Climate change good for the birds?

Research from a team at the Swiss Ornithological Institute, to be published in a forthcoming Proceedings B, a learned journal published by the Royal Society, reveals complex effects of global warming on migratory habits of birds. These effects may constitute a serious threat to some species – in particular those with longer migratory flights.

Over the past two decades spring temperatures in temperate regions have increased. This effect, due to global warming, has advanced the start of the reproductive season for many bird species. However, until now there has been hardly any information about the effects of global warming on other aspects of the annual cycle of migratory birds such as autumn migration.

Using the records of almost 350,000 captures of 64 different bird species in an Alpine pass in Switzerland dating from 1958 to 1999, Dr. Lukas Jenni and Dr. Marc Kery at the Swiss Ornithological Institute were surprised to find no uniform shifts in the timing of autumn migration but a complex adaptation that benefits some species over others. The birds were caught in ‘mist nets’ arranged in the pass during both day and night. After recording their details the birds were released to continue their migration.

“The Col de Bretolet site in Switzerland is 1920m above sea level and is a pure passage site for species migrating from a wide areas of Scandinavia, central and eastern Europe,” says Dr. Jenni. “This means that all birds recorded here are in transit and the data generated is not complicated by non-migratory movements of locally breeding birds.”

Short hop vs long-haul

“The data shows that long-distance migrants, those that winter south of the Sahara, appear to be leaving earlier,” says Dr. Jenni. “For instance the Pied Flycatcher, Willow Warbler and Garden Warbler now fly south almost a week earlier.”

In contrast, short distance migrants who winter in Southern Europe and Northern Africa have delayed their autumn migration. For example the Skylark now migrates seven days later and the European Starling nine days later.

Brood numbers

An additional complicating factor is that bird species with a variable number of broods migrate later than species with only one annual brood, probably because they have extended their breeding season. This increases their reproductive rate.

This long-term study suggests that effects of global warming are complex. “We believe that short distance migrants may benefit from global warming through higher reproduction and shorter migration,” explains Dr. Jenni. “In contrast, trans-Saharan migrants may not gain the same benefits. Global warming could thus be a serious threat to some long-distance migrants and one reason for the recent decline of such species in Europe.”

The timing of autumn migration is governed by three main factors: the end of the reproductive season; conditions in the breeding area after the breeding season is ended; and expected conditions in the passage and wintering grounds.

“For long distance migrants crossing the Sahara the migration could be constrained by the onset of the dry season in the Sahel region,” concludes Dr. Jenni. “This region suffers serious drought conditions. Migrant species would want to cross the region before the drought had started. The spring arrival of these species and the start of their breeding season is also constrained so they are loosing out to species with shorter migration patterns.”