Adult female brown booby

Tracking brown boobies and Ascension frigatebirds

Tess Handby and Beth Clark, University of Exeter 

For five weeks in September and October 2018, we visited Ascension to study the foraging behaviour of brown boobies and Ascension frigatebirds on the Letterbox peninsula. These seabirds were once isolated on cliffs, stacks and Boatswain Bird Island, but since the successful feral cat eradication in 2002-06, they are slowly returning to the mainland. There are now at least 93 nesting brown boobies on Letterbox and an amazing 66 frigatebird colonies, 22 of which were new for the 2018-19.

Our project complements previous GPS tracking on masked boobies, sooty terns and frigatebirds, feeding into a wider dataset that will inform the designation and management of a large-scale marine protected area in Ascension’s water. Protecting seabirds both on land and at sea is vital, particularly for the Ascension frigatebird as it is listed as globally vulnerable and exists nowhere else. This study is the first to track brown boobies on Ascension and the first to use altitude loggers to investigate Ascension frigatebird behaviour.

Female Ascension frigatebird with chick

To track the birds’ feeding behaviour, you first need to capture them. This involved Beth slowly and quietly approaching the target bird on its nest with a hand net. We then weighed and measured the bird before attaching a GPS tag to the central tail feathers. The bird is then released to return to the nest. To recover the data, we needed to recapture the bird after a few days. Seabird parents work together to look after the eggs and chick, taking turns to go out fishing. For some species such as frigatebirds, this leaves one bird on the nest for over a week awaiting their partner’s return. Parents must protect the egg/chick from the high heat and UV exposure and defend the nest from potential predators. Seabirds usually spend more time away from the nest when they are incubating an egg, as opposed to protecting a chick, as the chick will need feeding often.

For the frigatebirds, our GPS tags recorded the bird’s location every 5 minutes for up to 12 days. We recovered tags from 4 out of 5 of our tagged frigatebirds, but the altitude data will need processing before we can know how high they fly and learn more about the characteristics of their foraging behaviour.

For the brown boobies, the GPS tags recorded the bird’s location every minute for 2-5 days. We retrieved data from 17 tags, totalling 76 foraging trips (see map). This is an exciting result, providing valuable evidence of the marine area used by the brown boobies. Until now little was known about their local feeding behaviour. At other colonies around the world, the brown booby is a coastal feeder, relying on shallow diving to catch fish and squid. Given the deep waters around Ascension Island, it is interesting to see how they have adapted their behaviour for this habitat.

GPS tracks for 17 brown boobies with each colour representing a different individual

All of Ascension’s seabirds compete for scarce food resources in the surrounding ocean. Both booby species and frigatebirds eat the same food; flying fish and squid. However, the tracking data shows that they choose to travel very different distances in search of food, separating by species. We now know that the brown boobies are travelling the shortest distance from the colony (up to 143km) compared to any other seabird on Ascension Island. Given these shorter trips lasting only up to 12.6 hours, our data also shows that both the parents will return to the nest every evening, until the following dawn. On the other hand, frigatebirds travel the furthest, with one of our Letterbox birds reaching a distance of 677km from the island during a 4319km trip lasting 11 days.

This project was carried out as part of the Darwin Initiative-funded Ascension Island Ocean Sanctuary Project (DPLUS063). Beth and Tess received invaluable support from Department of Conservation and Fisheries, particularly from Eliza Leat and Sophie Tuppen.