Starlings forming fascinating formations over Tøndermarsken, south-west Jutland, Denmark

Migrant birds flexible on travel dates

A new study shows that migrating birds are able to keep their travel dates flexible

can respond to the effects of climate change by leaving earlier. However this doesn’t mean that birds will always arrive earlier.

The research, published online in Current Biology, reveals that Pied flycatchers which migrate don’t necessarily reach their destination earlier as they may experience travel delays due to harsh weather conditions on the final leg of their journey through Europe.

"We have been claiming for a while that migratory birds have difficulties in adapting to climate change because of their rigid and rather inflexible timing of spring migration; in Africa and South America, they cannot know when spring starts at their northern breeding grounds," said author, Christiaan Both of the University of Groningen in The Netherlands.

"This study shows that the timing of spring migration is flexible and that birds do respond to climate change, although in a rather indirect way: breeding dates have become progressively earlier, and birds are thus born earlier in the spring.

“We now show that the effect of early birth is also that the birds migrate early, and migration time has advanced over the last 25 years. The reason that the birds did not advance their arrival is thus not due to a failure to start migration earlier, but because circumstances at passage in southern Europe have not improved."

Pied flycatchers are one of the best-studied migratory bird species in the world. With records going back more than 50 years, researchers have been able to investigate the birds' reaction to climate change over time.

"Forests are characterized by a short burst of insects rather early in spring," Both explained. "If the birds miss this insect peak for raising their chicks, they do not produce enough offspring to keep up their population sizes."

Like many migrants, pied flycatchers must tackle a strenuous spring migration to reach their breeding sites which could be anything from 5,000 to 9,000 kilometres  away. As their wintering grounds in Africa become progressively drier, the birds somehow have to accumulate enough resources to fly about 2,000 kilometres across the Sahara desert and then recover enough to fly to their final destinations.

"Based on our calculations, they are covering the distance from Northern Africa to The Netherlands in about 6 days, and to central Sweden in about 12 days," Both said. "In some of the northern or eastern breeding grounds, the first birds often arrive when the breeding areas are still snow-covered. Earlier arrival probably means death because there are not enough insects to be found."

But on arrival birds have little or no chance to rest before breeding and nest building must begin and in most cases during the warm springs of the past decade, birds in The Netherlands have laid their first eggs 7 to 8 days after completing their journey.

Both's team found that the birds left their wintering grounds and made it all the way to Northern Africa 10 days earlier in the year in 2002 than they did in 1980. Still, they didn't arrive at their European breeding grounds any sooner.