422 BULLETIN OF THE 



into the Carolinian, and others over both the Alleghanian and the 

 Carolinian into the Louisianian. 



Some species which in general breed far to the northward of the 

 tropics, to which they retire in winter, are also known to breed on the 

 mountain- within the tropics (as Dendroeca coronata and Perissoglossa 

 tigrina), and doubtless many others will be found to do so when the 

 mountain faunae of these regions become fully known ; it being already 

 well ascertained that there is a migration from the plains and lowlands 

 to the mountains (more especially in the Tropical Realms) as well as 

 (in the northern hemisphere) from the south northward. 



VII. The lack of suitable food and the low temperature in winter in 

 northern latitudes are evidently the causes which impel so many species 

 to leave their breeding stations at that season to seek a warmer zone. 

 "While in most cases a degree of cold sufficient to cut off the supply of 

 food of any species, especially of the insectivorous ones, would of itself 

 also prove fatal to the birds themselves, it is by no means the case with 

 the baccivorous and graminivorous species, their winter migrations ap- 

 pearing to be primarily and principally controlled by the accessibility 

 of their food. This is evidently indicated by the irregular dispersion 

 in winter of such species near the boreal limit of their range at 

 that season, they being numerous where their food abounds and en- 

 tirely absent in the immediately adjoining districts.* 



VIII. The breeding range, as well as the migratory range, differs 

 greatly not only among the species of different families (nearly all the 

 species of some families having a wide range, while nearly all the spe- 

 cies of other families have a quite restricted range, as in the Cor- 

 vidcB and Ficidce, for instance, as compared with the Syhicolidce), but 

 also among those of the same family and even of the same genus. 

 The two extremes are well illustrated by the osprey or fish-hawk and 



* These remarks are illustrated by the winter distribution of the robin and the cedar- 

 bird in tin- Alleghanian Fauna, and by the sudden southward incursion of the snow- 

 buntings and other northern sparrows when deep snows suddenly render their food more 

 than usually difficult to procure in their usual winter reports. The early return of birds 



to their br ling stations, — their real homes, — as soon as the causes that impelled their 



winter migration are removed, is further corroborative of the same view. Most of even 

 the insectivorous species visit regions in winter whose average winter temperature dif- 

 fers but little from that of their breeding stations, and when the excessive heats of spring 

 and summer arrive in the, southern latitudes, they gradually retire again to their north- 

 ern breeding stations, keeping pace in their migration with the northward advance of 

 the summer warmth. 



