Eider - ages

Somateria mollissima

Most observers reporting during the years 1981 to 2006 noted the age of birds seen. 61% were classified as female, 15% as immature males and 24% as adult males. Adult males were rarely seen during the winter months but were more frequently seen among passing groups. Illustrating this point, of 74 adult males recorded during the period, 62 of them migrated past Strumble Head.

Graham Rees.

(Covers records up to and including 2008).

References

MATHEW. M. 1894. The birds of Pembrokeshire and its islands, R. H. Porter.

Great Northern Diver - ages

Gavia immer

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.

Graham Rees

(Covers records up to and including 2008)

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).