Stop-start, great timing: how birds chase an eternal spring

RESEARCHERS who tracked more than 3 dozen birds with sensors over thousands of miles have reported that instead of simply flying straight from their summer grounds in Denmarktotheirwintersiteinafrica,thebirds stretchedouttheirjourney,stoppingatseveral places along the way for weeks at a time.

The journey was exquisitely well timed to coincide with high levels of vegetation at each site. These habits, honed by thousands of years of evolution, probably helped them enjoy a good diet of insects on their trip. This may well be a common strategy among the world’s migratory birds, the researchers said, adding, however, that it may be under threat from climate change.

Traditionally, ornithologists caught a bird at its summer breeding grounds, put an identifying band on its leg, and then waited for someone to spot it wherever it ended up for thewinter.suchstudiessaidlittleaboutwhere the birds went between points A and B.

For the research published in Science Advances last week, Dr Kasper Thorup, a bird migrationexpertattheuniversityofcopenhagen andhiscolleaguesputlightweighttrackingdevices on common cuckoos to follow them throughtheirmigrations.asthebirdsmigrated between Denmark and Central Africa, they sentsignalstosatellitesshowingtheirlocation.

The researchers also tracked two smaller migratory species travelling from Denmark to sub-saharan Africa: thrush nightingales and red-backed shrikes. These birds are too small to carry the weight of satellite transmitters, so Dr Thorup and his colleagues fitted them with even tinier devices called geolocators. These sensors record only sunlight levels throughout each day. When the birds returned to Denmark, the scientists recovered the geo-locators and used each day’s data to plot the birds’ routes.

Studying38birdsinall,thescientistsfound they didn’t move directly from their summer grounds to their winter grounds. Instead, they flew for a few days, stopped somewhere for a few weeks, and then moved on again.

Red-backed shrikes, for example, leave Denmark and reach southeast Europe in August. In October, they go to East Africa. By December, they’re in southern Africa. And in April, they’re back in East Africa again.

Throughout their journey, the scientists found, the three species timed their flights so that they reached feeding grounds once they were abundant with vegetation. Dr Thorup suspects that the birds are able to make meals of local insect larvae feeding on the plants.

The new data show that even though the birds ended up in the same places in Africa, they sometimes followed different routes. It’s possible that birds somehow combine short-range flexibility with a navigation system, hard-wired into the brain, that guides them to the places where they can find the most food to eat.

But that strategy works well only when birds can be sure to find food at the same place at the same time each year. And climate change is altering the calculus.

In Northern Europe and North America, plants are greening up earlier in the spring. In Africa, rainfall patterns are shifting, changing the times at which plants put on new leaves and fruit. The researchers compared the migration routes of the birds to computer projections of how ecosystems will change in response to global warming over the next few decades. They concluded that climate change may make migrations much harder on the birds. CARL ZIMMER, THE NYT

Per Ekberg, via The New York Times Red-backed shrikes (above) and thrush nightingales were fitted with tiny geolocators that recorded sunlight levels, to track their routes as they migrated.