OCR Text |
Show 418 MR. R. B. SHARPE ON FALCO ARCTICUS. [Apr. 1, irregular shape of fig. 9 indicates that it is not perfect, and that it undergo a further change ; and although the marking is not of the same form as the feather placed in juxtaposition, they both have the same object in view, and are neither of them so very unlike fig. 10. The heart-shaped spot is, as far as we at present know, the limit beyond which adult Greenland Falcons "can no further go." To return to the question of the light and dark races, I would ask the advocates of that view of the question to which they would refer a specimen with the dorsal feathers as illustrated in figs. 7 and 7 a. The right-hand figure belongs to the young of the " light" race, but the left-hand one should be referred to the "dark" race. It is to me self-evident that fig. 7 is the previous stage of such a feather as fig. 9 a ; and it is equally clear that fig. 7 a is the antecedent stage of fig. 7. The tail (fig. 8) is also closely allied to fig. 11, but still shows traces of the markings visible on all birds of the " dark race," which have here not entirely dissolved; thus the tail, as well as the dorsal featherings, is intermediate between the two races, and to my mind illustrates their connexion. The sequence from the triply barred stage (fig. 6) to the longitudinal drop is not so clear, unless these changes are more rapid than we have at present supposed, or that the feather regains its longitudinal form by the gradual closing up and re-joining of the bars, which, from the irregular outline on the middle bar of fig. 6, seems not absolutely impossible. It appears likely, however, that a partial moult may intervene between the stages indicated in fig. 6 and figs. 7 and 7 a ; for I have noticed in some Accipitrine birds changes which induce me to believe in a rule which may be enunciated thus:- Rule 3. That in the progress to maturity, when the changes of plumage are very different from one another, the new dress still retains an indication of what the former one was like before the moult. Thus, if fig. 7 a is the result of a moult, it still shows evidences of the previous barred plumage, though they quickly disappear (fig. 7) and pass through the stages (figs. 9, 9 a, 10) to the final spots (figs. 10 a, 12, 12 a), which, from their irregular outline, might seem intent upon getting even smaller still. Let any one who doubts the possibility of markings such as those on the Greenland Falcon becoming gradually changed without an intermediate moult, study the changes exhibited by the common Sparrow-Hawk in its progress towards maturity. The general characteristic of the species of Accipiter is to have a striped plumage when young and a barred dress when old. But it is not generally known that this is effected by a gradual change in the markings of the feather, and not by an actual moult. Just as in the Greenland Jer Falcon we could tell the age of a bird by the state of the bars on the tail, so we can tell that of a young Sparrow-Hawk by the extent of the rufous edging to the feathers of the upper surface : if these are very broad and distinct, the bird is quite young; for they gradually wear off as it progresses in age. On the first appearance of the feathers from the downy covering of the nestling, |