Catching lesser black-backed gulls to attach GPS devices

One month later than my April visit, in mid-May the gulls had begun breeding, and it was now time to catch them to attach the GPS devices. Arriving at the island spring had sprung, the leaves were coming out, and the birds were much more active.

A lesser-black backed gull with GPS device mounted, held by Susanne

The easiest time to catch the gulls is when they are incubating their eggs. I used a ‘walk-in’ trap, where a wire cage is placed over the nest, with a door set to close when the bird enters the trap – to incubate its eggs. Once the bird is caught we ring it, and take various measurements and a few feather samples. The GPS device is then attached with a Teflon harness. For this period I had the help of Jonas Hentati-Sundberg from Stockholm Resilience Centre, who had caught gulls with the same method previously, and later from Susanne Åkesson.

The Baltic subspecies of lesser-black backed gull has darker plumage than the two western subspecies. We photographed the birds with a grey-scale to check birds were of the eastern type

As soon as the devices were out the data started rolling in – which is a big advantage of a wireless device, very different to my previous work using logging devices, where it was essential to recapture the bird. The data is looking very nice with some patterns already emerging, some individuals going to forage on farmland on Gotland, and other mainly foraging out at sea to the west of Stora Karlsö island.

Some of the first GPS tracks for lesser-black backed gulls breeding at Stora Karlsö. Some birds are foraging on fields on Gotland, others out at sea

In a future post I will talk more about some of the early data, and about the ‘wild-gull chase’ – when I went to check-out the fields that the gulls had visited on Gotland.

GPS track from a gull passing the nearby island of Lille Karlsö. It is moving left to right, apparently using coastal soaring for lift, to gain altitude as it passes the island, hugging the cliffs. Each orange point is a GPS position, and the line below these shows the altitude above the ground

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