The scientific mystery of how songbirds from one population are able to join other members of their own population at the same winter destinations following their long solo migration has been answered.
In a groundbreaking new study published in the journal Science, researchers have found that a combination of genetics and environment determines where migratory birds fly to for the winter.
By attaching tiny data loggers attached as backpacks to pied flycatchers, the team was able to record the birds’ routes over thousands of miles across land and sea.
This revealed that birds in different areas of Europe follow consistent routes to their wintering spots in Africa even when they are not the most direct.
The research was conducted by a team of European scientists led by the University of Groningen, and featuring the University of Exeter.
“Every autumn, billions of migratory birds leave their breeding areas to fly thousands of kilometres to warmer winter locations,” said Dr Malcolm Burgess, of the Centre for Ecology & Conservation at the University of Exeter, Cornwall. “In species such as geese, young birds learn migration routes from their parents, while in some other species they learn from companions with whom they migrate. But for songbirds that migrate at night, it is unknown what causes them to choose a particular place.”
The research team used incredibly light data logger trackers measuring light intensity and time fitted as backpacks to the birds. The birds were tracked from eight areas, ranging between Spain and Siberia – with Dartmoor the focus of the UK work. When the birds were recaptured, the team read the data to estimate the path they took and how long it took them to make their journey.
The results showed that in the autumn, all populations first flew to Spain and Portugal. There they made a longer stopover, before continuing in a non-stop flight of around 40 hours across the Atlantic Ocean to the westernmost tip of Africa.
Their migration routes then bent eastwards, with birds of different populations continuing their journey for varying distances: the Spanish birds settled in the westernmost part of the wintering range, whereas the Siberian birds flew 3,000 kilometres eastwards to spend the winter in Nigeria. While the Spanish breeding birds flew only 3,000 kilometres in autumn, the Siberian birds covered almost 13,000 kilometres because of the long detour they make via Spain and Portugal.
“It is not only remarkable that a small bird weighing just 12 grams covers such distances, but also striking that pied flycatchers from Siberia take such a long detour,” said Dr Burgess. “A more direct, less westerly route, in which birds cross the Mediterranean Sea via Italy and then fly over the Sahara, would save them about 4,500 kilometres and is used by a closely related species, the collared flycatcher.
“We believe that they use this route because it is an evolutionary remnant from the past, when during the last ice ages, pied flycatchers were restricted to western Africa and western Europe.”
To determine how flycatchers know where they should spend the winter in Africa, the researchers moved some of the birds from the Netherlands to southern Sweden. They did this by fostering Dutch eggs to Swedish parents. They also moved adult Dutch females to Sweden and there enabled them to breed with Swedish males, producing half-Dutch, half-Swedish young.
“It was a logistical nightmare,” said Koosje Lamers, a PhD candidate at the Groningen. “We worked 36 hours straight in the middle of a busy field season: catching birds and collecting eggs in the Netherlands one day, driving them to Sweden during the night in a van, and then setting them up in the Swedish study site the next day.”
Under normal circumstances, Dutch flycatchers were found to winter 500 kilometres further east in West Africa than their Swedish conspecifics. Dutch flycatchers that grew up in Sweden wintered roughly halfway between the normal Dutch and Swedish locations, while the hybrids had wintering areas further towards the normal Swedish ones.
“This study demonstrates that the location where flycatchers spend the winter is partly inherited and partly determined by the environment in which they grow up,” concludes Dr Burgess. “It is also remarkable that these wintering locations are reached via a shared route. What this tells us, for the first time, is that it is not the migration direction from the breeding area that is fixed, but possibly the length of the migration journey.
“The study also shows that this migratory behaviour is not learned from the parents. This knowledge is important for understanding how migratory birds can adapt to climate change. The timing of migration is changing strongly due to climate change, and whether birds can advance their timing is linked to where they spend the winter in Africa.”
Innate factors and ontogeny determine non-breeding areas of migrant songbirds, is published in the lastest edition of Science.