Climate Change Linked To Migratory Bird Decrease

"We predict that with increasing winter temperatures...the number of long-distance migratory bird species should decline," say Nicole Lemoine and Katrin Boehning-Gaese of Johannes Gutenberg University in Mainz, Germany, in the April issue of Conservation Biology.

The Earth's surface temperature has increased by about a degree F since 1860, and is expected to increase by as much as 10 degrees F more over the next century. Already, climate change is affecting plants and animals in many parts of the world: for instance, plants in Europe have a longer growing season, a North American marmot has a shorter hibernation period, and some migratory birds in Europe are starting to breed earlier.

Climate change could also affect the abundance and diversity of birds. The idea is that warmer winters could increase the survival of birds that live in an area year-round, which could give migratory birds more competition for resources such as food and nest sites when they return to breed in the spring – and that in turn could decrease the total number of migratory birds as well as the number of species.

To see if climate change affects the abundance and diversity of migratory birds, Lemoine and Boehning-Gaese analyzed existing bird census and climate data for the Lake Constance region of central Europe, which includes parts of Germany, Austria and Switzerland. The researchers determined the number of land bird species and the abundance of each species during two recent census periods (1980-81 and 1990-92). The researchers considered 300 species of land birds and divided them into three categories: residents, short-distance migrants (those that migrate an average of roughly 600 to 1,200 miles) and long-distance migrants (those that migrate more than 2,200 miles). There were 122, 80 and 108 species in each category, respectively.

While climate change did not affect resident or short-distance migratory birds, Lemoine and Boehning-Gaese found that it did affect the long-distance migrants. Between the two census periods, winters got warmer and the abundance of long-distant migrants decreased. Specifically, the average temperature of the coldest month increased more than four degrees F, and the abundance of long-distance migratory birds decreased by a fifth.

Ultimately, warmer winters will probably also decrease the number of long-distance migratory bird species in Central Europe, say the researchers. In addition, the birds' migratory behavior will probably evolve. The migratory behavior of bird populations can change in only a few generations, and several populations of wrens, skylarks and other short-distance migrants have stopped migrating in the last 20 years.