“Stop signs” for migrating songbird: variations in Earth’s magnetic inclination

Informing how birds know when and where to stop migrating, researchers using nearly a century’s worth of data report that the Eurasian reed warbler – a songbird that migrates between sub-Saharan Africa and areas throughout Europe each year – uses slight variations in Earth’s magnetic field as a kind of “stop sign” that signals when it’s arrived at its destination. Migratory songbirds set off on long journeys to return to their breeding grounds – trips that can occasionally span continents – and arrive each year with remarkable precision. Yet, while a great deal of research has focused on understanding how these creatures learn migratory routes and navigate them, it has remained a mystery how they know just where and when to stop migrating. It’s thought that birds use cues derived from parameters in Earth’s magnetic field – magnetic declination, inclination, intensity, and overall strength for a particular area – to guide their arrival. However, Earth’s magnetic field slightly shifts year over year, suggesting that the magnetic parameters used to define an individual’s natal and breeding site will occur in a slightly different location each year. Despite this, bird populations are still often able to return to within meters of their natal sites each and every year. To investigate whether fluctuations in Earth’s magnetic field can predict variation in the sites to which birds migrate, Joe Wynn and colleagues evaluated more than 80 years of ringing records for Eurasian reed warblers. The findings suggest that birds rely on magnetic inclination, or the specific dip angle between Earth’s magnetic field and Earth’s surface, as a “stop sign,” when relocating their breeding site. According to the authors, birds learn the inclination angle before departing these sites, which is subsequently used as a uni-coordinate signal that they’ve arrived upon return. Although several locations on Earth’s surface can have the same inclination, Wynn et al. show how birds solve this by stopping at the first place where the correct inclination is encountered on their inherited flight trajectory.

For reporters interested in trends, a May 2021 study in Science reported that when crossing the Mediterranean Sea and the Sahara Desert, great reed warblers that otherwise conduct their migratory flights strictly at night continue flying by day; but they climb to previously unknown daytime cruising altitudes – from roughly 2,000 meters to well above 5,000 meters above the Earth. These findings revealed a previously unrecognized migratory behavior in birds.