Extreme differences in flight altitude between day and night may have been an undetected pattern amongst migratory birds - until now. The observation was made by researchers at Lund University in Sweden in a study of great snipes, where they also measured a new altitude record for migratory birds, irrespective of the species, reaching 8 700 metres.
Great snipes are shorebirds that breed in Sweden, among other places, and spend the winter in areas near the equator in Africa. Previous studies have shown that great snipes make long marathon flights of up to 6 000 kilometres lasting 60-90 hours when they migrate between breeding sites in Sweden and wintering sites close to the equator.
In a new study published in Current Biology, the international research team describe how the great snipes fly at a higher altitude during the day than at night. The difference can be as much as several thousand metres. The birds regularly flew at an altitude of over 6 000 metres during the day, compared to an average altitude of about 2 000 metres at night. One bird even flew at over 8 000 metres for five hours during the autumn migration to Africa, reaching a maximum altitude of 8 700 metres.
"It is the highest flight altitude that has ever been recorded for a migratory bird", says Åke Lindström at the Department of Biology in Lund who led the study.
Researchers used small data loggers developed at Lund University and attached these to the great snipes in order to follow changes in flight altitude during the long flights. The record height of 8 700 metres is astonishing.
However, the researchers are even more fascinated by the pattern among migratory birds that they may have detected. A recently published study on great reed warblers, which was also led by researchers in Lund, found that on the few occasions during the migration when the small passerines prolonged their otherwise nocturnal flights into the day, they flew at much higher flight altitudes during the day than at night. This occurred when great reed warblers crossed inhospitable terrain such as the Sahara Desert and the Mediterranean Sea.
The considerably larger great snipes do the same. However, not only when they fly over so-called ecological barriers such as deserts and seas, but also when they fly over the tropics and over Europe.
"Other species that that make long migratory flights are also likely to use this day-and-night rhythm. We may well be tracking a general pattern, it will be up to future studies to show this", says Åke Lindström.
If it transpires to be a pattern amongst many migratory birds, it would enhance our understanding of which environmental factors are important for migratory birds. This knowledge may in turn bring us closer to explaining the great variation in the behaviour of these birds. Why do some species migrate at night and others during the day? Why do some birds only fly short distances at a time while others, such as great snipes fly for several days in a row?
As yet, no one knows for certain why great snipes and great reed warblers fly at a higher altitude in the day than at night during migration. The research team mention three explanations as the most probable: birds can navigate more easily via landmarks, they avoid birds of prey, the cold temperature at high altitudes helps prevent overheating during strenuous exercise under the blazing sun.
"Our main line of inquiry is that they fly at a high altitude to cool down, but we must be humble and acknowledge that there may be other or additional explanations", concludes Åke Lindström.