The Irish wintering population of Great Northern Divers Gavia immer has been estimated at between 1,000 and 1,500 birds. Four sites qualify as internationally important, i.e. regularly supporting 50 or more birds (> 1% of the estimated west European wintering population), of which the shallow seas around the Mullet Peninsula, Co. Mayo is one, with a five-year mean of 54 birds between 1996/97 and 2000/01. From 2002–10, a single count in flat calm conditions was made each January or February of Great Northern Divers in Blacksod Bay, off the southeast coast of the Mullet Peninsula, and from 2005 similar counts were made in Broadhaven Bay, off the northeast coast; in total, these recorded an average of 98 birds. Additional counts suggested wintering numbers were augmented in spring by moulting birds, and that from late February over 250 may be present in the area. These counts, I-WeBs data, and other incidental records, all suggest that the Irish wintering population of Great Northern Divers exceeds 1,500 birds.

Thompson (1851) wrote of the Great Northern Diver Gavia immer in Ireland that it “is a regular winter visitant to the coast, remaining from five to six months, and is occasionally met with in summer”; that statement is still relevant today. They are the most numerous species of diver in winter, being present from September to April, particularly off the south, west, and northwest coasts (Hutchinson 1989). The Irish wintering population has been estimated at between 1,000 and 1,500 birds (Lack 1986), although no systematic surveys have been conducted for the species, and this figure is considered an absolute minimum (Crowe 2005).

Recent estimates of local numbers are based on systematic counts of waterfowl under the Irish Wetland Bird Survey (I-WeBS), a joint project of the National Parks and Wildlife Service and BirdWatch Ireland initiated in 1994/95 (Boland et al. 2010). I-WeBS data found that three ‘sites’ qualified as internationally important for Great Northern Divers, i.e. regularly holding 50 or more birds, or 1% or more of the estimated west European wintering population of 5,000 individuals (Wetlands International 2006). These are Inner Galway Bay, the Mullet Peninsula, and Donegal Bay, which held five-year winter means of 105, 54 and 50 birds, respectively, between 1996/97 and 2000/01 (Crowe 2005). A more recent assessment found that these sites still qualified as of international importance, along with Ballinskelligs Bay, Co. Kerry, on the basis of 70 birds recorded in 2003/04, while the five-year mean for the Mullet Peninsula increased to 91 birds between 2003/04 and 2007/08 (Boland 2010), possibly due to increased coverage and changes in observers (H. Boland pers. comm.).

However, wintering Great Northern Divers can be difficult to detect from land in all but the calmest sea conditions, and are not adequately monitored through the I-WeBS, since counts are conducted on predetermined dates irrespective of weather conditions (Crowe 2005). The aim of this study was to achieve at least one count of Great Northern Divers in the relatively sheltered waters east of the Mullet Peninsula between late December and February during flat calm conditions. In addition, when conditions and time allowed, an effort was made to count the same area in spring (March to May).

Special thanks to Helen Boland for extracting and providing I-WeBS data on Great Northern Divers, and to Martin Heubeck for commenting constructively and greatly improving earlier versions of the manuscript. Thanks also to two anonymous referees for their constructive comments.

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