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. cundicans, to which F. 

 holbcelli in its mature state bears some resemblance (as noticed by 

 Holbbll) ; 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 my Falco holbcelli. 



Professor Schlegel has truly remarked that the difficulties of 

 studying Birds of Prey are enormous ; and this opinion I can-indorse 

 from my 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 

 and F. 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 endea- 

 voured 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 takan 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. : — 



Bule 1. That no two birds of the same species absolutely follow 

 the exact sequence of change in their assumption of the adult plu- 

 mage. 



llule 2. That not every species of the same genus gains the adult 



* Qf. my notes on Petrocossyphus, in P. Z. S. 1872, p. 406, and in Sharpe and 

 Dresser's ' Birds of Europe.' 



