273 Prof. S. F. Baird on the Distribution and 



ance in the Rocky Mountains. The following is a list of those 

 collected by Mr. Drexler at Fort Bridger (about in lat. 41°, long. 

 110°) in the centre of the Rocky Mountain range, nearly all of 

 which have been found still further to the north-west, toward 

 Puget Sound. The birds found at Fort Bridger probably 

 arrived there by way of the Platte, those of Washington Territory 

 by both the Platte and upper Missouri. 



Although thus extending westward, almost, if not quite, to 

 the Pacific, along the northern boundary they appear to always 

 return the way they went, as none of the species have yet been 

 met with in California. 



I have added to each species the locality on the Missouri River 

 up to which it was observed by Dr. Hayden in one of his early 

 explorations. 



Species of Eastern birds found at Fort Bridger. 

 Tyrannus carolinensis. Fort Union and Yellowstone. 



Turdus fiiscescens. 

 T. swainsoni. 

 Siurus uoveboracensis. 

 Dendroeca coronata*. 



Setophaga ruticilla. Fort Pierre. 



Vireo olivaceus. Fort Union (Mr. Audubon). 



Mimus carolinensis. Fort Lookout. 



Zonotrichia leucoplirys. (Not found further west). 



Quisealus versicolor. Fort Benton (Pearsall). 



It will be sufficiently evident, as most birds change their resi- 

 dence from winter to summer, and vice versa, that, unless we 

 devote especial attention to their distribution during the breeding- 

 season, we shall not be able to mark their boundaries with pre- 

 cision. Species which go north to the Arctic circle to nest, 

 return to mix, in Mexico, Guatemala, or the West Indies, with 

 species resident in those countries, or of short migration, and 

 are followed part way in their southern flight by Arctic birds 

 starting from localities stdl further north. The case is quite 

 different with reptiles and most insects and mammals, of which 

 a few species only change their residence or leave their place of 

 birth, not in obedience to the instinct of reproduction, but of 

 necessity caused by overcrowding, the search for suitable food, &c. 

 * Found by Dr. Suckley on Puget Sound. 



