18 BRITISH BIRDS. 



thoroughbred bird has a white tail at all ages, and entirely white under- 

 parts in the adult ; the upper parts below the head are sparingly marked 

 with tear-shaped spots. In the young the feathers of the back are brown, 

 with white margins and bases, and a few longitudinal streaks appear on 

 the head, breast, and flanks. In birds that are not thoroughbred, the 

 spots on the back gradually broaden until they become bars ; and examples 

 may be found showing every intermediate form between a few spots on the 

 tail and flanks and a perfectly barred tail and flanks in the adult, and in 

 the young with the addition of spots on the breast. Where the back is 

 barred and the thighs are streaked only or white, it is the so-called dark 

 race of the white Jer-Falcon, Sharpe's intermediate stage between young 

 in first plumage and adult of that bird. When the thighs are barred and 

 the breast white, it is Sharpe's adult F. holbcelli ; and when, in addition to 

 the barred thighs, the breast is spotted, it is Sharpe's supposed intermediate 

 stage between young in first plumage and adult of that bird. The white 

 edges to the feathers of the back in the young o£ these half-bred forms 

 have become pale brown, and every feather of the underparts has a con- 

 spicuous brown longitudinal streak in the centre. All these intermediate 

 forms are found in Greenland, and are connected with another series of 

 intermediate forms, also found in Greenland, with the Iceland birds, F. 

 islandus, difli'ering but little from the preceding in first plumage, but always 

 being streaked on the breast in the adult. The changes I have described 

 are also accompanied by a greater development of the dark spots on the 

 head, which, in the thoroughbred F. gyrfalco, are almost distributed over 

 the entire feather, causing the head to look nearly uniform dark brown. 

 In western North America intermediate forms occur between the Iceland 

 and Norwegian birds*. The selection of any one of these intermediate 

 forms is purely arbitrary ; and between the two extreme forms it is just 

 as easy to make ten subspecies as two. Even in such a comparatively small 

 series as that in the British i\Iuseum, intermediate forms are found upon 

 which ornithologists dififer in opinion as to which race they should be 

 referred. 



Three at least of the four principal forms of Jer-Falcon above enu- 

 merated have occurred at various times in the British Islands. From the 

 manner in which the several forms of this Falcon have been confounded, 

 it is extremely diflScult to apportion the " large Falcons " that have so often 

 visited our shores to their respective subspecies. It is very evident that 

 the white Jer-Falcon was well known as a British bird a century ago ; and 



* Compare P. Z. S. 1870, p. 384, where Newton refers them "without doubt'' to F. 

 islandicus, " though belonging to the darker phase of that form," with P. Z. S. 1875, p. 115, 

 where Dresser asserts that, if the American specimens had not unfortunately been ssent 

 back, every one then present could have convinced himself of their specific identity w ith 

 F. yyrfalco. 



