150 THE BIEDS OF MONGOLIA ETC. 



true F. sacer ; whilst on F. hendersoni, as also on our three specimens the 

 breast is milk-white, marked with narrow and triangular small spots. The 

 bill is black at the point and bluish at the base, and has only on the lower 

 mandible a yellow mark, which colour is predominant on both mandibles 

 in our three specimens. Consequently the young-plumaged F. hendersoni is 

 nearer to F. sacer^ which, however, is sufficiently developed to be separated 

 as a species. 



Measurements: — 





Length. 



Width. 



Wing. 



Tail. 



Gape. 



Tarsus. 



Middle toe. 





in. 



in. 



in. 



in. 



in. 



in. 



in. 



,3 . 



. 18-5 



33 



14-8 



7-8 



1-23 



1-9 



1-78 



2 . 



. 22-5 



37 



16-6 



93 



1-27 



216 



2 



Henderson's Falcon was found by us wherever we went, from Kiachta 

 down to the sources of the Yantze-kiang ; but it was most numerous in 

 winter in the Zachar country and about Koko-nor, which locaHties abound 

 with Alpine hares ; and these, at least in winter, form its principal food. 



This Falcon also attacks birds, such as Syrrhaptes paradoxus, usually when 

 the latter are drinking. Once it threw itself upon a hare which we had 

 started, and followed it, constantly swooping down upon the animal and 

 hitting it with the beak; after every blow the hare stopped and went on 

 running after a time, until we lost sight of it and consequently could not tell 

 how the attack terminated. 



The Mongols and Tanguts do not train these Falcons for sport ; at least 

 we never saw it during our travels. 



14. Hypotriorchis subbuteo, L. Socol Cheglock, 



We met with it occasionally in Mongoha, about Muni-ul and Hara- 

 narin-ul, also at Northern Halha and Urgey ; but in neither of these localities 

 is it common. It has also occurred at Lake Hanka and the river Sufun. 



