1274 U.S. NATIONAL MUSEUM BULLETIN 23 7 part 3 



Because unusual observational opportunities are required to recog- 

 nize such events, we are less aware of them than the facts probably 

 warrant. But it is this sort of random exploration of extralimital ter- 

 ritory that helps account for the ability of so many species to colonize 

 new territory as soon as it becomes suitable. We know, today, that 

 the margin of the tundra which forms the northern limit of the white- 

 crown's range has been both farther north and farther south during 

 the last 10,000 years, and that the range and the population of this 

 species have varied accordingly. 



The eastern white-crown is best described as a subarctic and alpine 

 zone bird, but so extensive is its range that only ecological character- 

 ization is really helpful. This has seldom been done and as a result 

 "life zone" pigeon-holings are rampant with apparent contradictions. 

 The only full awareness of the ecology of this bird in all the material 

 before me is Harrison F. Lewis' excellent description, in a letter of 

 July 1, 1963: "In coasting along the north shore of the Gulf of St. 

 Lawrence from west to east, in latitude slightly north of 50°, one enters 

 the breeding range of the white-crowned sparrow abruptly on passing 

 Saint Genevieve Island, the easternmost of the Mingan Archipelago. 

 The reason for the abruptness of this boundary is that the Mingan 

 Islands, which border the coast for some 50 miles, are formed of lime- 

 stone, as is, also, much of the adjacent coastal mainland. Eastward 

 from Saint Genevieve, the mainland and the coastal islands are of 

 acidic pre-Cambrian rocks, such as granite and gneiss. The vegeta- 

 tion of the limestone belt is much superior to that which grows on the 

 adjacent pre-Cambrian rocks, and the bird life reflects this; for ex- 

 ample, the ovenbird nests on the limestone of the Mingan region, but 

 not beyond. West of Saint Genevieve I have only a few records of 

 sporadic occurrence of the white-crown in the nesting season." 



This Mingan region is an outlier of Paleozoic rocks. In his monu- 

 mental photoreconnaisance of the vegetation and physiography of 

 the Labrador-Ungava peninsula, F. Kenneth Hare (1959) marks it as 

 the eastern terminus of the Laurentide massif and considers it an 

 outlier of the coastal tundra that fringes the southeast coast of Lab- 

 rador. Climatologists bring the 55° F. July isotherm, which transects 

 the peninsula from west to east, to the coast just east of this region, 

 and forms a rough boundary between the open subarctic woodland 

 to the north, the closed-crown forest to the south and west, and the 

 scrubby forest characteristic of the eastern tip of the peninsula at 

 this latitude. 



It is open, stunted tree growth and brush that attracts nesting 

 white-crowned sparrows. At Goose Bay, Labrador, for example, 

 I found them nesting only in the open, often burned, black spruce 

 and dwarf birch on the high, sandy delta of the airport plateau. In 



