Common Scoter winter off the coast of Western Europe, from northern Norway to Western Sahara. Spring migration sees birds moving north into the Bay of Biscay and then east across the North and Baltic Seas before an overland route to their breeding grounds in northern Scandinavia and Russia.
A recently published web article by The Sound Approach records the nocturnal migration of Common Scoter over the Iberian Peninsula and also includes this map of the breeding range:
© The Sound Approach 2017
The Sound Approach team mapped hypothetical migration routes, plotted from nocturnal sound recordings, as green dotted lines on the map above. By joining up the green dotted lines that pass from Iberia to those that pass across the English Channel, to those that represent the Baltic flyway, one can plot a path that passes across south-east England and potentially, Oxfordshire. Although these green lines are hypothetical migration routes, they neatly explain the pattern of records of Common Scoter in Oxfordshire.
Common Scoter in Oxfordshire is considered to be an annual scarce passage migrant, see the table below. An analysis of records submitted between 2002 – 2012 show three peak periods of occurrence. The first is in spring, with frequent records in March and April, which coincides with birds moving from wintering areas towards their breeding grounds.
The second peak of Common Scoter records in Oxfordshire is in July. These mid-summer records are noteworthy, as the table reveals that one of the best months for recording Common Scoter in Oxfordshire is outside of the expected spring and autumn passage periods.
This spike of mid-summer records may be explained by the complex post-breeding behaviour of Common Scoter. After breeding Common Scoter move to favoured moult sites. In the UK large gatherings have been reported off the east coast of Scotland and in Carmarthan Bay. The numbers involved are far greater than the small UK breeding population of under 200 pairs, so must involve birds from other breeding areas. In Europe, an extensive moult migration takes place, with large gatherings of moulting Common Scoter in the Baltic, the eastern North Sea and off western France (The Migration Atlas, BTO, p.689) . The number of records of Common Scoter in Oxfordshire in July could be explained by birds moving towards moult sites after breeding.
The main bulk of Oxfordshire Common Scoter records are from the autumn and early winter period of September to December. This coincides with the movement of birds from their moult sites to their wintering grounds. Studies of the British breeding populations show that birds disperse widely to a range of wintering sites. The Icelandic population is the same: birds from the same breeding areas spend their winters in different wintering areas over a large geographical range. As such, birds from a wide range of breeding areas will move south and west in autumn and some of these birds may be attracted to large water bodies within the county as staging posts on their migration. Virtually all records of Common Scoter come from Farmoor Reservoir, in the period 2002 – 2012 there was just one exception, a bird at LWV pit 60 on 18th September 2006. As summer approaches, so will post-breeding Common Scoters, heading to a reservoir near you.