Speed Traps: Using Radar to Study Migrating Birds

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Short-distance migrants (such as the European Robin shown here) and long-distance migrants appear to use different strategies for arriving at and leaving their breeding sites. Image by chasnz.

Radar was initially developed in the mid-20th century for detecting aircraft.

Since then, radar has found use in both military and civilian aviation, as well as with traffic police, and weather weather forecasters.

Now, science is  calling the technology into use for research into bird migration.

New Migratory Bird Research Using Radar

Traditionally, researchers have investigated bird migration using visual observations, trapping and ringing .

More recently, they have also used radio tracking by attaching small transmitters to the birds under study.

Now, scientists from Lund University in Sweden have turned to radar for their research into the nocturnal migration of passerine (perching) birds.

DecodedScience asked Cecilia Nilsson, a PhD student in evolutionary ecology, and a member of the research team, about their methods. She told us, “We use tracking radar to measure flight speeds because it is really the only way to very precisely measure flight speed of free flying birds that are too small to carry transmitters” she said.

Cecilia further explained: “It is a very good technique for measuring individual behavior as you can accurately track birds several kilometers away without having to handle them in any way. It is especially appropriate for the study of nocturnally migrating birds, as they of course are extra difficult to observe at their high altitude flights in the night“.

The Falsterbo peninsula (circled), at the extreme south-west tip of Sweden, is an important stop-off point for migrating birds. Image: courtesy of U.S. Passports and International Travel

The team wanted to discover how migration behavior varied between short-distance migrants (such as the European Robin, Goldcrest and various Thrushes), and long-distance migrants (most Warblers).

They therefore set up a radar installation at the Falsterbo peninsula, on the Southern tip of Sweden.

This is a migration hotspot, as it’s the last land available before the birds have to make a sea crossing to Denmark or Germany.

Identifying Short and Long-Distance Migrants

An experienced radar operator was able to interpret the characteristics of the radar echo, the altitude, airspeed and direction, in order to identify those signals that related to migrants.

So how did they identify what sort of migrants were being tracked? This was made easier due to existing knowledge about their migrating habits.

Ringing records from the Falsterbo site showed that short-distance migrants tended to arrive at their breeding sites in early spring, and did not leave for their wintering sites in Southern Europe until late autumn (fall).

In contrast, long-distance migrants, did not arrive until late spring, and were already leaving by early autumn, for the longer return trip to sub-Saharan Africa.

By continuing their observations through the whole spring and autumn migrations, the team used this difference in timing as a guide to identify whether they were observing short-distance or long-distance migrants.

The Early Bird…

This research confirmed that migrants fly faster in spring than they do in the autumn. Researchers think that the first to arrive in the spring will have the better choice of nest sites, territories, mates and other resources. This is like being at the front of the line at the January sales.

The team also found that this difference in speed between spring and autumn is more pronounced in short-distance migrants that those flying longer distances. The resulting report suggested that this was due to the short-distance migrants being more ‘relaxed’  about their autumn flight.

Since they had less far to go to their wintering grounds, the team thought that they could be flying more slowly in order to conserve energy.   The research team felt that using radar in this way had improved their understanding of the factors that affect bird migration, and they expect to go further with this innovative technique.

As Cecila Nilsson told DecodedScience: “Currently we are in the process of planning future projects and looking for new and interesting sites to place the radar“.

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