Tracking godwits

Research on godwit migration involves tracking or following the flight path of the birds. Scientists like Dr Phil Battley and Jesse Conklin from Massey University do this by putting a tracking device on the bird that enables them to follow where the bird goes from a distance. There are several methods of tracking.

Colour banding

The simplest form of tracking is by colour banding. This involves capturing the bird and attaching a coloured band to its leg to identify it. A band may also include a number, which further helps to identify the bird. Using colour and number bands means people need to look for and identify the birds in various places. This isn’t always possible, so the scientists decided to try satellite tracking.

Satellite tracking

This involves a satellite transmitter that sends a radio signal into space where it’s intercepted by a satellite and then transmitted back to Earth. The satellite determines where the bird is, so it can give very accurate information. Two godwits – E3 and E7 – became famous in 2007 when they were tracked by satellite and followed by people on the internet as they made their journeys across the world. Books were written about them including a children’s story called E3 Call Home by Janet Hunt.

However, there are problems with satellite tracking. They require battery power, and there’s a limit as to how long they last for. They are very expensive – one transmitter costs about NZ$5,000. They are heavy – weighing 10–25 grams. Some of these transmitters need to be surgically implanted within the bird. This means capturing the bird and getting a specialist vet to perform surgery to insert the transmitter. Other satellite transmitters work through solar power and are strapped onto the back of the bird. They’re the size of matchboxes and have aerials coming off them. The problem with this transmitter is that, when the bird loses weight through flight, the transmitter has a tendency to fall off.

Geolocators

Geolocators are small and light so they are not such a burden for the bird. They weigh about 1.5 g. The main problem with them is that the birds need to be caught again when they return from their journey to get the information off them. Geolocators are sensors that record light and dark. Scientists use this information to work out how long the days and nights are and, from this, work out the latitude and longitude of where the bird might be. Their accuracy is within 200 km.

Capturing through canon netting

In order to attach colour and number bands, satellite devices or geolocators, the birds need to be caught. This is often done using a cannon net, which is a long net the researchers hide along the tideline where the birds come to roost. When the birds are in place, cannons fire the net out over the birds. The workers move fast to attach devices or put the birds into containers if they are to have surgery. They handle the birds carefully and for a minimum amount of time.

Sometimes only one bird (or a small group) needs to be caught, for example, the recapture of a bird carrying a geolocator. In that case, it may be possible to catch just a few birds with a smaller net, but more often, many birds have to be caught, even if just a few birds are needed. The ‘extra’ birds can be released quickly. It seems that, if birds have been caught before, they are harder to catch the second time.