As temperatures in central Europe have risen, blackcaps have arrived earlier at summertime breeding areas and departed later for their winter homes. Some researchers have predicted blackcaps would also migrate over ever-shorter distances, and in some cases stop altogether, allowing them to save energy and concentrate on finding food and mates. But this hadn’t been tested.
To gauge the birds’ migratory energies, Pulido and Berthold removed a few hundred blackcaps from the local population each summer. As captive birds are restless during the time they would typically be migrating, the researchers used them to measure the duration of wild migrations. These dropped slowly but steadily between 1988 and 2001, in keeping with predictions.
In a second part of the study, Pulido and Berthold bred the most sedentary blackcaps. They wanted to accelerate the natural trend, seeing in a few years what would normally take decades. From this, they extrapolated that some blackcap populations could stop migrating altogether within 40 to 50 years. Other birds may do the same.
However, shorter distances may only be an option for some species. Blackcap migration spans a relatively modest 1,000 miles, and sometimes less. For birds that travel thousands of miles, with no hospitable territory between their destinations, there may be no middle ground.
“Adaptation requires a large population. Otherwise they’ll go extinct,” he said.
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