As summer turns to fall, many European birds begin to prepare for their long migratory flights south. But some have a longer journey than others. A few of the most impressive migrators—the Common Cuckoo, the Red-backed Shrike, and the Thrush Nightingale—begin in Northern Europe and fly to Sub-Saharan Africa, where they'll spend their winters foraging on bugs and seeds before returning in the spring.
Researchers have always inferred migration patterns based on where birds wind up, but identifying the motivations that underlie migrating birds' decisions has proven difficult. Since biologists have historically been unable to track individual animals, it's been hard to say why they choose to stop in any one spot over another. As such, scientists have long suspected that some birds "surf the green wave," following the growth of new plant life as they move about. But proving this idea has been difficult. In recent years, though, innovations in tracking technology have allowed researchers such as Kasper Thorup of the Natural History Museum of Denmark to see whether these hunches are correct. As it turns out, evidence suggests they are.
To better understand why birds migrate the way they do, Thorup and his colleague tracked 38 cuckoos, shrikes, and nightingales from Southern Scandinavia to their destinations throughout Africa. On the eight cuckoos, they used radio telemetry tags, which could be tracked by orbiting satellites. And for the 18 shrikes and 12 nightingales, they used loggers that measure light levels. Using recorded information about sunrise and sunset on each day, the researchers could then calculate the bird's latitude and longitude.
These species weren't just important for the study because they migrate long distances—they also migrate alone. Many birds simply follow the leader, but these birds must make movement decisions by themselves. So, by comparing their migration routes with satellite-derived estimates of vegetation, Thorup could discern what features of the landscape the birds were using to guide their movements.
As suspected, he found that during the autumn migration birds weren't simply sticking to a certain preferred climate. The birds selected their wintering sites based on local resources, and when those conditions changed, they found new foraging grounds nearby. But the birds used different strategies to do it. The results were published recently in the journal Science Advances.
According to the study, the cuckoos preferred wintering in places that had high vegetation density, or "greenness," all year long. Shrikes and nightingales wintered instead in spots with higher-than-average greenness only while they were present. Unlike the Cuckoos, they migrated to places with vegetation density that changed seasonally. The researchers say that the two strategies could be related to the birds' foraging preferences: Cuckoos eat insect larvae while nightingales gobble up smaller arthropods and shrikes prefer feasting on adult insects. Each of these invertebrates might be more likely to occur in areas of low or high vegetation seasonality, respectively.
As some of the most mobile animals on the planet, birds have different habitat requirements than terrestrial migrators. Wildlife corridors—tracts of land connecting habitats separated by human activity—are typically considered the domain of earthbound creatures. But birds require connectivity, too. It's just that their corridors might allow for patches of habitat separated by hundreds of miles.
A better understanding of such migratory preferences could help scientists and conservationists predict how birds will be affected by climate change. As Sub-Saharan Africa adjusts to warmer temperatures, these avian migrators may have trouble locating suitable sites for overwintering. "It's likely that climate change will mean that the distribution of seasonal resources will change, and there is a chance that this will disrupt bird migration," Thorup says. "In 80 or 100 years' time, things will look different from how they look now.”
Whether the birds would be able to react to these changes and choose new stopover or wintering sites is not known. Biologist Barbara Frei, a postdoc at the University of Ottawa, says that it isn't clear from the data whether the birds are actively selecting these sites or they simply have an innate preference for a certain sort of vegetation density. "We cannot say that the birds are selecting sites on the basis of their greenness, simply that there is a significant correlation between site greenness and the individuals' presence at specific times," she says. If the birds' innate migration decision-making strategies are flexible enough, however, then perhaps they could revise their travel routes should they ever need to.
In the meantime, Frei is excited by Thorup's research for other reasons. She'd like to see his methodology used for pinpointing areas where bird migration sites do not currently coincide with optimal habitats. "Are these ecological traps where birds simply must return, despite being unsuitable?" she says. For example, waterfowl stop each year in Central California to rest and refuel on their long journeys, but agricultural fields have replaced the wetlands they prefer. By finding these mismatched landscapes, ecologists could identify and restore critical sites for migratory birds as they make their way across the country or world.