Some birds follow a built-in migration schedule across continents

Red-backed shrikes, small songbirds in northern Europe, show that birds migrate with remarkable precision. Each year, these birds travel about 12,400 miles between Scandinavia and southern Africa.

Instead of wandering across continents, shrikes follow consistent routes divided into repeated flight segments and carefully timed rest stops.

A new study shows that this schedule likely comes from a sophisticated genetic program rather than on-the-spot decisions.

Tracking birds across the year

The work was led by Sissel Sjöberg, an evolutionary ecologist at Lund University (LU) in Sweden. Her research focuses on how internal migration programs interact with experience to shape birds’ routes and schedules across years.

Sjöberg and colleagues attached multi-sensor data loggers weighing about one gram to 15 red-backed shrikes breeding in Denmark.

The tags are used for biologging, a method that uses tiny sensors placed on birds to record behavior and reconstruct each night’s flight.

“We can now follow a bird’s location throughout the year,” said Sjöberg.

Bird migration in stages

Across a full year, the accelerometer data revealed six clear flight segments separated by longer stopovers at key regions.

During spring, the birds spent about 270 hours spread over 43 nightly flights to cover roughly 6,835 miles back to Europe.

In autumn, three segments carried them from Denmark through southeastern Europe, across the Mediterranean and Sahara, to Sahel and southern Africa.

Despite individuals flying alone, total spring flying time differed by only about six percent among birds, demonstrating how consistent the schedule remained.

Advanced genetic migration program

Researchers see the detailed schedule as evidence that shrikes rely on a built-in circannual clock that governs their year-long migration.

In many bird species, research shows that such year-long clocks govern migration, molt, and reproduction.

Comparing earlier cage experiments with these wild tracks, the team saw real flights clustered into repeatable travel segments across the year.

“Their genetic migration program may be considerably more advanced than we previously thought,” said Sjöberg, noting how detailed the inherited plan appears.

Internal timing of bird migration

The shrikes studied were adults returning to their breeding territories, so their internal programs had been shaped by earlier journeys.

Previous tracking studies on the same species show that individuals repeat their departure and arrival dates with striking consistency between years.

Young shrikes probably begin life guided almost entirely by this genetic script, and then later layer map knowledge from real landscapes onto that template.

Understanding how much migration depends on flexible behavior versus an inherited rulebook is key to predicting whether populations can adapt to rapid environmental change.

A schedule with a strong structure

Although lab birds often show nightly restlessness during migration seasons, the wild shrikes spent most nights resting or foraging instead of flying.

So-called odd flights, short nocturnal trips taken during long stopovers or winter, made up about three percent of total annual flying time.

These rare excursions likely reflect local housekeeping, such as adjusting territories or tracking temporary food shortages, rather than changes in the migratory plan.

When the researchers set those odd nights aside, the remaining flights form a strongly structured schedule whose timing and duration match closely between individuals.

Earth’s magnetic field 

Many migratory birds appear to sense the geomagnetic, Earth’s magnetic-field-based environmental pattern, giving them information about both direction and position.

Experiments with Eurasian reed warblers in Russia showed that cutting a key nerve prevented birds from noticing a virtual 620-mile displacement.

Shrikes probably combine this map sense with their internal schedule – responding to particular magnetic signatures by starting, ending, or intensifying successive flight segments.

Other cues such as star patterns, odors, and landmarks likely work alongside magnetic information, giving experienced birds multiple ways to locate each segment.

Why this precision matters 

Shrikes depend on reaching good stopover and wintering sites when insects are abundant, so precise timing can boost survival and breeding success.

Earlier annual cycle work on this shrike population showed intense daytime activity during the breeding season, when parents must feed chicks.

If birds arrive late to these seasonal peaks, they may find fewer insects, weaker body condition, and less time to raise young.

The tiny variation the team measured between individuals suggests that natural selection has already fine tuned this timing very tightly.

New tools are reshaping research

Before miniaturized devices, scientists relied mainly on bird band recoveries and a few radar studies, which offered only rough outlines of migration.

The shrike loggers sampled acceleration every five minutes through the year, storing summaries that allowed researchers to identify each sustained flight reliably.

“It is an impressive achievement for a little bird,” said Sjöberg. Shrikes weigh only between 25 and 30 grams.

As devices get lighter and more capable, scientists expect to expand tracking to additional species, longer journeys, and bigger samples.

Data that is missing

Because the tags must be retrieved the following breeding season, the data come from a relatively small set of returning birds.

First-year birds that never return to their birth site remain invisible to this method, leaving the structure of their inaugural journeys unknown.

Because the tracks include only birds that survived, deadly events may be missing from the graphical summary of activity from logger readings.

Even a small backpack and harness could alter flight effort or predation risk, so ongoing studies check for subtle tag effects on behavior.

Wider patterns of bird migration

Red-backed shrikes are not unusual in migrating between Europe and Africa, so their internal scheduling may resemble that of many other long-distance songbirds.

Similar tracking of other species suggests that some migrants follow surprisingly narrow corridors and repeat much of their annual schedule.

If genetic programs fix migration timing strongly, rapid environmental changes could leave birds following schedules that no longer match food or weather conditions.

Detailed flight records from individual shrikes give ecologists the data needed to model how migratory populations might respond to future landscapes and climates.

The study is published in the journal Proceedings of the Royal Society B Biological Sciences.

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