Most of the many millions of songbirds that migrate every year between Europe and Africa fly by night and spend the daytime hours resting and eating. Some species, which normally only fly by night, occasionally fly for over 24 consecutive hours to avoid having to stop in inhospitable locations with insufficient access to food, such as deserts and seas. The great reed warbler, Acrocephalus arundinaceus, is such a species. During its month-long migration, it can fly for up to 34 consecutive hours without landing.

A research team from Lund University in Sweden, University of Copenhagen and the Nature Research Centre in Vilnius has used unique data loggers developed at Lund University to study these long migratory flights almost minute by minute. The researchers used the data loggers, weighing no more than 1.2 grams, to track 14 great reed warblers which were captured during the summer at Lake Kvismaren, close to Örebro in Central Sweden.

The data loggers collected data over a full year, thereby providing the researchers with valuable information. As the great reed warblers migrated to and from Africa, the data loggers continuously stored information about altitude, activity and the sun’s ascend and descend. The altitude measurements were recorded once per hour and every five minutes the data loggers saved information about whether the birds were flying, looking for food on the ground or resting. 

Based on this information, the researchers calculated that the great reed warblers flew at an average altitude of 2400 metres at night. On the few occasions on which the birds flew for more than one consecutive night, they climbed 3000 metres at dawn to fly at an average altitude of 5400 metres during the day. They remained cruising at that altitude until dusk, when they dived 3-4000 metres and continued their flight at an altitude of around 2000 metres the following night.

Sissel Sjöberg and Dennis Hasselquist at Lund University led the study, and the results came as a surprise to them and their colleagues. They describe the great reed warblers’ behaviour as very consistent, indicating that the birds may have to fly at more than twice their night-time altitude to manage migrating during daylight hours at all. 

There is no definitive explanation as yet. However, the research team rules out the previously predominant explanations for choice of altitudes in migratory birds: winds and air temperature.

“At altitudes over 2000 metres, winds and air temperatures do not change merely going from day to night or vice versa”, says Sissel Sjöberg.

Nor are there any high mountains along the migration route that could push the birds up to extreme altitudes. Two possible explanations could be that during daytime, the great reed warblers see further from a great height and that they can avoid birds of prey. But the researchers also suggest another explanation: the birds climb to more than double the altitude during the day to avoid becoming overheated when they are exposed to the additional effect of the sun’s heat radiation.

“It is colder higher up in the air and when the birds rise to 5400 metres, they reach a layer of air with a temperature of around minus 9 degrees centigrade. That is 22 degrees colder than the altitude at which the birds fly during the night”, says Sissel Sjöberg.

“The wings of migratory songbirds beat several times per second, so they are working extremely hard, which makes their bodies very warm regardless of whether they are flying by day or by night. But if they fly by day, they are also exposed to the heat from the sun’s radiation; that is what we suggest they are compensating for when they climb to a much colder layer of air at daytime. It is likely that they would not manage to fly by day without becoming overheated if they did not climb to these extreme altitudes”, says Dennis Hasselquist.

The researchers are continuing to study the great reed warblers. In time, they hope to find answers to the question of why the birds behave in this way, which might also help to explain why most birds that migrate to the tropics during the winter fly almost entirely by night, whereas they are active during the day and sleep at night for the rest of the year.