Global warming effects on bird migration

Global warming effects on bird migration

Due to climate change, thousands of migrating birds are spending less time in Africa or are shortening their migration distances to spend the winter in southern Europe. Some have even become completely sedentary. 

Common crane in migration flight from the Pyrenees.

The warmer the climate, the less the birds migrate. Traditionally, hundreds of millions of birds spend the winter in Africa and return for the spring to Europe. But the winters on the Old Continent becoming milder and milder, and the food resource sufficiently abundant, some decided not to inflict on themselves the journeys of a long and perilous migration. Brave sandstorms in the Sahara, turbulence over the Mediterranean or the dangers of wind turbines and power lines? Very little for these few thousand birds, which no longer see enough interest in it.

Among the white storks, for example, some no longer systematically make the journey south of the Sahara for the winter. “Little by little, their wintering area moved up towards Spain and even France, for many on the Atlantic coast. Several thousand individuals even stay all year round in the same place, betting on a mild winter,” explains Jérémy Dupuy, ornithologist at the League for the Protection of Birds, and one of the authors of the next “Atlas of birds migrants from France”, to be published in 2022 by Biotope. “As long as there is no snow, the food resource remains accessible. Storks can thus frequent landfills, or wetlands in search of invertebrates.

However, it should be noted that this phenomenon is in the minority. “Between 200,000 and 300,000 individuals still pass through the Strait of Gibraltar in autumn, heading for the Sahel. The majority of storks thus continue to migrate in sub-Saharan Africa. But we can imagine that this tendency to settle down or to migrate less far develops if the winters continue to get milder in Europe” continues the ornithologist.

DUCKS INCREASINGLY HOMEBODY

The white stork is not the only species to have certain groups of individuals who decide to shorten migration distances. The common crane, which usually goes to the Iberian peninsula to winter, is sometimes content with the south west of France, the Camargue or the lakes of Champagne. Another example: the goldeneye. Traditionally, this duck flees the freezing temperatures of Scandinavia in winter to come to France. But in recent years, this is less and less the case. “In 1990, there were between 2,000 and 3,000 of these ducks in France in winter. Today there are only 1,000 of them. Yet their overall population is not decreasing - it's simply due to the fact that they now winter much further north,” explains Jérémy Dupuy.

For now, the decision to become sedentary or to shorten its migration is individual to each bird. “What governs this decision is not necessarily known but we can hypothesize that the physical condition at the time of departure plays a role. Usually, and the vast majority of the time, birds that do not migrate die. But if they survive, it may lead to new migration strategies the following year. The mild winters that follow one another allow this strategy to continue.

Like the white stork, the cattle egret now winters heavily in France. But some harsh winters can lead to very high mortality.

This bet - to stay further north and face a supposedly tolerable cold rather than endure the migration - carries risks. “In the event of a very harsh winter, this surprises and kills the migrants who have become sedentary, who are not used to such temperatures. However, this is not a danger for the survival of the species, because in the case of the white stork for example, the majority of the birds are still in Africa and will therefore return in the spring,” explains Jérémy Dupuy.

UP TO TWO MONTHS LESS IN SUB-SAHARAN AFRICA

Other migratory birds are also adapting to climate change without stopping to migrate south of the Sahara, but spending less time there. This can be up to two months less for some birds, as shown in a recent study published in Global Change Biology , by a team of British and Gambian researchers. Among the species concerned: the nightingale or certain species of passerines, such as the fitis warbler. They leave later and return earlier, in order to take advantage of the abundance of spring food in Europe.

This is coming faster and faster. What raise questions among the authors of the study. "This could create competition for food in fall and winter between birds that have always been sedentary and migratory birds that stay longer," said Kieran Lawrence, one of the study's authors, in a press release . “In addition, the ecosystems of the sub-Saharan regions where these birds normally spend the winter could change, since migratory birds will be less present to eat insects, or disperse seeds and pollen.

Wheatear: the entire breeding population from Greenland to Siberia winters in sub-Saharan Africa.

But according to Jérémy Dupuy, these changes in migration habits are not currently a cause for concern. In any case concerning the populations of migratory birds. “Overall, we perceive this rather positively. This means that birds are able to adapt to climate change.

And to recall: “It is above all the disappearance of habitats that causes the collapse of certain populations, among others. The mechanization of agriculture, the disappearance of hedgerows, and the use of phytosanitary products are indeed among the greatest dangers for a good number of bird species. And, migratory or not, it is difficult for them today to escape these threats.