The purpose of our visit to Ile aux Cochons* was to undertake a pilot study of the foraging ecology and at-sea distribution of South Georgian diving petrels (’jojos’) to compare with the closely related (and similar-looking) common diving petrels (’plon plons’) that we had studied on Ile Mayes the previous week. The two islands are only 6 km apart, both have the same introduced mammal fauna (i.e. mice, after rabbits were eradicated from Ile aux Cochons about 15 years ago), and both have a diverse and abundant seabird fauna. But there were many differences in the abundance of individual seabird species between the two islands.
There are about a dozen species of burrow-nesting petrels that breed on islands in the Golfe du Morbihan, but some are so rare or difficult to find there that they are not monitored or studied. Ile Mayes has been the focus of seabird research by CNRS Chizé in the Golfe for more than 30 years, with biologists monitoring breeding success of dozens of pairs of grey petrels, white-headed petrels, blue petrels and thin-billed prions every year. In addition, the population dynamics of these four species plus (the smaller) common diving petrel and Wilson’s storm petrel are monitored through mist-netting near the hut.
In contrast, there has been very little seabird work on Ile aux Cochons, but a previous survey of petrel burrow contents (using a burrowscope and call play-back) along transect lines had revealed two sites where there appeared to be concentrations of South Georgian diving petrels. This was all the information we had to start the study, and we had little knowledge of how difficult (or easy) it would be to find breeding burrows, whether the nest chambers would be accessible for study, or whether the birds would be amenable to study without deserting their nests.
We began searching for jojo burrows soon after arriving on the island, but after an hour of searching had found only burrows of Antarctic prions (absent or rare on Ile Mayes) and blue petrels. Both species are abundant on Ile aux Cochons, plus thin-billed prions were also present in areas with deeper soil.
Night-time is the best time to search for petrels, as they return to their burrows at night and most species call loudly from the air or ground. With the exception of the two species of diving petrel, the petrels on both islands were very wary when on the colony surface, disappearing rapidly down their burrows to avoid prowling skuas, and trying to avoid our headlamp beams. Fortunately most species are readily distinguished by their calls, and so a knowledgeable ear is all you need to get an idea of which petrel species are present and how abundant they are.
The noisiest species at night on Ile aux Cochons were the Antarctic prions (especially the 5-6 pairs that nested under the hut) and white-headed petrels, although we never saw the latter species on the ground. The blue petrels were relatively quiet as their breeding season was well advanced, with adults busy feeding large chicks. Both species of diving petrel were vocal, with the common diving petrels breeding in deep burrows on the most densely-vegetated steep slopes, where they responded readily to mimicry of their crooning calls. We also located burrows of white-chinned petrels (the largest species present) and Wilson’s storm petrels (the smallest species) by following their calls.
Great-winged petrels (which are closely related to the New Zealand grey-faced petrel) are another common breeding species on Ile aux Cochons. However, as they breed in winter, the main evidence of their presence in January was pairs of wings of fledglings that had been killed by skuas when the petrel chicks emerged from their burrows before departing the island. Another species that we failed to see (though they are likely to breed in small numbers) was the Kerguelen petrel. They favour very wet sites, with swimming-pool entrances to their burrows, so mainly breed on the very wet western side of the archipelago, about 80 km away. We found a few likely looking burrows in seepages at the south-western end of Ile aux Cochons, but did not have a chance to visit the site at night. Grey petrels (another winter breeder) are apparently absent or rare on Ile aux Cochons.
We did eventually find jojo breeding burrows on the first day, on a sparsely-vegetated plateau 5-10 minutes walk from the hut (now named ‘le Plateau des Jojos’). Antarctic prions also nested on the plateau, although their burrows could usually be identified (and avoided) by their larger entrances. Many of the jojo burrows were among large rocks, with inaccessible chambers, but we eventually found about three dozen accessible burrows with single chicks (mainly) or eggs during four days of intensive searching.
Our first priority was to attach tiny GPS logger tags to the backs or tails of nine breeding birds, as we wanted the birds to complete a feeding trip away from the island and return before we departed the island, so that the tags could be retrieved and the data downloaded. The tags provide very accurate data on where the birds have been, but can hold only a few days’ data. They are attached to the birds’ feathers with tape and glue, and so would be shed when the birds moulted after the breeding season if we failed to retrieve them all. But the birds were very obliging, and we were delighted to recover all nine tags, with the last bird caught about midnight on our last night on the island.
The next priority was attaching small geolocater tags to the legs of ten birds. These tags capture less accurate data than GPSs (they measure light vs dark against time of day, allowing rough assessment of latitude and longitude based on day length and the time of sunrise and sunset, whereas GPS tags get location data by triangulation from satellites). The advantage of geolocater tags is that they are smaller and cheaper, and can store months or years worth of data, allowing estimation of where the birds feed both when breeding and between breeding seasons. Before our study, nothing was known about the winter at-sea distribution of these small seabirds of the southern Indian Ocean.
The geolocater tags were attached to a leg band and so will not be lost when the birds moult. But the birds must be recaptured next breeding season for the tags to be removed and their data downloaded. We attached all these tags to birds caring for chicks (one adult per pair) at ten burrows all located close to each other, to increase the chance of the birds being recaptured. Most petrels are very faithful to their burrow, using it year after year. Any birds that change burrow (or mate) are likely to move only a few metres.
Our final task was to trial the installation of artificial nest boxes at burrows with chicks, to see if the adults accepted the changes to their burrows. The boxes used were designed in New Zealand and built in France, and we hope that they will facilitate longer-term study of the birds. The nest chambers inside the boxes can be accessed easily without damaging the burrow by simply lifting the double lids (natural burrows can collapse if opened too often, or the entrances may become too large, allowing larger petrel species to take over the burrow). We installed four of the boxes gradually over four days, and were delighted to find all four pairs accepted the changes, and their chicks continued to be fed and gain weight.
We returned to Port aux Français very pleased with what we had achieved during a busy seven days, and having learnt a lot about working with this smaller, less well known diving petrel species.
*Note that there is a larger island of the same name in the Crozet Islands.
Te Papa curator of vertebrates Dr Colin Miskelly’s participation in seabird research programmes on the French subantarctic island groups of Crozet and Kerguelen was at the invitation of Dr Charly Bost of the CEBC laboratory (Chizé) of CNRS (Centre National de la Recherché Scientifique), France, and was supported by the Institut Polar Français Paul Emile Victor (IPEV) and Te Papa.
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