Dr Phil Battley, from Massey University, explains how birds are kept track of through satellite tagging. He describes and compares two ways of doing this – using backpacks with solar panels and internal transmitters.
Dr Phil Battley
A satellite tag is a very different thing to a geolocator. It’s a big unit, it’s powered by batteries or by solar panels, and they weigh quite a lot, so whereas a geolocator weighs maybe 1½ grams, a satellite tag would weigh anything between 10 grams and 25 grams, so it’s a heavy thing to put on a bird, but what we’ve found actually is that the heaviness probably isn’t as big an issue for the bird as where on the body that transmitter is.
We tried two different types of transmitters. One if them has a solar panel on it. That means that it doesn’t have to have as big a battery, so it can be lighter. And this goes on the outside of the bird, so it’s like a little box about the size of a matchbox with an aerial coming off it, and that gets strapped onto the back of the bird with a harness that loops around and goes between its legs.
There are different ways of putting a transmitter on a bird but we didn’t want to put it up by the wings because the wings are doing so much beating and flying that we then had them shifted further back on the body in the hope that the airflow wouldn’t be as disrupted as well. But the problem with making a harness is that these birds double their weight before they migrate so the harness holding the transmitter on has to be loose enough to allow the bird to get really fat, but it still has to hold on when the bird is really skinny. That’s a difficult balancing act.
The other type we tried were ones that go inside the bird, so this is one where you need a specialist bird vet. The bird is put under an anaesthetic so it goes to sleep, and then a little bit of surgery is done on the bird and a transmitter is put into the abdomen of the bird. Birds have air sacs all through their body, so this could just fit into one of these air sacs, and then the aerial has to come out through a little hole in the skin. So this is something that you can’t just go and do, you need a specialist bird vet, it’s a very indepth sort of procedure, but we’ve put those on, and counter-intuitively, it was those implanted transmitters that work the best.
What we found with the backpack ones was that the birds we put those on took off on migration, got part way there and then they dropped out and went to an island, and then we would lose track of them because what would happen is that their transmitters would fall off once they landed on this island and they were skinny. But some of those birds were seen again in Australia and back in New Zealand, so we know the birds did OK. They survived even though the transmitters had fallen off. The birds with the implants are the ones that gave us the long-distance information on the flights.
Pete & Judy Morrin Productions