ICELAND FALCON. 47 



semble each other, so much so, that it is often not easy at 

 first sight to separate them, especially as the Icelander, like 

 the Greenland Falcon, is subject to a considerable amount of 

 variety in the prevailing shade of tint, and it is quite possible 

 that examples of the true Gyr-Falcon have occurred in these 

 islands, and have been mistaken for the commoner form. 

 As a rule, however, it may be asserted that in the Iceland 

 Falcon the crown of the head is lighter, and generally much 

 lighter, in colour than the back, while in the Gyr-Falcon the 

 crown of the head and the back are of the same hue, or the 

 former is darker. In the Gyr-Falcon, also, there is com- 

 monly a very perceptible black mystacial streak or patch, 

 which in adults of this form is often as much developed as 

 we find it in the Common or Peregrine Falcon, and the 

 coloration generally is darker than in the Icelander. The 

 late Mr. Hoy, who was well versed in Falconry, and seems to 

 have been the first English writer to clearly distinguish the 

 two forms, has pointed out (Mag. Nat. Hist. vi. p. 108) some 

 other differences. The Icelander, he says, rather exceeds the 

 Gyr-Falcon of Norway in size ; the tail is considerably 

 shorter ; the wings are, in proportion, longer, and the head 

 is larger, so much so, that, in modelling the hoods for 

 trained birds of the two kinds, falconers use different blocks. 

 Whether all these distinctive features can be established on 

 the comparison of a large series of specimens, is perhaps 

 uncertain, but it does appear that in some parts at least of 

 the structure of the two forms there exists a remarkable 

 difference of proportion, which does not seem to have been 

 hitherto noticed. The average length of the sternum and 

 coracoid in Falco islandus, as ascertained by the careful 

 measurement of six female specimens, not specially selected 

 for the purpose, in the Museum of the University of Cam- 

 bridge, is 5-46861 in., while the average length of the same 

 bones in as many specimens of F. gyrfalco of the same sex, 

 and in the same Museum, is 5-06383 in. This would at 

 once show that the Icelander has the longer body of the two, 

 by nearly half an inch; but the difference becomes more 

 striking when it is found that the breadth of the sternal 



