Study: Siberian Willow Warblers Migrate Incredible 8,000 Miles One Way

A small songbird called the Siberian willow warbler (Phylloscopus trochilus yakutensis) holds a long-distance migration record in the 10-gram weight category — with the tiny birds flying around 8,000 miles (13,000 km) or longer to reach their destination.

The Siberian willow warbler (Phylloscopus trochilus yakutensis). Image credit: Kristaps Sokolovskis.

The willow warbler (Phylloscopus trochilus) is a medium-sized, slender leaf-warbler between 11 and 12.5 cm long and weighing 7-15 g.

The bird is a migrant inhabiting extensive parts of northern Europe and Asia for breeding and wintering in sub-Saharan Africa.

“Willow warblers use a variety of forested habitats for breeding and may occur in high numbers in parts of their breeding range, being the most numerous breeding bird in e.g. Sweden,” said Lund University’s Professor Susanne Åkesson and colleagues.

“Three subspecies occur across its range: Phylloscopus trochilus trochilus and P. t. acredula meeting in a hybrid zone in central Sweden, and P.t. yakutensis occurring farther to the east in Siberia.”

“P.t. trochilus and acredula are known to have different migration phenotypes, route choices and winter destinations in west and southeast Africa, respectively, while much less is known about the route choice and winter destinations of P.t. yakutensis.”

Simulated autumn migration routes of willow warblers using alternative compass mechanisms and aiming to the wintering ground (first winter stop) through: (a) direct route or (b) an intermediate goal area (stopover region). In panel (A) only the Sun compass route (b) crosses the identified stopover region (filled black triangles) and neither of the two possible solutions for the magnetoclinic route (d and d*) could reach the wintering area. In panel (B) all routes have a solution that brings birds to the stopover region before switching the compass direction toward the wintering ground (black circle 1). However, for the magnetoclinic route the switch must occur earlier and at higher latitudes (between black circle 2 and 3) than for the other compass mechanisms (black circle 1). (c) a sun compass can produce alternative routes when the bird’s internal clock partially adapts to the local time (i.e. partial longitudinal time-shift), but an intermediate goal area would still be necessary to cross the stopover region. (d) alternative magnetoclinic routes can be generated from locations with alternative geomagnetic inclination values (i.e. different starting points) for which a change in compass on route must always occur not earlier than 85-95 degrees E longitude. For all the panels, departure location (filled black circle) is the breeding area where birds where tagged. GLS data for the three birds tracked in this study are also reported. Image credit: Sokolovskis et al, doi: 10.1186/s40462-018-0138-0.

Professor Åkesson’s team used geolocators to track the migration of Siberian willow warblers from their eastern part of the range in Siberia to wintering areas in sub-Saharan Africa.

From their breeding habitats in eastern Siberia, Siberian willow warblers fly to staging areas located in south-west Asia and the eastern Mediterranean.

From there, they continue to their winter destinations, located in Kenya and Tanzania — a distance of around 8,000 miles.

“I think it’s fascinating — they are so small and migrate at least 8,000 miles one way,” Professor Åkesson said.

“There are no other studies that show that birds of that size can migrate that far. Even more impressive perhaps is that they make the journey alone in their first year of life.”

In addition to the record for long-haul migration, the study is also the first to show where Siberian willow warblers spend the winter in east Africa.

Moreover, the researchers compared alternative compass routes with the routes the birds actually take.

Based on this, they identified two alternative mechanisms that the willow warblers can use during their long migration — a solar compass that compensates for time shifts during the migration, and a magnetic compass based on the assumption that the birds can measure the inclination angle of the Earth’s magnetic field.

The findings appear in the journal Movement Ecology.

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Kristaps Sokolovskis et al. 2018. Ten grams and 13,000 km on the wing – route choice in willow warblers Phylloscopus trochilus yakutensis migrating from Far East to East Africa. Movement Ecology 6: 20; doi: 10.1186/s40462-018-0138-0