18/3.] MR. R. B. SHARPE ON FALCO ARCTICUS. 417 



plumage by the exact sequence of change followed by another allied 

 species. 



There can be no doubt that when we thoroughly understand the 

 changes of plumage undergone by species of birds, a new light will 

 be thrown upon ornithology as regards the relation of geographical 

 races and subspecies. 



In treating of the Greenland Falcon, one difficulty always pre- 

 sents itself; and that is, the almost impossible chance of getting 

 specimens correctly sexed and dated ; and in a study of this kind this 

 is half the battle. To capture a specimen, and watch the gradual 

 changes in confinement, would doubtless afford some clue; but the 

 species under consideration would be an unsuitable one for experi- 

 ment, as there can be little doubt that confinement in England 

 would produce more or less the consequences of arrested develop- 

 ment in the plumage, by destroying the need of assimilative colour- 

 ing which induces the species to become white in the regions it 

 inhabits. 



Fig. 2 represents the centre tail-feather of the bird from whose back 

 I took the first feather (fig. 1 ); and it will be noticed that the lower bars 

 give traces of approaching dissolution. The way in which this takes 

 place is well illustrated in fig. 3, which is the tail-feather of a slightly 

 older bird. Fig. 4 represents the back of a young bird changing 

 from its first into its second plumage : x is the old feather, very 

 similar to fig. 1 ; and the darker feathers are the new ones being 

 donned. Fig. 3 is the middle tail-feather of this identical specimen ; 

 and thus it appears that the commencement of the great change of 

 tail takes place about the time of the first moult. Fig. 5 is a feather 

 taken from a bird not yet fully adult, but in full clean-moulted 

 plumage ; it has a tail in the same stage as fig. 3, and is doubtless 

 very little older than the specimen whose tail is thus represented in 

 the Plate : in fact it is in the full plumage indicated by the new 

 feathers (fig. 4), and shows its slightly advanced age by the greater 

 extent, of the white indent. The step from this stage of the dorsal 

 feathers to the next (fig. 6) is tolerably evident ; for here the bars, 

 indicated in the previous stages, are quite complete. A bird thus 

 marked is the adult of the "dark race" of Mr. Gould, which we 

 have thus followed from its young to its perfect plumage. 



It will perhaps reuder my argument more intelligible if, for the 

 present, we leave aside the feathers represented in figs. 7, 7 a, and 8, 

 and proceed atonce to the consideration of the so-called "light race" of 

 F. candicans. It is a remarkable fact that, although there exists great 

 difference in the tail-feathers in the dark race, the light form should 

 have the tail nearly uniform white in both young and old. Thus 

 figs. 9, 9 a are taken from the back of a bird in the " tear-dropped " 

 plumage, which is supposed to be the young, and figs. 10, 10 a, 12, 

 12 a, are all from the backs of very old birds. With this last fact 

 I perfectly agree ; but so far from considering the feathers figured 

 as 9, 9 a to be those of a young bird, I consider that they are the 

 sign of a very old specimen, only one whit less old than the one from 

 whose back the feathers figs. 10 and 10 a have been drawn. The 



Proc. Zool. Soc— 1873, No. XXVII. 27 



