Manx shearwaters, small seabirds that cross the Atlantic in search of food, can suffer a form of jet lag that pulls them off course. Researchers at the University of Oxford found that the birds' internal body clocks can drift more than an hour away from local time on fast flights, causing navigational errors on the way back to their nests.
Patrick Lewin, who led the work, said the same basic process drives jet lag in people. The team has avoided the term itself because people associate it with health effects, but the underlying mechanism is the clock shift. "It's having these physiological and behavioural effects, where the birds are mistiming their behaviours," Lewin said. He added that other long-distance travellers, possibly including some insects and bats as well as birds, are likely to experience the same clock-shift effect.
Manx shearwaters breed on islands around the UK and Ireland. While parents are raising a chick, they may fly more than 1,000 kilometres west to reach rich fishing grounds in the mid-Atlantic. Once they have food, they fly back in a fairly straight line, diverting only to skirt around landmasses such as Ireland. Some birds can sense Earth's magnetic field, but the shearwaters do not appear to rely on it. Lewin's team attached small magnets to the birds, designed to disrupt geomagnetic navigation, and the magnets had no effect.
Instead, the evidence points to a sun compass. A bird that knows the time can keep a course by adjusting its orientation relative to the sun. Many animals, from insects and fish to turtles and birds, use this method, and earlier experiments showed that when their internal clocks are reset with artificial light and darkness, they head in the wrong direction. The researchers saw a natural version of that experiment in the shearwaters' own flights.
The logic works like this. A shearwater flying east needs to adjust its heading as the sun moves. If it travels 15 degrees of longitude eastward, local time advances by an hour, but its internal clock still reads the earlier time. The sun therefore no longer sits where the bird expects it to be. Birds flying rapidly east should veer slightly clockwise over the course of a day, in the absence of other cues.
To test this, the team analysed GPS tracks from nearly 4,000 foraging trips recorded over decades by other researchers. When Lewin and his colleagues examined the flight paths, they found that birds on rapid return flights tended to deviate clockwise once they were out of sight of land. The team considered other explanations, including the Coriolis effect and winds, but "the one thing that best predicted the observed changes in shearwaters' orientation was always this sun compass effect," Lewin said.
The effect is small. After a 1,000-kilometre flight, a bird would need to fly only about 5 extra kilometres to reach its colony, which Lewin said is probably negligible for such efficient long-distance fliers. That may explain why the shearwaters have not evolved a way to avoid jet lag. Ken Cheng of Macquarie University in Sydney, who was not involved in the study, praised its robust dataset, noting that analysing many tracks allowed the researchers to test different models.
The findings may matter more for animals near the poles. Long-distance bird flights are typically migrations that run mainly north to south, so jet lag is unlikely to be a major problem for most species. Near the poles, however, the distance needed to cross 15 degrees of longitude becomes very short, and many animals could be affected, perhaps including polar bears or Arctic foxes travelling on sea ice. Lewin noted that in parts of the year, near-polar regions have 24-hour daylight or darkness, which can break down circadian rhythms in some species. The study was published in the journal Science.
27
