[Contributed by Dr Steffen Oppel, Senior Conservation Scientist, RSPB - all photos are credited to Steffen Oppel]
In February/March this year my colleague Mark Bolton and I visited Ascension Island to help the AIG Darwin Project and seabird teams to track seabirds.
We focussed on two large species - Masked Boobies and Ascension Frigatebirds.
Masked Boobies - above
Ascension frigatebird that will soon be ready to fly - above.
We studied their foraging ranges at sea using minature GPS loggers that were taped to the tail feathers of the bird. These loggers weigh around 20 g and do not affect large seabirds, but they can record a precise location every two minutes for more than one week, and thus reveal the incredible foraging journeys of seabirds in great detail.
The loggers (barely visible on the upper end of the tail)
Masked booby tracking
On this trip we captured 28 masked Boobies on 20 nests. We then returned about one week later to recapture the birds and retrieve the loggers from their tails. None of the loggers failed us, and we recovered almost all of them, except for 5 birds that had left after their nest had failed. The birds from which loggers were retrieved were not lighter at recapture than at first capture, which indicated that the logger attachment did ot affect the foraging behaviour of the birds.
Together with data collected by the RSPB in 2011, a total of 63 foraging rips from 41 individual Masked Boobies are now available. The foraging trips by the birds are spectacular, and cover virtually the entire exclusive economic zone around the island! The birds appear to take off in a random direction and fly up to 340 km away from Ascension, foraging as they go along, and then return to the colony 1-3 days later. Most birds did not fly off into the same direction of subsequent trips, and the two members of a pair also went into different directions. So, there does not seem to be a 'hotspot' where they forage around Ascension - they just go as far as they can and explore the huge expanse of ocean to find food for their chicks.
Foraging facts:-
Ascension Frigate bird tracking
Tracking these birds was slightly more complicated, because the birds only nest on Boatswainbird Island.
After all the researchers climbed the ladder, it was time to hoist equipment from the boat onto the island
We managed to take the boat out to Boatswainbird Island 5 times to deploy 45 GPS loggers on frigatebirds with small chicks and egss. We also deployed an experimental camera on the back of a female frigate bird to get a 'birds-eye-view' of a foraging trip.
Capturing the frigate birds unproblematic, but re-capturing the birds to retrieve the loggers was more difficult. The main challenge was that adult birds left their chicks unattended for extended periods, so the birds that had our loggers attached to their tail were not on Boatswainbird Island when we were there! However, we still managed to retrieve 19 of the loggers and 1 of 3 cameras.
From the 19 retrieved GPS tags we downloaded a total of 44 complete foraging trips. Incubating adults spent longer at sea and travelled further than adults that had to take care of a small chick. Although some birds repeatedly took off into the same direction, the general pattern of space use was similar to boobies in that birds set off in all directions and used virtually the entire exclusive economic zone around Ascension.
Record facts of Ascension Frigatebirds
While much more needs to be learned about the remarkable foraging journeys of Ascension's seabirds, it appears that sustainable management of all the waters in a large (>300 km) radius around the island will be necessary to ensure that these birds can continue to raise their chicks here. We found no area whether either frigatebirds or boobies concentrated, and there may simply be no reliable foraging hotspot that has a guaranteed supply of fish to make the journey to the place worthwhile.
Frigatebirds are aerial acrobats despite their large wings