ORNITHOLOGICAL EXPLORATIONS. 353 



they gradually invaded those outlying regions where we have made their 

 acquaintance. It will suflBce for our present purpose to start from the 

 region east of Lake Baikal, the so-called province of Transbaikalia. 

 The effort of the species to extend the limits northward and eastward 

 forced them to move eastward gradually along the southern slopes of 

 the Jablonnoj and Stanovoj Mountains, finally, by degrees, reaching 

 the Okotsk Sea at Udskij. From here they only had the narrow slip 

 of land between the sea and the high Stanovoj Mountains, which they 

 could not cross. Year aftei" year the pioneers advanced a little farther 

 to the northeast. Finally the swallow reached a latitude beyond which 

 it was not comfortable for it to proceed. But a glance at the map (p. 359) 

 will show us that the sea separating it from Kamtschatka is too insignifi- 

 cant an obstacle for the swift swallow, which found suitable quarters 

 and plenty of food in the mosquito-ridden peninsula, where it found no 

 very nearly related comi>etitor. The wagtail, on the other hand, could 

 stand the northern climate better, and j)ushed on until the outposts 

 reached the shores of Berings Strait, across which they will probably 

 continue their emigration before long. That Motacilla ocularis in reach- 

 ing Penschinsk did not extend southward over the j)eninsula may be 

 explained by the fact that it found Kamtschatka already occupied by 

 the closely allied Motacilla lugens. 



On a former occasion I have made the remark that Kamtschatka is 

 not only interesting on account of the birds it possesses, but, perhaps, 

 even more so on account of those which are absent from its ornis. To 

 illustrate this more fully we shall undertake a comparison wi'uh the avi- 

 fauna of the district at the mouth of the Amur Eiver and at Udskij 

 Ostrog, giving in a table all the species especially mentioned as col- 

 lected by V. Schrenck and Middendorff" in the former localities, which 

 are situated at about the same latitude as Petropaulski, and not known 

 to occur in Kamtschatka. Such forms are excluded, however, which 

 may be regarded as representing each other, as for instance, the corre- 

 sponding races of Urogallus, the different forms of woodpeckers, &c. 

 I have retained his nomenclature without any changes, since in many 

 cases it is difficult to decide, without specimens, the exact name of the 

 species which he collected. 



15861 Bull. 29 23 



