The delayed arrival of migratory songbirds in northern Europe last year was caused by a drought in Africa and suggests that the species may not be able to adapt to rapid climate change, Danish scientists said.

The extensive 2011 drought in the Horn of Africa had significant consequences for European songbirds such as the thrush nightingale and red-backed shrike. These birds normally arrive in northern Europe in the spring to mate and stay through the summer to take advantage of ample food resources.

But last year, they arrived a week later than normal, as they got delayed in their migration route from southern Africa by a longer-than-usual stopover in the Horn of Africa, which was plagued by a severe drought at the time.

Using tiny backpacks fitted to the birds, the scientists tracked them and were able to reconstruct 26 migration routes and stopovers. The backpacks, which weigh only 1 gram, showed that the birds spent a week more than expected in the Horn of Africa. Other birds that didn't pass through there arrived in Europe at the expected time.

The red-backed shrike. The small "backpack" capsule on its leg is an electronic device that allows scientists to follow its migration. Photo courtesy of Per Ekberg.

"At first we thought we were so unlucky that the birds were delayed, but then we realized that this might actually give us an opportunity," said Anders Tøttrup, an associate professor at the Center for Macroecology, Evolution and Climate at the University of Copenhagen. "Because of the drought, the birds would have needed longer to feed and gain energy for their onward travel, causing delayed arrival and breeding in Europe. This supports our theory that migrating animals in general are dependent on a series of areas to reach their destination."

The scientists are now planning to expand their research to track other populations of red-backed shrike.

"We want to see how this may change over time, as they are really dependent on these few stopover sites," Tøttrup said. "We are trying to model how the migration system may look in the future."

Rapid climate change will likely have a significant effect on the birds' migration patterns, he said.

"As the climate changes, we can model the environmental predictors that we have to see how these stopover sites may change," Tøttrup said. "We can see that a lot of migratory birds follow what looks to be optimal feeding conditions. They go to the Horn of Africa because it's a good time to be there in the spring right after the rainy season."

The birds' late arrival in 2011 led to delayed breeding. Although this hasn't yet affected the size of the population, the scientists said it was possible that they haven't yet observed the full effect of the delay.

"The birds didn't adjust to the drought; they just stayed there longer," Tøttrup said. "Their system doesn't seem very flexible."