PALAEARCTIC ELEMENT IN NEW WORLD AVIFAUNA 425 



the unexplored tundra in the New World arctic is vast enough to 

 hide a small local breeding population of the Curlew Sandpiper. 

 It may be less likely for some of the larger species, but certainly 

 cannot be discounted. Every year many thousand Eastern Dowitch- 

 ers {Limnodrormis grisens griseiis) pass on migration along our 

 East Coast, but as of 1957 the A.O.U. Check-list can only say of 

 this large shorebird, "Breeding range uncertain, but presumed to be 

 in the interior of the Ungava Peninsula." Thus these so-called 

 accidentals may yet be shown to be an established component of 

 the New World avifauna. 



We turn next to those Old World groups known to be established 

 in the Americas. Mayr listed approximately twenty families of New 

 World birds as being of Old World origin — approximately, since 

 family limits are a subject for continuing debate among ornitholo- 

 gists. He divided these roughly into three groups; Early, Fairly 

 Early, and Recent Immigrants. Two principal types of evidence 

 are used to classify these groups. The fossil record, of the relatively 

 few species for which it is at all adequate, is naturally the best 

 evidence, but note the warning of Darlington (1957, p. 238) : "Birds 

 are a good example of the fact that, in zoogeography, a poor fossil 

 record interpreted too literally is almost worse than no record at 

 all." The other type of evidence is more inferential, based on the 

 relative numbers of species and genera in the New and Old Worlds, 

 the degree to which the New World forms have differentiated 

 taxonomically, and the distance into the Americas they have 

 penetrated. Mayr's tripartite chronological division is of necessity 

 arbitrary and, particularly for large families, may be misleading in 

 suggesting a lesser number of invasions than has probably occurred. 

 Mayr circumvented the latter difficulty by subdividing the rep- 

 resentatives of some families, as the Corvidae (crows and jays) 

 and the Turdidae (thrushes) into older and newer invasions. Even so, 

 additional subdivision beyond Mayr's seems desirable. For instance, 

 Mayr pointed out, on the basis of both fossil and Recent taxonomic 

 evidence, that the typical owls, family Strigidae, though almost 

 certainly of Old World origin, must have reached the New World 

 very early. Fragmentary remains assigned to an allied but extinct 

 family, Protostrigidae, are known from the Eocene of Wyoming, 

 and a Lower Miocene species from South Dakota is identified with 

 the living genus Strix (this and other data on fossil birds are chiefly 



