Many of us have seen large flocks of birds such as geese flying overhead and wondered, where are they going? When migrating, many birds travel incredibly long distances. Birds use one of two strategies to make these journeys: flapping flight and thermal soaring. Thermal soaring involves riding rising air currents (thermals) up and then gliding down, meaning that the birds don't have to flap their wings, thus saving energy. When it comes to migratory birds, there are two distinct categories. Firstly, there are terrestrial migratory birds, such as cranes, and seabirds, such as gulls. Secondly, there are birds that only use thermal soaring while migrating and those that switch between flapping and soaring. It was thought that the thermals required for soaring fight were not available over the sea, so that birds either avoided travelling across water or resorted to flapping as they flew over. This was based on the observation that many migratory birds seem to avoid the ocean on their routes. However, a global team of researchers from Israel, the USA, Russia and South Africa recently showed that this is not the case for the common crane (Grus grus), suggesting that many birds may be able to take advantage of thermal soaring while crossing the sea.
Sasha Pekarsky and colleagues decided to monitor the migration strategies of the common crane, as they are the heaviest bird that uses both energy-saving soaring and expensive flapping flight during migration. To test their idea that birds may be able to use soaring flight when crossing the sea, the researchers fitted 44 common cranes in Western Russia with GPS dataloggers, which allowed them to determine each bird's location and altitude, and whether it was flapping or gliding at any individual moment. Between 2018 and 2021, the researchers collected over 1500 hours of observations during the cranes’ migratory flights between western Russia and Africa, and back, crossing both land and sea (Black or Mediterranean) during the spring and autumn migrations.
Reviewing the measurements collected during the crane's lengthy flights, the team realised that the birds use a combination of thermal soaring and flapping flight over both land and sea. The amount of time that the cranes spent soaring varied depending on which region the birds were in, soaring on rising thermals most over the desert and least over the sea. The season also affected how much time the birds spent soaring on rising thermals, with the birds soaring more over the sea in the autumn. Additionally, the amount of energy saved when soaring was the same whether the birds were crossing over land or sea. The team also realised that the birds switched from flapping to soaring flight over the sea when they could ride thermal updrafts generated in the wake of a cold front following a low-pressure system passing over the water.
Considering the pressure systems that allow cranes to benefit from thermal soaring, it now looks as though the apparent lack of migration routes over the sea has more to do with fewer low-pressure systems, and cold fronts, crossing over seas rather than a complete lack of rising air currents, which are essential for energy saving thermal soaring. Thanks to these observations, we now have a better understanding of the impact of environmental and atmospheric conditions on the migration strategies of birds. These findings are important for the field of avian migration as it shows that other birds may also fly over the seas and oceans. The results not only provide a novel view of avian migration but also position birds as sensitive indicators of meteorological events such as cold fronts.
