The greatest advances were found among short-distance migrants that winter in Europe or North America: about 1.5-2 days per decade. Long-distance migrants that winter in the tropics have also advanced the start of their migration, but only by approximately 0.6-1.2 days per decade.
“Based on changes in median migration dates, birds have on average advanced their spring migration by a little over a week since the late 1950s”, says Aleksi Lehikoinen from the Finnish Museum of Natural History, University of Helsinki.
Some species show much greater advances. For instance, Whooper Swans now arrive in Finland about two weeks earlier than in the 1980s.
The advances in spring migration dates are not equal across the migration season. Early migrants of a given species have advanced their migration dates more than late migrants within the species’ migration season. First migrants have the highest pressure to arrive at their breeding grounds as early as possible, whereas late migrants are typically non-breeders, which have no rush to move north. This asymmetry has led to an overall increase in the duration of migration.
Source: Spring migration is now earlier in European and North American birds