Radio-telemetry data were collected on rafting Manx Shearwaters Puffinus puffinus at Skomer (southwest Wales), Rum (northwest Scotland) and Bardsey (northwest Wales) between 2003 and 2005. These were used to investigate whether Manx Shearwaters tend to raft adjacent to their breeding areas and whether rafts move closer towards shore as the evening progresses. On Skomer and Bardsey, there was a tendency for birds to raft in an area roughly adjacent to where they bred, although they did not raft exclusively opposite their breeding site. On Rum, birds breeding at two different locations appeared to show different preferences for rafting areas. However, it was difficult to draw conclusions from this, as signal coverage around the island was poor, and the breeding locations were close together. At all three islands, there was strong evidence that birds tended to move closer inshore as the evening progressed.

Many nocturnal Procellariids form rafts, or dense flocks of birds, on the sea adjacent to their breeding colonies from late afternoon onwards, before coming ashore at nightfall (Brooke 2004). Species of shearwater known to engage in such rafting behaviour include Cory’s Shearwater Calonectris diomedea, Streaked Shearwater C. leucomelas, Manx Shearwater Puffinus puffinus, Flesh-footed Shearwater P. carneipes, Great Shearwater P. gravis, Sooty Shearwater P. griseus and Short-tailed Shearwater P. tenuirostris (Brooke 2004). Nocturnal attendance at the colony is thought to be a predator avoidance strategy (Brooke & Prince 1991; Mougeot & Bretagnolle 2000; Keitt et al. 2004), but the function of rafting is unclear. It is possible that the birds have difficulty in timing their return to their colony from their distant foraging grounds to precisely coincide with nightfall, so they assemble in rafts until it is safe to come ashore (Warham 1996; Brooke 2004). Most Procellariids in nearshore rafts do not feed, or only do so if schooling fish are present (Lockley 1942; Warham 1990), but rafts may provide an arena for courtship behaviour and other social interactions, as well as maintenance behaviour such as preening and resting (Brooke 1990; Warham 1996).

Rafts can comprise thousands of birds and it is assumed that they include both breeding and non-breeding birds, as both visit the colony after nightfall (Furness et al. 2000). Anecdotal observations indicate that rafting birds are generally more restless during windy conditions, frequently flying and re-alighting to maintain position (Brooke 2004), suggesting that raft position is of importance to participating birds. It is thought that Manx Shearwaters may raft adjacent to where they come ashore and their rafts might tend to approach the shore, once darkness falls (Brooke 1990). Based on visual observations of birds coming ashore close to where they had been rafting, Furness et al. (2000) also assumed Cory’s Shearwaters tended to raft opposite their breeding site.

Most observations of rafting have been anecdotal, and to our knowledge, there have been no detailed studies of rafting behaviour itself. Recently, we used radio-telemetry to determine how far Manx Shearwater rafts extend offshore, as part of a project to inform the issue of possible extensions to breeding colony Special Protection Areas into adjacent marine areas (McSorley et al. 2008; Wilson et al. in prep.). In this paper, we use those data to further investigate whether Manx Shearwaters tend to raft adjacent to their breeding areas and whether rafts move closer towards shore as the evening progresses.

All work was carried out under licence from the Countryside Council for Wales, Scottish Natural Heritage and the British Trust for Ornithology, and was funded by the Joint Nature Conservation Committee. Thanks to Skomer and Skokholm Islands Management Committee, Scottish Natural Heritage, Ynys Enlli Trust and Bardsey Bird and Field Observatory for permission to work on the islands, and to the wardens for their assistance. The fieldwork involved many individuals, to whom we are very grateful. Advice on radio-tracking techniques was gratefully received from Peter Smith and Brian Cresswell (Biotrack Ltd), and Robert Kenward, while the manuscript was improved by comments from Chris Perrins.

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