NATURAL SCIENCES OP PHILADELPHIA. 



95 



blue birds marked "jeune." It is the same with specimens from 

 Copenhagen and elsewhere. Now it being understood tiiat, as I 

 have above said, the age of the ])ird may be detected from the 

 color, and, still better, from the direction of the markings, it will 

 then be evident that in a large series you have what at first sight 

 appears to be almost every step from the nearly pure white phase 

 — wliieh some consider, tliough I do not, to be the F. arcticus of 

 Holboll — to the dark-colored F. lahradora of Auduljon, and it is 

 not easy to see how thej^ can lie distinguished. Easy it is, how- 

 ever, on trial. Sort out all the specimens with white l)ills and 

 claws (tlie white is often flesh-colored owing to extravasated 

 blood), and then you will have Falco candicans. Then turn all 

 the other specimens on to their l)ellies, and lay in one heap those 

 that have the tops of their heads not darker than their backs, 

 and on another those that have the tops of their heads not lighter 

 than their backs. The first of these heaps will be F. islandicus, 

 and the second F. gyrfalco. You will have perhaps some five per 

 cent, that this test will not reach, and this remainder will require 

 further comparison; but I am much mistaken if the "moustache" 

 will not enable 3^ou to distriluite tlie balance. Then you may 

 look at the labels — always being, of course, suspicious of French 

 geography — and I think you will have something like this result : — 



BILL AND 

 CLAWS. 



White or ( 

 pale flesh < 

 color. ( 



Dusky 

 horu 

 color. 



PLnjIAGE ABOVE. 



White with dark markings. 



' Top of head not"! 

 diii'ker than 1 



back ; raous- | 

 tache light. J 



Dark 

 with 

 liglit 

 mark- 

 ings. 



Top of head not~l 

 ligliter than I 

 hack ; raous- f 

 tache heavy. J 



F. eandieans,^ 



r 



F. islandicus,'^ 



LOCALITIES. 



Adult. 



N. Greenland, 

 N. parts of fur 



countries, 

 Siberia ? 



Southern 



Greenland 

 and Iceland.' 



■f. gyrfalco, -{ 



I Norway, 

 Sweden, 

 Finland, 

 Siberia ? 



Immature. 

 Iceland, British 

 Islands, Nor- 

 way, Sweden, 

 Canada, U. S., 

 " Siberia" 



(Pallas), 

 (in Mus, Berol), 

 " Amoor" 



(Schrenck). 



British Islands, 

 Norway, Ice- 

 land, Labrador, 

 Canada, U. S. 



Norway, Swe- 

 den, Holland, 

 N. Germany. 



I P. S. 6 May, 1871. Since this letter was written Prof. Baird has kindly 

 sent me some spcchnens from Alaska to examine. These are adult, and 

 differ from Icelandic examples only in being slightly darker. (P. Z. S. 1870, 

 p. 384.) "Alaska" should, therefore, be added to the localities named above. 

 1871.] 



