For 20 years, Christiaan Both and his colleagues have been studying multiple populations of flycatchers in the Netherlands. For even longer, the small migratory birds have undertaken the epic journey from their winter home in Africa to their breeding ground in Europe. The flitting flycatchers arrive, recoup their strength and lay their eggs--a rhythm timed to the seasonal abundance of caterpillars. Now climate change has moved up the period of caterpillar plenty and produced catastrophic declines among birds that undertake their migration too late.
Throughout the world, warming earlier in spring has induced plants to flower, trees to bud, insects to appear and birds to breed accordingly. Caterpillars time their appearance to the budding of oak trees in the Netherlands. According to Both's research, caterpillar population now peaks 16 days earlier than in 1985, when the study began. This is important for flycatchers--Ficedula hypoleuca--because their chicks rely on caterpillars as their primary food source. Already, the birds have begun to breed 10 days earlier.
But that is not enough. Because migration cues in Africa have not changed, the birds are arriving too late to successfully breed, Both's team found. As a result, flycatcher numbers in areas of the Netherlands with the earliest abundance of caterpillars have declined by 90 percent. This matches data from nesting sites across Europe and even the population rhythms of other birds: great tits only have a second brood in seasons where caterpillars appear late. Second broods have become rare in the same areas showing steep declines in flycatchers.
"Mistiming as a result of climate change is probably a widespread phenomenon, and here we provide evidence that it can lead to population declines," the researchers write in a paper presenting the findings in today's Nature. The natural clock that has governed the migration of these birds for centuries now runs a little too fast for the flycatchers to catch up.