By following free-flying birds in Falsterbo we are hoping to get a better understanding of a number of migratory behaviors and to get an insight in what decisions the birds make under certain circumstances such as time of day, special weather conditions or availability of different orientation cues. The possibility to combine the telemetry studies with other methods, such as caged orientation experiment and analysis of stable isotopes, additionally increase the opportunities and this information could provide unprecedented insight into the behavior of migrants during stopover, the importance of access to compass cues and departure decisions.
Migratory hot spot
The Falsterbo peninsula is located in the southern westernmost tip of Sweden and a huge number of birds pass through the area during the migratory seasons, following the last spit of land before flying out over the Baltic Sea. The peninsula is confined by the sea and of a manageable size, so the region is an ideal place to carry out telemetry studies.
This system uses coded transmitters which enables us to track up to several hundred birds simultaneously at a single frequency and to individually identify them by the pulse rate (code) of the transmitter. The smallest available transmitters to date weigh 0.29 g, allowing us to track birds weighing as little as ~6 g, if keeping the transmitters weigh below the recommended 5% of the body mass of the bird. These transmitters have an average lifetime of about three weeks, which is sufficient to track migratory birds that rarely stay for longer periods at a stopover site.
The telemetry system consists of three fixed stations with a total of thirteen antennas that cover the area and continuously search for radio signals. By integrating the signal strengths of the transmitters received by the individual antennas of each station per time interval (6 min), we are able to determine the direction relative to the station for each bird at any given time.
Whenever possible, integrating these directions from all three stations will make it possible to triangulate the position of each bird on the peninsula. The last direction of each bird recoded by the system provides us with its time of departure and vanishing bearing. The possibility to follow the radio transmitters with a handheld tracking device throughout the day additionally allows detailed visual observations of the birds’ behavior in order to further elucidate their decisions during migration