Even during the Ice Age, birds migrated between their breeding and wintering grounds
Billions of birds around the world migrate in search of breeding and wintering grounds each year. Their migratory behaviour is primarily influenced by the prevailing climate. Until now, the question as to whether birds migrated during the last ice age, which preceded the current warm phase, has been disputed. Researchers at the Max Planck – Yale Center for Biodiversity Movement and Global Change have used computers to model the development of bird migration over the last 50,000 years. Their simulations indicate that birds have been migrating since the last ice age. However, the ways in which birds have reacted to climate change differs around the world. The results could help with predictions about the effects of human-induced climate change on migratory birds.
Depending on the season, some bird species migrate over huge distances to take advantage of the varying environmental conditions for themselves and their offspring. This allows them to benefit from the high food supply in one area whilst raising their offspring before migrating to other regions with more favourable living conditions in the autumn. Cuckoos from Kamchatka, for example, migrate to Angola to spend the winter there, which is the longest annual journey of migratory birds known to date.
The climate has changed dramatically over the last 50,000 years: the present warm period with its comparatively high temperatures was preceded by an ice age that lasted tens of thousands of years, during which glaciers covered vast regions of the earth. Until now, most researchers believed that birds lived as resident species during such periods and only migrate during warm phases, yet they have found it difficult to investigate the origins of bird migration as it leaves hardly any traces.
Computer reconstructs history of bird migration
That is why, together with colleagues from the USA, Great Britain and France, Martin Wikelski of the Max Planck Institute of Animal Behavior in Radolfzell and his team have created computer simulations of bird migration. Based on the climate over the last 50,000 years, they developed a computer model that calculates the global distribution of environmental factors that are important for bird survival. The model uses a cost-benefit analysis in which the energy expenditure during the migration must not outweigh the energy gain at the destination. This allows them to reconstruct bird migrations in the past.
To test its reliability, the researchers input current climate and environmental data into the computer model, which correctly predicts the current distribution of all known bird species. They then fed climate data from the past 50,000 years into their model, which can reconstruct the flora, and therefore potential food sources for birds, that existed during various epochs.
As expected, birds brooded closer to the equator during colder periods. There were significantly fewer resident bird species further north, particularly north of the 50th parallel. However, the results also show that birds migrated back and forth between summer and wintering grounds during the last ice age. “Therefore”, as Wikelski explains, “rather than beginning after the last ice age, the origins of bird migration must go back much further”.
According to the model, the ways in which migratory birds react to climate change varies from region to region. In Europe, Asia and Africa, for example, there were about as many migratory bird species during the last ice age as there are today: whilst the flight distances varied over time, they remained relatively constant on average. The land masses in North and South America are distributed differently between north and south, which is why there were 20 percent fewer migratory bird species there during the last ice age. “Apparently”, says Wikelski, “these were resident species during the ice age and only began to migrate afterwards. And”, he continues, “at an average of 500 kilometres, the migratory routes were also 40 percent shorter”.
Additional threat from human-made climate change
So bird migration has continued even during major climate changes. “Animals have a deep-rooted ‘wanderlust’: as soon as the conditions for reproduction or survival are better elsewhere, they will migrate”. On the other hand, it is still unclear how human-caused climate change is affecting bird migration: not only is it progressing at a faster rate than previous climate change events, but the living conditions of birds are also deteriorating in many other ways such as loss of habitat and food sources.
It is almost certain that the current climate change poses an additional threat to numerous species and could accelerate the dramatic loss of birds. “About a billion migratory birds die en route between Europe and Africa each year”, says Wikelski, “and we don’t know exactly where or why”. The computer model can now help to predict the effects of global climate change on bird migration.
Wikelski wants to use the international Icarus project to find the exact reasons for these losses. Icarus is a satellite-supported animal observation system that uses tiny transmitters mounted on the birds’ backs to collect data around the clock throughout their lives, including their migratory treks. “Hopefully”, says Wikelski, “Icarus will soon reveal the greatest threat to the survival of migratory birds as well as where they fare best”. The system is scheduled to go into test operation in March and to be available for research projects as of August.