Io6 Loomis on Birds of Chester County, South Carolina. I April 



extremity of winter range, particularly in boreal species, varia- 

 bility arising from southward progression is not to be mistaken 

 for variability arising from shifting in lines of movement. The 

 southern limit of many winter birds is not sharply defined any 

 more than the northern limit is, there being a soil of reserved 

 ground between the extremes of southward movement of a 

 species. 



Where there is exceptional movement it is maintained that 

 there is exceptional cause. Such movement and cause were 

 well illustrated when the hordes of Prairie Horned Larks were 

 forced soutli into this region in the winter of 1876-77, when the 

 snow lay on the ground for a length of time unprecedented in 

 my experience, covering up the food-supply so that the greatest 

 abundance occurred in the wake of the snow, as it receded 

 northward. 



As great movements of Snowy Owls and Evening Grosbeaks 

 take place when there is no unusual cold some other immediate 

 cause than failure of food through sudden severity must be 

 sought for. While local abundance in the Snowy Owl may not 

 improbably be due sometimes to variability in lines of movement 

 of isolated communities, coming from the southern frontier of 

 the breeding range, the wide-extended invasion of the autumn 

 of 1876 (Rep. Bird Migr. Miss. Vail., p. 123 ; Deane, B. N. O. 

 C, II, p. 9) can only be imputed to unusual migration southw.-ird. 

 Because the effect and not the cause of the movement was wit- 

 nessed, there is no necessity for assigning it to the domain of 

 inexplicable mystery. It should be kept constantly in mind that 

 we view the migratory movements of this Owl from the south- 

 ern portions of its range and that more than two thousand miles 

 intervene between Grinnell Land, where it is reported as breed- 

 ing abundantly, and the northern boundary of the United States. 

 As the species is a regular seasonal migrant, it is not extraordi- 

 nary that there should be variations in abundance in the United 

 States, that the migrations of the numbers should be protracted 

 further south in some seasons than others. The incursion of 

 Robins in January, 18S7 (1. c.) with the advent of snow, lessen- 

 ing the food area, proves that there may be migration at the 

 South in a bird that thrives where the mercury freezes when 

 food is plenty. Hence, if some Snowy Owls remain in the high 



