226 Trotter, Land-Bird Fauna of X. E. America. [juiv 



phenomenon of migration may possibly have originated as an 

 extension of a once more southerly breeding range of various species 

 of birds which followed the widening zone of green and the develop- 

 ment of insect life northward in the wake of the receding ice sheets. 

 The individuals of a species that spread farthest to the north be- 

 came the migrating element, passing over the intervening areas. 



The present land bird fauna of Nova Scotia may have been 

 derived from two faunal stocks — a more primitive boreal one that 

 has occupied the region from remote times, and a later transition 

 fauna which has invaded the peninsula since the reelevation of the 

 previously sunken isthmus. This somewhat venturesome state- 

 ment appears to be borne out by certain facts. Many of the boreal 

 types belong to genera of widespread distribution in both the 

 Nearctic and Palaearctie regions. Such for example are Pinicola, 

 Carpodacus, Loxia, Spinus, Sitta, Regains, Certhia, and Parus. 

 These may have occupied the region even during glacial conditions, 

 for glaciers do not preclude a forest growth and food would be 

 abundant during the short breeding season. Furthermore, these 

 forms probably spread around the subarctic zone in Pleistocene 

 times when possibly, as many geologists believe, a more extensive 

 land relation existed between the eastern and western continental 

 land masses than at present. Indeed, these genera may have a 

 still older history, dating back to the Middle Tertiary, with a more 

 decidedly polar distribution, but this is purely speculative and we 

 have no evidence, fossil or otherwise, in support of this view. 

 These boreal types, as we know, are wide rangers and the glacial 

 winters would find them foraging to the southward, along the 

 borders of the crowded Austral life zone. Narrow straits would 

 offer no barrier; more than likely there was a much greater land 

 area and wider land connections than at present when these hardy 

 species made their seasonal shifts through the then subarctic forests 

 of the region now embraced by Lower Canada and the northeastern 

 United States. The present irregular movements of these birds 

 may possibly be the result of a habit of wandering widely in search 

 of food, impressed upon them by the precarious conditions of 

 existence during the Glacial Period. 



The purely American element in the present boreal fauna, such 

 as the Thrushes, the several species of Wood Warblers, the Junco, 



