Red-throated diver - winter

Gavia stellata

Red – throated Divers have been recorded all around the outer coast in winter, normally 1 – 10 at most localities but some places have been more favoured, where up to 50 have sometimes been recorded , such as the Strumble Head tide races, various parts of St Bride’s Bay and off Amroth.

They have also shown an ability to temporarily accumulate at what were presumed to be good food sources and hence demonstrated a great deal of mobility within the region. Occurrences of this nature were up to 75 at Goultrop Roads 1st – 19th January 1996, 60 there on 19th January 1997, up to 40 in January 1998, peaking at 85 on the 23rd, 60 on 16th December 1998 decreasing to 22 by 31st December. A similar attraction was noted at Amroth in 1993 when 200 were present on 20th February, decreasing to 25 by 27th February but then 145 there on 6th March.

Mathew (1894), Lockley et al (1949) and Lockley (1961) emphasised the importance of the Milford Haven waterway for Red – throated Divers. Post 1961 none were recorded there until 1981, since when they have been noted in 18 years, up to 2006.

Most records have involved single birds, occasionally two together, with three at the Gann on 1st April 2006, six Monk Haven/ Gann area 3rd – 19th January 2003 and seven Dale Roads on 10th February 2002. All records refer to the region between Dale Roads, Angle Bay and South Hook Point, save for singles at Picton Ferry 17th January 1981, Llangwm 12th October 1992, Carew-Cresswell 12th December 1993 and 10th March 1985, Garron Pill 20th March 1997 and five Carew River on the 5th December 1982.

The Red – throated Diver is predominantly a marine species in Pembrokeshire, with just four singles being recorded on fresh water, viz. Bicton Reservoir 9th April 1967, Hayston Hall November 1985 and Llys y fran Reservoir 21st December 1993 and 16th January to 22nd February 1986.

Red-throated diver - 1980s winter atlas

Gavia stellata

The BTO atlas covering the winters of 1981-82, 1982-82 and 1983-84 shows a similar distribution to that described above.

The darker the colour, the higher the relative total count for each 10km square.  The darkest blue represents 8+ birds, with the highest count being 16 seen at Strumble.

Red-throated diver - Sea Empress oil spill

Gavia stellata

The tanker “Sea Empress” grounded outside the Heads approaching the Milford Haven waterway on the 15th February 1996. Approximately 72,000 tonnes of crude oil and 360 tonnes of heavy fuel oil spilled into the sea between the 15th and 21st of February. Further fuel oil was spilled when the ship was moved to Belfast.

The waterway within Milford Haven was heavily coated and much of the oil was carried south and east to affect the south Pembrokeshire coast and Carmarthen Bay. The offshore islands were not heavily contaminated nor was St Bride’s Bay, with no oil reported north of St David’s Head.

123 Red – throated Divers were known to be in Pembrokeshire waters in January and February prior to the oil spill, the largest concentrations being 10 at Fishguard Harbour, 17 at Frainslake, 20 off Amroth and 75 in Goultrop Roads. Of these, only the Amroth birds were in the badly oiled area, as were five at Freshwater East.

Ten oiled birds were collected from the south coast of Pembrokeshire but a further 49 from the nearby affected coasts of Carmarthenshire and West Glamorgan (SEEC, 1996). Those at Goultrop Roads disappeared and may well have been involved in the movement of 96 passing Strumble Head, going into Cardigan Bay, on the 25th February.

References

SEA EMPRESS ENVIRONMENTAL EVALUATION COMMITTEE, Initial Report, 1996.

Red-throated diver - passage

Gavia stellata

A marked autumn passage of birds travelling from north to south has been recorded since at least 1980, spanning the months August to December, though singles were recorded in 2 years on the 11th July and once on the 3rd July.

This movement has been well documented at Strumble Head. August records were confined to the latter part of the month, 19th – 31st, totalling just six birds over the period 1980 – 2006.The main passage was between October and December and is represented below in graphical form. These are the mean annual values calculated from the total number of birds recorded for the period 1980 to 2006, grouped into six day intervals.

Only once in 27 years did the passage continue into January, when 40 passed on the 6th in 1991, thereafter no movement being detected during that month. The number of birds passing on any one day was generally small but over 30 was recorded five times in four different years, the maximum being 44 on the 4th December 1993.

The December peak was obscured in some years, with passage ceasing during gales. It is not known whether these held up birds within the comparative shelter of Cardigan Bay which continued their migration when the wind moderated, or the gales displaced birds from further south to seek shelter in the Bay, or possibly whether both reactions took place.

A passage of smaller volume than in the autumn has been noted in spring, March to 30th May. This has not been well documented but has involved birds passing westwards off St Govan’s Head, northward past the offshore islands of Skokholm and Skomer and eastwards past Strumble Head.

The Red – throated Diver has only been recorded in Pembrokeshire once in June, when two were seen at Skomer on the 18th in 2003.

Red-throated diver - ages

Gavia stellata

Very few observers have reported whether their sightings involved adults or immature birds, perhaps not surprisingly considering the circumstances in which they are often seen. However in the autumn it is easy to differentiate those in full summer plumage and those showing traces of summer plumage, from those in winter and juvenile plumage.

This was recorded systematically at Strumble Head during autumn passage between the years of 1980 and 2008. The proportion showing signs of summer plumage was 91 % in September, 50 % in October and 3 % in November (latest on 23 November), the total number of birds examined being 483, 588 and 533 respectively. This clearly indicates that adults pass earlier than the bulk of younger birds.   

Graham Rees

(Covers records up to and including those for 2006).

Red-throated Diver - 1894

Species account from M Mathew, 1894, The Birds of Pembrokeshire and its islands

Click to read more ...