HOW many Manx shearwaters…?!

If you’re not convinced by recent evidence that the Manx shearwater population of Skomer Island was much larger than previously thought, maybe this will help!

This thermal imaging video footage was filmed in mid-June 2013, an hour after nightfall in the skies over North Haven, the very densest part of the breeding colony on Skomer Island. Some birds will have been visiting their nesting burrows to take their turn incubating the egg or bringing food to newly hatched chicks, others non-breeders prospecting for their own nest site and calling raucously. While shearwaters are masterful flyers over long distances, foraging hundreds of kilometres to bring food to their chick and migrating huge distances to the South Atlantic, the end of the film (a bird on Skokholm Island) shows they’re pretty ungainly on land – that’s why they’re nocturnal, to avoid predators like great black-backed gulls.

The size of the breeding population on Skomer has puzzled ornithologists in recent years, with the 1998 estimate of 101,000 breeding pairs trebling to 316,000 pairs in 2011 (Perrins et al. 2012). A team from the University of Oxford, the University of Gloucestershire and the Wildlife Trust for South and West Wales played the calls of male shearwaters down nest burrows to see who was home: nearly all the males call back (clear off!), whilst the females stay quiet (hmm, who’s that?!) . Each pair of Manx shearwaters raises a maximum of one chick per year, with around 60-75% of eggs laid resulting in a successfully fledged chick. So that’s around 600,000 breeding adults raising 250,000 or so chicks – add a few non-breeders to the mix and you have maybe a million Manx shearwaters on, under and around the island in late summer. Most day visitors never see one of these nocturnal wanderers, only their burrows and the odd unlucky shearwater predated by gulls or ravens.

So is this a seabird success story or just too good to be true? Perrins et al.’s team were understandably puzzled by their result, concluding that the population is unlikely to have increased so much, even over 13 years, due to the recruitment of new breeding birds. Although the 2011 estimate was more reliable than that of 1998, it’s no easy task to estimate the size of a breeding population on the scale of a whole island. Given the number of birds visible on this night in June, there’s no doubt that Manx shearwaters are very numerous on Skomer Island – the continued efforts of ornithologists and field workers will allow us to see how the population fares in future.


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