It is a well-known fact that several migratory land bird species take on a tremendous journey between breeding and wintering grounds when the seasons change. But how do they cover these vast distances spanning hundreds of kilometers without taking a single break?
Scientists at the German Max-Planck-Gesellschaft discovered how they do it with the most comprehensive research of sea-crossing behavior to date.
Until now scientists were unsure on how land birds crossed vast bodies of water - (Image Credit: Favious via Shutterstock / HDR tune by Universal-Sci)
Flying over open water can be hazardous to terrestrial birds (a.k.a. land birds). Land birds, unlike seabirds, cannot rest or eat on the water; thus, sea crossings must be done as uninterrupted flights. For decades, bird-watchers thought that big land birds could only make short sea crossings of fewer than 100 kilometers and avoided flying over open water.
Nonetheless, modern advancements in GPS monitoring technology have called that assumption into question. Researchers tracked the global migration of five big terrestrial birds (a.k.a. land birds) that make long maritime crossings with help from GPS technology. The results revealed that many land birds routinely fly hundreds or even thousands of kilometers over open seas and oceans as part of their migration.
This puzzled researchers as flapping wings is an energy-intensive action, and large, heavy terrestrial birds would be unable to sustain uninterrupted flapping flight for hundreds of kilometers.
According to past studies, birds may be able to sustain such long flights employing tailwind to their advantage. Doing this right will save them a lot of energy. Recent research determined that a single species, the osprey, used rising air thermals known as "uplift" to soar over the open sea.
An Amur Falcon (one of the birds that was studies) resting on a pole. When crossing vast distances over the sea no such resting spots present themselves - (Image Credit: Unnikrishnan Anilkumar via Shutterstock / HDR tune by Universal-Sci)
In a new study, scientists studied the ocean- and sea-crossing behavior of 65 birds from 5 different species to get the most comprehensive understanding of how land birds survive long flights over open water.
The team examined 112 sea-crossing tracks gathered over nine years of research to identify the criteria that birds use to choose their migratory paths over the open sea. To make this study feasible, scientists over around the entire world donated their tracking data.
The collected data not only confirmed the significance of tailwind in enabling sea-crossing behavior but also showed that uplift is often utilized to save energy during these uninterrupted trips. Suitable uplift means less drag, making sea crossing less energetically demanding.
Dr. Elham Nourani, first author of the study, stated that uplift was assumed to be feeble or even absent over the open sea. She and her team now showed that this is not the case. Instead, the researchers found that migratory birds change their flight routes to profit from the best uplift and wind conditions when they cross the sea. With help from these winds, they are able to sustain hundreds of kilometers of flight.
Autumn migration trajectories - Image Credit: Elham Nourani / Max Planck Institute of Animal Behavior (Click on image to enlarge)
During its yearly migration from Japan to Southeast Asia, the oriental honey buzzard, for example, travels 700 kilometers over the East China Sea. The almost 18-hour uninterrupted sea trip takes place in fall when airflow conditions are ideal. According to Nourani, these birds can soar as high as 1 kilometer above the sea surface using uplift.
Because the climate is changing worldwide, scientists aim to devote future research to the potential impacts of climate change on migratory land birds.
Further reading:
Why do birds sing? (Universal-Sci)
Want to save millions of migratory birds? Turn off your outdoor lights in spring and fall (Universal-Sci)
The interplay of wind and uplift facilitates over-water flight in facultative soaring birds (Proceedings of the Royal Society B Biological Sciences)
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