Sunday, 16 September 2007

Pecs from the east?

The past week has seen the arrival of two species that are traditionally considered to be of North American origin; Pectoral Sandpiper and Buff-breasted Sandpiper. However, looking at a map of records of these two species from the past 7 days seems to contradict this theory, for Pectoral Sandpiper at least. The westerly distribution of the Buff-breasted Sandpipers (14 individuals at 8 locations, shown in blue) fits nicely with what is expected from a Nearctic vagrant wader. The distribution of Pec Sands (18 individuals at 16 locations, shown in red), on the other hand, shows a bias towards the east & south-east; not what is expected and perhaps hinting at the direction of the birds' origins. There is certainly no reason to suspect that the birds in the west, e.g. on Lewis and in Co. Derry, are not of American origin, but it seems far less likely that the 3 birds in the south-east & the 5 birds in East Anglia arrived direct from the USA.

A potential explanation for this pattern of occurrence is given by Lees & Gilroy in British Birds 97 (December '04), pp.638-646, where they explain that many of the Pectoral Sandpipers we see in autumn could very well be birds from the far west of the species' range in northern Siberia travelling along a newly set migration route to wintering grounds in Africa.

Like the large numbers of Yellow-browed Warblers passing through the UK each Autumn and making their way down through western Europe, these 'pseudo-vagrant' Pecs just go to show how little we still know about the movements of wild birds!