Satellite tracking has been used to monitor the movements and survival of large birds and mammals for about 15 years, but the technology is rapidly improving and tag sizes are decreasing. We are now in the exciting position where satellite tags are small enough to be safely used on birds weighing as little as 150g. There is huge interest in understanding woodcock migrations and satellite tags provide the opportunity to follow individual birds accurately in near real-time.
The satellite tags we use weigh 9.5g and are mounted on the lower back using an elasticated leg-loop harness. Each tag consists of a tiny satellite transmitter, a battery and a solar panel to keep the battery charged. The tags transmit a signal containing their identity for 10 hours in every 58-hour period as long as the battery is sufficiently charged. The signal is received and relayed by earth-orbiting satellites and the tag is triangulated using the satellite positions and Doppler shift of the transmitted signal. Position fixes are typically accurate to between 150m and 1km.
So far, we have received useful information from 24 satellite tags fitted to woodcock at six wintering locations during March 2012 and March 2013. We have recorded birds returning to seven countries to breed: Norway (2), Sweden (4), Finland (1), Poland (1), Latvia (3), Belarus (1) and Russia (western 9, central 3). Total distance migrated has ranged from just over 1,000km (620 miles) for our Scotland-Norway migrants to around 7,100km (4,440 miles) for the three birds breeding in central Siberia. The average distance flown to the breeding grounds has been close to 3,000km (1,860 miles).
As well as providing information on where the birds are breeding, the satellite tags reveal the routes and stop-over sites that they use. Insights from satellite tags include:
- Departure times – typically mid-March to early April from winter sites, late September to late October from breeding sites.
- Mixing of migrants at wintering sites – birds tagged at the same winter site often headed to different breeding grounds.
- Extraordinary distances – three birds have travelled distances of 6,180-7,100km (3,860-4,440 miles) to breeding sites in Siberia. One male, tagged as an adult in 2012, is estimated to have flown at least 38,000km (23,750 miles) during his life!
- Journey times - times taken to reach breeding sites have varied from three weeks (northern Norway) to eight weeks (central Russia).
- Site fidelity – evidence from a few of the satellite-tracked woodcock followed for more than one year indicate the use of exactly the same winter and breeding sites each year.
More on our woodcock satellite tracking can be found on the Woodcock Watch website.