Gavia immer – TROCHYDD MAWR – Winter visitor and passage migrant. Recorded in every month
Appears to have decreased since the 1940s as both Mathew (1894) and Lockley et al. (1949) stated it was sometimes numerous, particularly in Milford Haven. Dickenson and Howells (1962) noted groups of up to 20 and recorded mainly immature birds. Although 26 in a day have been seen passing Strumble Head during autumn passage, in winter there is a scattering of ones and twos around the outer coastline with occasional concentrations at favoured spots; for example, up to five have been seen in Fishguard Harbour, five in St Bride’s Bay and seven in the Strumble Head tide races, with a remarkable 43 there on 6 January 1991, following prolonged gales. It is no longer numerous in the Cleddau Estuary. One or two still occur fairly regularly between Dale and Hakin, with up to five on occasions, but they rarely penetrate further up river. It is seldom seen on fresh water, with just a few records from Llysyfran reservoir and Bosherston Pools.
There is a small spring passage from March to May and a large autumn passage from September to December, birds in full summer plumage occurring particularly in May and September but sometimes in other months.
Dickenson H. & Howells R.J., 1962, Divers in Wales, Nature in Wales 8: No 2.