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Show 416 MR. R. B. SHARPE ON FALCO ARCTICUS. [Apr. 1, give it a much lighter and more mealy aspect, but, above all, by its pure white chest, which is devoid of all streaks or tear-shaped drops as in the true Iceland Jer Falcon. It is altogether a much lighter bird than its congener, especially on the head, which is white, with a few blackish lines. The male bird in the national collection is beginning to lose the bars on the two centre tail-feathers, where they are breaking up into minute frecklings, which I never found to be the case in the Iceland Jer Falcons I have examined. This dissolution of bars on the tail has been noticed in F. candicans, to which F.. holbcelli in its mature state bears some resemblance (as noticed by Holboll); but that species loses its mottled or irregularly barred tail before it becomes very old-long before it attains the age of the specimens which I have made the types of m y Falco holbcelli. Professor Schlegel has truly remarked that the difficulties of studying Birds of Prey are enormous ; and this opinion I can indorse from m y own experience. But the adult birds at least of all the northern Falcons, ought to be easily recognizable to any one who will take pains ; and in studying Accipitres nothing but the greatest amount of patience will produce a satisfactory result. A Greenland Jer Falcon can be told at any age by its white flanks and thighs, which have never more than a small longitudinal streak of brown along the shaft, whereas in the other three species these parts have large dart-shaped marks almost worthy to be called cross bars. The Norway Jer Falcon has a uniform dark head, whereas F. islandus andcF. holbcelli have both white heads narrowly streaked with black. Of the differences between these two birds I have spoken above ; and I believe that by following these points any careful student will make out the species. I must add that, after much laborious study, I have failed to find a character by which the young of the three last-named species may be distinguished one from another. It will be noticed above that I have alluded to the " so-called" dark race of the Greenland Jer Falcon ; and I may at once state that I believe this race has no existence at all in nature. I have endeavoured to trace the sequence of Hawks during their progress towards maturity ; and by the help of a good series of birds I am able to offer to the Society to-night some interesting facts bearing on the changes of plumage, on which subject I have already made a few remarks*. The accompanying Plate (fig. 1) represents a feather taken from the back of a very young Falco candicans; and in a study of this species the middle tail-feathers must always be taken as indicative of the age of the specimen : in the old bird the tail is always pure white, and in the young one is irregularly but completely barred. In cousidering the progress of Accipitrine birds towards maturity, it is necessary to bear in mind two rules, viz. :- Rule 1. That no two birds of the same species absolutely follow the exact sequence of change in their assumption of the adult plumage. Rule 2. That not every species of the same genus gains the adult * Cf m y notes on Petrocossyphus, in P. Z. S. 1872, p. 496, and in Sharpe and Dresser's ' Birds of Europe.' |