds, and (2) what adjustments in migration
strategies this involved. We found that in years with early snowmelt
egg-laying in multiple populations advanced, but only two waterfowl
populations also advanced arrival in the Arctic. In contrast, arrival in
the Arctic generally advanced with time, even when snowmelt or egg-laying
dates did not advance. Earlier arrival with time was mostly explained by
populations travelling to the Arctic faster, likely spending less time at
stopover sites. Inability to forecast conditions in the Arctic may limit
birds to adjust migration timing to annually varying snowmelt, but we show
that several species, particularly waterfowl, are able to travel faster
and advance timing of migration over the years. The question remains
whether this reflects adaptations to Arctic climate change, or other
factors, for example environmental changes along the migratory route.