Greenland Wheatears drift further apart from Northern

This nominate Northern Wheatear can now be proven as such by its measurements and stable isotopes. Photo: Steve Young (www.birdsonfilm.com)
This nominate Northern Wheatear can now be proven as such by its measurements and stable isotopes. Photo: Steve Young (www.birdsonfilm.com)

Criteria for identifying Greenland Wheatear have been further refined in a new study.

An intangible test of British birders' skills is the ability to confidently identify a Greenland Wheatear, the subtly different subspecies of Northern Wheatear which passes through Britain and Ireland every spring at a slightly later time than the birds found in Britain itself. The birds are notoriously difficlut to identify in the field, and only birds at the extreme ends of each form's ID criteria should really be assigned to subspecies. Four of the currently accepted subspecies of Northern Wheatear are recorded from the Palearctic, but two are seen in Britain: Greenland Wheatear, Oenanthe oenanthe leucorhoa, which breeds in Greenland, Iceland and north-east Canada, and nominate Northern Wheatear, breeding in Europe, Siberia, Alaska and north-west Canada.

The same two subspecies also pass through the German island of Helgoland, the well-known rarity hot-spot, at the same time of year. A team from the Institute of Avian Research in Wlhelmshaven, Germany, trapped Northern Wheatears at several migration watchpoints in Europe known for their high turnover of migrants: Heimay, Iceland, Fair Isle, Scotland, Gamvik, Norway, Helgoland and Wilhelmshaven, Germany, Rybachy, Russia, Ventotene, Italy and the British protectorate of Gibraltar. The researchers took standardised measurements of wing length, seven to nine primary feathers, tarsus, morphometric wing shape, bill to skull length and tail length for both sexes of wheatear; they also collected tail feathers during the breeding season in the hope that a comparative stable isotope analysis would reveal the natal areas of migrating birds, when correlated with measurements.

The workers hoped to pin individual birds down to four populations: preseumed Greenland birds, Icelandic birds, Scandinavian birds and those migrating along the Baltic sea coast, presumed to be from European Russia. A discriminant function analysis was used to statistically distinguish between population measurements with an accuracy of between 87 and 96 per cent, and resulted in clustered distinctions between all four groups of migrants. Presumed Greenland migrants in particular, clearly stood out from Icelandic birds on Helgoland, and for the three isotopes measured, all four groups showed measurable differences.

It became apparent that measurements can be used to differentiate between populations with a great deal of accuracy. Greenland birds, known particularly for their long-distance almost non-stop over-water migration to West Africa as far south as Mali, are confirmed to have more pointed wings, and the results for all four population groups correlate well with what has been gleaned from ringing recoveries. Greenland Wheatear has been reaffirmed by this study as a diagnosable subspecies using the discriminant formulae derived from the study, and has an eastern bias to its migration, with most presumed leucorhoa being trapped at Fair Isle, Helgoland and Gibraltar.

The present study throws no further light on actual field identification of Northern Wheatear populations, but will certainly refine the criteria in the hand.

Delingat, J, Hobson, K A, Dierschke, V, Schmaljohann, H and Nairlein, F. 2010. Morphometrics and stable isotopes differentiate populations of Northern Wheatears (Oenanthe oenanthe). Journal of Ornithology DOI:10.1007/s10336-010-0599-4.