Tuesday, 5 November 2013

Geolocators vs Satellite Tags

Since 2012, the GWCT Wetlands team has tagged 24 woodcock with satellite transmitters. The movements of these birds have been transmitted back to us in near real time, and as well as a large amount of valuable data, they have provided a great deal of interest to the thousands who have viewed their migrations on the Woodcock Watch website.

But these few satellite-tagged stars of the show are only a small proportion of the woodcock caught and tagged. Though they may provide an exciting and easily-accessible insight into the movements of our birds, there is no escaping the fact that satellite tags are expensive, costing around £3000 each. It’s with this in mind that we have looked at alternative methods of gaining data on woodcock migration routes, and found one, in the form of the geolocator.

Geolocators are tags that are even smaller and lighter than satellite transmitters. They are also a fraction of the cost - we can buy around twenty geolocators for the price of a single satellite tag. They work by logging light levels, from which time of sunrise, sunset and day length can be determined.

These data can be used to calculate an approximate longitude and latitude for the bird twice a day. Geolocators are accurate to 60-150 km and whilst this is far less accurate than a satellite tag, it is an acceptable margin of error for a bird that may migrate 2000–7000 km.
Geolocators do present one difficulty however. The tags store information on the birds’ whereabouts but do not transmit it – the tags must be retrieved and the data downloaded. The chance of recapturing the same woodcock from one year to the next – considering the several thousand miles it may have travelled in the meantime – probably seems like finding a needle in a haystack.

Amazingly, however, it is possible, and this is thanks to the fact that woodcock tend to be very site faithful. Woodcock will return to the same wintering sites year on year and on some occasions even the same field. We have deliberately deployed these geolocators at sites where we have previous knowledge of site fidelity from ringing and are able invest time in recapture efforts each winter.

As well as recapturing live birds, we have also had several tags returned to us by those that have shot woodcock wearing them. It is with this in mind that we share the following information so that those who shoot woodcock are able to recognise the tags and return them to us.
  • The tags are very small, approximately 8 x 20 mm.
  • The tags may be almost entirely covered in feathers as they sit flat on the back. The light sensor should be visible, protruding from the feathers on a small stalk about 20 mm long. It is possible that this stalk could break off but the tag itself may still contain useful data so please check beneath the feathers.
  • The tags are mounted to the lower back and held using leg loop harnesses. These pass under both legs and hold the tag above the preen gland and between the wings in an area that does not impede preening or flight. A tag can be easily removed by stretching the elasticated loops back over the legs.
  • The tags have been deployed in small numbers in Norfolk and in north-east Scotland. Tags have been deployed in large numbers in Cornwall, and, with the help of the Woodcock Network, in Mid-Wales. People who shoot in these areas are far more likely to encounter geolocators.

To date, just over 100 geolocators have been deployed since 2010 and 18 recovered. People who recover a geolocator can return it to Andrew Hoodless, GWCT, Burgate Manor,
Fordingbridge, Hampshire, SP6 1EF or email: woodcock@gwct.org.uk for more information.
Both geolocators and satellite tags are valuable tools in monitoring woodcock migration, each with their own benefits and disadvantages. Whilst satellite tags are able to provide us with very accurate and easily-accessed data, the geolocators are a cheaper way to increase sample size. The success of our geolocator study hinges on achieving reasonable return rates and every tag that we are able to recover will make a big difference.

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