Gavia immer – TROCHYDD MAWR – Winter visitor and passage migrant. Recorded in every month
The Great Northern Diver breeds across the sub Arctic region of the Americas, Greenland and Iceland and on rare occasions has bred in Scotland and as far east as Svalbard. Those that winter in the east Atlantic do so as far south as the Bay of Biscay and Iberia.
As with many other species, it is difficult to assess the Great Northern’s status in Pembrokeshire in the past. The accounts of previous authorities have been largely anecdotal and prone to expressions like “sometimes numerous” and “frequently seen”.
Dickenson and Howells (1962) must have felt sufficiently able to interpret the past to state “From the numbers recorded in the last decade it is possible that a decrease has taken place since the 1930’s.” The decade they referred to was 1952 to 1961, when an average of three per annum was recorded, the maximum in any year being nine in 1959.
The average recorded per annum remained at three until 1982, the maximum in a year being six in 1972. By 1983 an increasing number of observers were putting the results of their observations into the communal record pool and, from then until 2006, the average number recorded reached 61 per annum, with the most in a year being 119 in 1999:
There was a marked increase in the number of active observers during the 1980’s and 1990’s, they possessed ever improving optical aids and most had the discipline to contribute their observations to the communal record. These are likely to be the factors that explain the upsurge in records, rather than there being an increase in the number of Great Northern Divers visiting Pembrokeshire.
Plumage – Observers have seldom included an age assessment in their reports unless noting adults in summer plumage. Great Northern Divers have a prolonged post breeding moult period stretching from September to January, consequently birds in summer plumage have been recorded throughout autumn passage, occasionally as late as the end of December. Of a total of 646 birds closely scrutinised passing Strumble Head in autumn, between 1981 and 2008, 48% were in full or showed variable vestiges of summer plumage. A few birds have been recorded in full summer plumage in the spring, from late April.
Spring passage has been relatively small, mostly recorded as departure dates from wintering areas, normally completed by late March to early April. Occasionally presumed migrants have briefly appeared at some of these same places in late April up to 22nd May and others have been noted at or passing headlands and the offshore islands from late March to 26th May. Later occurrences were: one in the Gann – Lindsway Bay region throughout May until at least the 21st June 1991 and two at Ceibwr on 22nd June 1997.
The variation in county annual totals seems to have had more to do with variation in the number of passage migrants recorded than with the number of wintering birds. Autumn passage has been noted along the coast and at the offshore islands, being most consistently documented at Strumble Head annually between 1983 and 2006.
The earliest recorded there was on 28th July 1991. August records were all of single birds, on the 31st in 1985, 24th in 1986, 28th in 1989, 23rd in 1994, 28th in 1994 and 27th in 1995. Numbers built up in September, the main passage taking place from October to December:
In winter they have been found in the offshore waters all around the coast from Cemaes Head to Amroth, including Fishguard harbour.
Most sightings have been of single birds or sometimes two together, with occasionally up to seven in areas such as St Bride’s bay, Fishguard Harbour and the Strumble Head tide races. The unusual gathering of 43 at Strumble Head on 6th January 1991 was probably due to birds seeking a lee shore during a period of prolonged SW gales.
Great Northern Divers have regularly been seen within the Milford Haven / Cleddau Estuary waterway during the winter months, January to April and October to December. The average number recorded between the years 1983 and 2003 was five, the maxima in a year being 10 in 2002 and 12 in 1999, which probably included some migrants in transit.
Most records were from the area between Dale and Hakin, including the Gann, Angle Bay and the Pembroke River. Singles have been seen further upstream at Castle Pill, Llanstadwell, Neyland, Lawrenny, Cresswell and Carew Rivers, including the tidal Carew Mill Pond, Garron Pill, Beggars’s Reach, Llangwm and Landshipping to Hook Reach area.
The only fresh water occurrences on record are of one at Bosherston 9th – 13th November 1986, two at Llys y fran Reservoir 16th January 1986, with singles there in 1983, 1984, 1985, 1999, 2001, 2002 and 2006.
Graham Rees. Pembrokeshire County Bird Recorder 1981-2007
DICKENSON. H and HOWELLS. R. J. 1962. Divers in Wales, Nature in Wales 8.
DONOVAN. J and REES. G. 1994. Birds of Pembrokeshire, Dyfed Wildlife Trust.
LLOYD. B. 1929-1939 Diaries, National Museum of Wales.
LOCKLEY. R. M, INGRAM. C. S. and SALMON. H. M.1949. The birds of Pembrokeshire, West Wales Field Society.
MATHEW. M. 1894. The birds of Pembrokeshire and its islands, R. H. Porter.