The 2008 Manx Shearwater Census on Bardsey by Richard Else, July 2008

The whole island breeding population of Manx Shearwaters on Enlli is to be surveyed over a three-year period, beginning in 2008. This year I have undertaken the census of roughly a third of the island, from the southern tip, up the West coast and lowlands as far North as Carreg. The rest of the island will be surveyed over the next two years. Once this is done we will have an accurate count of the breeding Shearwater population, as well as detailed maps showing exactly where the birds are nesting.

Manx Shearwaters are a notoriously difficult species to survey, due to the fact that they nest in burrows and only visit the colony after dark. Estimating the population involves counting all the burrows on the island that appear to be occupied. This method is not perfect as identifying occupied burrows from empty ones is not always possible. For this reason another census technique is being used. This involves playing a tape recording of male Manx Shearwater song (a sound familiar to regular visitors to the island) into every burrow, and if a male bird is at home incubating, he will sing back in response. All the burrows that sing back are counted, and from that figure a population estimate can be calculated. The results of these two census methods will give a good idea of the present population.

The last complete census was done in 2001, when the breeding population was found to have risen rapidly to between 10,000 and 16,000 pairs. I have recently completed the 2008 census work, and it seems, surprisingly, that there has been a slight decline in the population since 2001; although of course we can’t tell for certain until the rest of the island has been counted in 2009/2010.

I am also currently monitoring the breeding success of the shearwaters by visiting a sample of occupied burrows periodically to assess their progress. At the moment most eggs have recently hatched and the burrows contain small, fluffy, grey chicks. These will be ringed and weighed in the next few weeks.

© Copyright BBFO 2008