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.