Whilst stable-isotope analysis provides information on the origin of migratory woodcock at a broad geographical scale, we still know very little about the routes flown by the birds. Using miniature data loggers called geolocators, we are learning more about the stop-over locations and journey times of migrants en route to different breeding grounds.
These tags are much smaller and cheaper than satellite tags and have the potential to yield similar information on migration routes. They have been used successfully to follow arctic terns to Antarctica and work by logging daylight levels, from which the time of sunset and sunrise can be determined. Day hours and the time of relative midday can then be used to estimate latitude and longitude respectively to an accuracy of about 100 km, an acceptable margin of error for a bird that may migrate 2,000–6,000km.
One disadvantage of geolocators is that they must be recovered to download the data. We have therefore selected deployment sites carefully, based on those where we have previous ringing and recapture data, to ensure reasonable returns. Return rate of geolocators to date has been about 15%, with both live captures and recoveries from shooting.
The following amazing insights have been provided by geolocators:
- Migration strategy – a series of long, fast flights of 600-1,100km (375-690 miles), broken up by stops en route typically lasting at least 10 days.
- Flight speed – average about 30 km/h (19 mph), but can reach 93 km/h (58 mph).