Cranes soar on thermal updrafts behind cold fronts as they migrate across the sea

Thermal soaring conditions above the sea have long been assumed absent or
too weak for terrestrial migrating birds, forcing large obligate soarers
to take long detours and avoid sea crossing, and facultative soarers to
cross exclusively by costly flapping flight. Thus, while atmospheric
convection does develop at sea and is utilized by some seabirds, it has
been largely ignored in avian migration research. Here we provide direct
evidence for routine thermal soaring over open sea in the common crane,
the heaviest facultative soarer known among terrestrial migrating birds.
Using high-resolution biologging from 44 cranes tracked across their
transcontinental migration over 4 years, we show that soaring
characteristics and performance were no different over sea than over land
in mid-latitudes. Sea-soaring occurred predominantly in autumn when large
water-air temperature difference followed mid-latitude cyclones. Our
findings challenge a fundamental paradigm in avian migration research and
suggest that large soaring migrants avoid sea crossing not due to absence
or weakness of thermals but due to their uncertainty and the costs of
prolonged flapping. Marine cold air outbreaks, imperative to the global
energy budget and climate system, may also be important for bird
migration, calling for more multidisciplinary research across biological
and atmospheric sciences.