THE BIRDS OF MONGOLIA ETC. 159 



On comparing the Mongolian C. apus with European specimens, we could 

 not detect any difference, except the somewhat paler coloration of the former, 

 for which reason Mr. Swinhoe (P. Z. S. 1871, p. 345) made the new species 

 C. pekinensis ; but the Daurian and Baikal birds do not differ at all from 

 the European ones, not even in the shading of the plumage. 



They arrive in S.E. Mongolia about the middle of May, and leave about 

 the end of August; and during our voyage from Ala-shan to Urgey, through 

 Gobi, we met with the migrating C. apus throughout the month of August. 

 Then, again, on the 1st of September, at a distance of about 150 versts south 

 of Urgey, we saw at twilight a great flock of C apus migrating in a south- 

 westerlv direction. 



28. Cypselus pacificus. Lath. 



Cypselus pacificus, Gould^ Birds of Australia^ ii. pi. xi. 



Was found by us throughout S.E. Mongolia, Ala-shan, and Kan-su. 

 Contrary to the preceding species, the present one inhabits only the highest 

 mountains, and there the most rocky, wild, and inaccessible localities. Only 

 in the lofty Kan-su does it breed in the central and lower zones, but is 

 numerous there also in the alpine regions at an absolute height of 12,000 feet, 

 or even higher. 



By its habits, character, and voice (the latter only being somewhat 

 weaker) Cypselus pacificus does not differ from C, apus. 



During whole days these birds are flying round, shooting through the 

 air and about the rocks and cliffs ; but in the mornings and evenings they 

 go into the steppes, w^here they usually fly close to the ground, occupied in 

 catching insects. 



C. pacificus arrived in S.E. Mongolia very early in 1872. In the Muni-ul 

 mountains we observed the first specimens on the 12th of April, which at 

 once took possession of their old quarters. They reappear in Kan-su about 

 the middle of May, and take their departure towards the end of August. 



