Manx Shearwaters' Sun Compass Skewed by Internal Clock Drift
A Science study of 3,688 GPS-tracked foraging trips finds that Manx shearwaters' internal clocks drift during long east-west flights, bending their Sun compass.
The story in focus
A new Science study led by Patrick Lewin at the University of Oxford analyzed two decades of GPS tracks from Manx shearwaters and found that the birds' internal clocks fall out of sync during long east-west flights, skewing their Sun compass. Errors reached about 15 degrees on homing flights.
- The study analyzed 3,688 foraging trips by 1,045 Manx shearwaters tracked with GPS tags since 2004.
- On 335 homing flights out of sight of land, the birds veered in directions consistent with Sun compass predictions.
- Navigational errors reached up to about 15 degrees; a 1,000-kilometer eastward flight produced an average error of roughly seven degrees.
- Crosswinds, the Coriolis effect and magnetic cues fit the observed data poorly, according to the team's calculations.
- Lewin cautioned that the non-experimental design limits causal claims.
Two decades of tracking data
Manx shearwaters nest in burrows on coastal islands around the North Atlantic, mostly around the UK and Ireland, and fly dozens to more than 1,000 kilometers into the Atlantic to feed. To return, they are thought to use a map sense, possibly based on smells, and the Sun as a compass.
Because the Sun moves across the sky, Sun navigation requires an internal clock. The birds recalibrate that clock at every dusk and dawn, so each degree of longitude covered between sunrise and sunset leaves it four minutes out of sync with local time. Researchers led by Patrick Lewin at the University of Oxford drew on GPS tracks from 3,688 foraging trips made by 1,045 birds since 2004.
They separated homing flights, when birds fly fast and straight, from short, wiggly foraging flights. For each homing flight, they calculated how far east or west the bird had traveled since its last sunrise and predicted the resulting compass bias, then compared those predictions with the birds' actual deviations.
Compass errors and alternatives
Across 335 homing flights made out of sight of land, the birds veered as the Sun compass predictions indicated. Observed time shifts reached about an hour at most, with most around 10 to 15 minutes. Navigational errors reached up to about 15 degrees; for a shearwater flying 1,000 kilometers east in a day, the average error was roughly seven degrees, adding about five kilometers to the trip if the bird keeps checking its position.
The team calculated biases that crosswinds, the Coriolis effect or a magnetic compass might introduce, and none fit the data well. Because the study was not a controlled experiment, Lewin argued that causality requires caution.
Next questions
The study could not determine whether the birds experience other jet lag symptoms such as poor sleep or hunger at odd hours. Lewin said the next step is to find out whether the time shifts affect when birds forage, how much food they bring home, and their breeding success. The team also raised the question of whether the effect occurs in other animals and whether species that routinely experience jet lag have evolved ways to cope.
How large were the shearwaters' navigational errors?
Errors reached up to about 15 degrees. For a bird flying 1,000 kilometers east in a day, the average error was roughly seven degrees, adding about five kilometers to the trip if it keeps checking its position.
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