ON NOUTJI AMERICAN ZOOLOGY. 189 



accidental visiters, and conclude with some authors that the 

 species properly belonging to a district are only those which breed 

 within its limits, we shall then find that in North America the 

 number of breeding birds increases as we go northwards, up to 

 the 62nd degree of latitude, where the woods begin to thin off. 

 Even on the verge of the barren grounds, near to the arctic 

 circle, as many species breed as in the neighbourhood of Phila- 

 delphia, though in the latter locality some birds rear two or 

 more broods in a season, which is not the case in the north. 

 The Prince of Musignano states the number which hatch near 

 Philadelphia, near the 40th parallel, at 113, while fourteen de- 

 grees farther north, at Carlton-house, on the Saskatchewan, the 

 number amounts to 149, and the difference would no doubt be 

 greater in favour of the latter place were its ornithology more 

 thoroughly investigated ; but all the species included in our 

 estimate were detected in tiie course of a single spring by Mr. 

 Drummond and myself. 



The amount of species which reside the whole year in any 

 one place has no direct relation to the numbers which breed 

 there, but is regulated chiefly by the winter temperatures, or, in 

 Humboldt's phrase, by the course of the isocheimal lines ; and 

 it seems evident that it is the diminution of supplies of food, 

 and not the mere sensation of cold, which occasions birds to 

 migrate from the high latitudes on the approach of winter. 

 After the spring movement, the feathered tribes are often ex- 

 posed in the fur countries to much lower temperatures than had 

 occurred before their departure in autumn ; and the eagle and 

 other kinds which soar above the summits of the highest moun- 

 tains, do not appear to be inconvenienced by the rapid change 

 of climate to which they thus subject themselves. All the 

 birds which feed on winged and terrestrial insects and worms, 

 such as the fly-catchers, vireos, and warblers, must migrate 

 from the northern regions, as well as most of the aquatic and 

 piscivorous tribes, the suctorial tenuirostres, and all the gralla- 

 tores, which thrust their bills into soft spungy soil in search of 

 food. The wood-peckers, though insectivorous, are more sta- 

 tionary, because the larvae of the xylophagous beetles, on which 

 they subsist, lodging in trees, are as accessible in winter as in 

 summer ; but the colaptes auratus, which feeds mostly upon 

 ants, and the picus varius quit the snow-clad fur-countries in 

 winter, while they are permanent residents in the more southern 

 districts. 



The only bird seen at Melville Island, in latitude 75° N., 

 during winter was a white one, supposed to be the strix nyctea, 

 or it may have been a wandering falco islandicus, both these 



