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 tbe 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. holbelli; 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 of 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, differing 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 Museum, intermediate forms are found upon 
which ornithologists differ 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 difficult 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. 
tslandicus, “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 sent 
back, every one then present could have convinced himself of their specific identity with 
F. gyrfaleo. 
