THAT ORKNEYS THRUSH: WAS IT FROM NORTHERN ALASKA - SHOULD IT BE IN SOUTHERN CALIFORNIA?

                                               

Varied thrush - Did Orkneys bird mistake Orkneys for Los Angeles County? 

    

AMID  speculation about the origins of the Varied Thrush that recently spent a few days in The Orkneys, a paper published more than 120 years ago might offer some pointers.

American ornithologist Joseph Grinnell identified plumage differences that led him to conclude that were two separate breeding races in Alaska - one on the coast and one inland.

Below is an abbreviated version of the report that he wrote for the April 1901 edition of The Auk, quarterly journal of the Nuttall Ornithological Society.

"Well-marked differences exist in the case of the female between the race of the Varied Thrush (Hesperocichla naevia naevia) breeding in the humid area around Sitka on the South-east coast of Alaska and that of the race (Hesperocichla naevia meruloides) breeding in the dryer interior region in the north of the same state. 

"The Sitkan race is characterised by a predominance of deep browns, restriction of white or light markings, and by a shorter and more rounded wing.

"By contrast, the northern and interior race (Hesperocichla naevia meruloides) has a much greyer and paler coloration, greater extension of white markings, and a longer and more pointed wing. 

"Unfortunately I have no male birds from Sitka, except juveniles, but three spring males from the Kowak Valley in the North-west of Alaska when compared with late-winter males taken from northern California are of a lighter slate colour dorsally and slightly paler tawny beneath. 

"The females of this species appear to be much more subject to protective coloration, so-called, than the males, and it is therefore reasonable to expect climatic variations to be more pronounced in the females than in the males, especially when the climate of the sunnier habitat is of an extreme nature. 

"In the winter home of Varied Thrushes, there is also a different distribution of the two races, but their latitudinal relation is reversed.

"Thirty-five skins from Los Angeles County, in southern California, are all but one referable strictly to birds (meruloides) from northern Alaska while the majority of the winter skins from the coast region of central and northern California are of the Sitkan race (naevia). 

"In other words, though its summer habitat is more northerly, meruloides migrates further south in winter than naevia. Its migration route is much the longest. 

"Naevia's apparently much shorter migration route probably extends at most between the latitudes of Sitka and Monterey."

On the basis that the meruloides race has the capacity to migrate longer distances, is it fair to assume that The Orkneys bird is from northern Alaska and should, by rights, now be in southern California? 

Grinnell ends his article thus: "The wing-contour seems to offer a criterion by which to judge the length of the annual migration of a bird - by the study of further material I hope to arrive at some more definite conclusions in this regard."