Every autumn, billions of songbirds abandon their breeding grounds and strike out for wintering territory thousands of kilometers away — and somehow, individuals from the same breeding population tend to end up clustered together when they get there. Birds that hatch in the Netherlands keep running into other Dutch-bred birds in Africa each winter, while Spanish-bred birds settle into an entirely different patch of the continent. Exactly how a bird that migrates alone, at night, with no one to follow, manages to find that specific destination has puzzled biologists for decades. A new study in Science, led by researchers at the University of Groningen, finally supplies part of the answer: it’s both nature and nurture, working together.
The pied flycatcher, the small songbird at the center of the study, weighs about as much as a few sheets of paper — roughly 12 grams — yet travels between 3,000 and 13,000 kilometers each way to reach its wintering grounds in west Africa. A large European research team, coordinated by PhD student Koosje Lamers and fellow researcher Janne Ouwehand at the University of Groningen under the supervision of Christiaan Both, fitted flycatchers from eight breeding locations spanning the species’ range, from Spain to Siberia, with small dataloggers to track their migration routes in detail.
The tracking revealed a route shared by every population, regardless of where the birds started: all of them first flew to Spain and Portugal, paused there, and then launched into a roughly 40-hour, nonstop flight across the Atlantic to reach the westernmost edge of Africa. Only after that shared Atlantic crossing did the routes diverge, bending eastward by different amounts depending on a bird’s origin. Spanish-bred flycatchers stopped relatively close to where they crossed, settling in the most western wintering grounds; birds from Siberia continued much farther east, eventually reaching Nigeria. The distances involved were strikingly lopsided: Spanish breeders flew only about 3,000 kilometers in total, while Siberian breeders covered nearly 13,000 kilometers, almost entirely because of the long detour through Spain and Portugal before heading south and east.
“It is remarkable that these pied flycatchers from Siberia take such a detour,” said Lamers. “A less western route, for instance, crossing the Mediterranean Sea near Italy and then crossing the Sahara, would save them some 4,500 kilometers.” That shorter path isn’t hypothetical — the collared flycatcher, a close relative, already uses it to travel from Central Europe to Africa. Lamers suspects the pied flycatcher’s roundabout route is a holdover from a much older migration pattern. “So it is plausible that this strange detour is an evolutionary remnant from the past, when during ice ages, the pied flycatchers only appeared in the western part of Africa and Europe.”
The more pointed question — whether a bird’s wintering destination is hardwired by genetics or shaped by where and how it grows up — required something beyond tracking data. The team carried out a cross-fostering experiment, removing eggs from Dutch nests and having Swedish foster parents incubate and raise the resulting chicks in southern Sweden. They also relocated some female Dutch birds to Sweden directly, producing offspring that were half Dutch, half Swedish by parentage. Tracking all three groups — Dutch-raised Dutch birds, Swedish-raised Dutch-egg birds, and the mixed-parentage offspring — let the researchers tease apart genetic inheritance from the influence of where a bird is actually raised.
The results pointed to both factors mattering. Untranslocated Dutch flycatchers wintered around 500 kilometers farther east in west Africa than Swedish birds did. Dutch-egg birds raised by Swedish foster parents ended up wintering roughly halfway between the typical Dutch and Swedish locations — evidence that upbringing nudges the outcome even when genetics doesn’t change. Meanwhile, the mixed-parentage offspring, who were genetically half Swedish, settled slightly closer to the standard Swedish wintering grounds than the cross-fostered Dutch-egg birds did, suggesting genetic inheritance also plays an independent role. “So, it is probably not the case that the direction of the migration is inherited, and differs per location,” Lamers said. “Instead, it is probably the length of the route that is fixed.” In other words, every population may be following the same general southwesterly path — through Spain, across the Atlantic, then east — with genetics and environment together fine-tuning how far along that shared path a given bird ultimately settles.
The study also ruled out one tidy explanation: that young flycatchers simply learn the route by watching their parents. That can’t be the case here, because juveniles set out on their first migration well after their parents have already left, with no opportunity to follow them.
The findings carry a practical edge beyond resolving a long-standing question in animal behavior. As climate change shifts the timing of bird migration earlier or later, a population’s ability to adjust may depend partly on where in Africa it happens to spend the winter, since conditions there affect when birds are ready to head north again. And because the study showed that new combinations of breeding ground and wintering location can emerge — as demonstrated by the cross-fostered Dutch eggs that ended up wintering somewhere their genetic relatives never had — it raises the possibility that migratory routes themselves have more flexibility, generation to generation, than previously assumed.
The new findings extend a line of research stretching back to classic cross-breeding experiments with another European songbird, the blackcap, which first demonstrated decades ago that migratory direction and distance can be inherited and that hybrid offspring display intermediate migratory behavior. The Groningen-led study adds environment to that genetic picture, and does so in a species that, unlike the blackcap, makes one of the longer and more demanding transoceanic journeys in the songbird world.
Endnotes
- Koosje Lamers et al., “Innate factors and ontogeny determine non-breeding areas of migrant songbirds,” Science, June 25, 2026, DOI: 10.1126/science.ads0532.
- University of Groningen, “Migratory birds find their wintering spot in Africa thanks to an interplay between genes and environment,” news release via EurekAlert!, June 25, 2026.
- Peter Berthold and Ulrich Querner, “Genetic Basis of Migratory Behavior in European Warblers,” Science 212, no. 4490 (1981): 77–79 — foundational cross-breeding evidence for the genetic inheritance of migratory direction and distance in the blackcap, Sylvia atricapilla.
- Peter Berthold and Andreas J. Helbig, “The genetics of bird migration: stimulus, timing, and direction,” Ibis 134, no. s1 (1992): 35–40.