On satellite telemetry and mortality

In 1996 the first Swedish Ospreys were equipped with satellite transmitters, small electronic devices that allow tracking the movements of individual birds in nearly real-time. For the first time the impressive migratory movements of this raptor bird could be mapped, along with details about stopover behaviour, timing of migration, migration speeds, etc. The satellite tracking program has continued ever since (still running today), in which many individuals of different species of birds have been included. The research has provided many new insights in the migratory behaviours of birds, and resulted in two PhD theses (Nils Kjellén and Roine Strandberg).

Adult female Marsh Harrier “Fe06-66” has just been equipped with a satellite transmitter. The small electronic device allowed us to track the movements of this bird for the rest of its life. This particular female disappeared under suspicious circumstances in France. Most likely she was (illegally) shot by a hunter.

One of the downsides of satellite telemetry is that it is a rather expensive technique, and thus only a small number of birds can be followed every year. Because of these small sample sizes it is extra unfortunate to lose a bird that we are tracking. However, when, where, and under what circumstances migrating animals die is interesting information in itself. It is almost impossible to collect data on this important topic in any other way. Every case of mortality is thus sad but interesting at the same time.

Last locations obtained from the female Marsh Harrier Fe04-53, during her second autumn migration. She started to cross the Mediterranean Sea on the 27th of September under good metrological conditions. Midway the crossing she suddenly stopped moving, and she remained at this location for more than a day. Subsequent signals were received from the south coast of Spain, where the transmitter moved westwards at the speed of a car and disappeared into a small town. The last position was received from a large building. This bird was presumably shot by poachers that await migrating birds in small boats.

After 12 years of tracking migrating raptor birds a sufficiently large dataset on had been accumulated in order to look for patterns in when and where migrants die. In a recent paper we have focussed on risks associated with the crossing of the Sahara Desert, a formidable barrier for billions of Eurasian breeding birds wintering in African. We evaluated about 90 tracks across the desert of Ospreys, Marsh Harriers, Honey Buzzards and Hobbies. The passage turned out to be very hazardous for juvenile birds; during their first autumn migration about one third of the young birds died while attempting crossing the desert. For adult birds only 2% direct mortality was recorded per desert crossing, which still contributes significantly to their annual mortality. Additionally, adult birds showed in nearly half of their crossings aberrant behaviours, like abrupt course changes, slow travel speeds, interruptions and retreats from the desert, indicating problems during the passage. Aberrant behaviours during the crossing in spring resulted in late arrival at breeding grounds and an increased probability of breeding failure (carry-over effects). The Sahara Desert is thus indeed a true barrier for migrating (raptor) birds, and has a major effect on has a major impact on both survival and breeding success.

Juvenile Osprey “Juv61-07” succumbed just before it had completed the crossing of the Sahara Desert. This happens to about one third of the young birds in their attempt to overcome this almost everlasting dry environment.

The death of an Osprey in the desert as seen from GPS location fixes. This particular transmitter was sending for several weeks from exactly the same location. It is impossible for the birds to drop their transmitter, stationary positions thus indicate death.

A collaborator in Mauretania succesfully located a juvenile Osprey that failed to cross the desert. The transmitter was returned to Lund and can thus be used to track another hopefully longer living Osprey.

The paper on risks associated with the crossing of the Sahara Desert can be found at:
http://rsbl.royalsocietypublishing.org/content/early/2009/11/25/rsbl.2009.0785.short?rss=1

(Roine Strandberg, Raymond H.G. Klaassen, Mikael Hake & Thomas Alerstam. 2009. How hazardous is the Sahara Desert crossing for migratory birds? Indications from satellite tracking of raptors. Biology Letters).