430 K. C. PARKES 



have been derived from two incursions of the same stock. The de- 

 tails of these invasions may be inferred to have been as follows. 

 The first immigration took place prior to the Pleistocene glaciation 

 by birds belonging to the widespread Old World species that we now 

 call Laniiis excubitor. With the advent of glaciation this species 

 moved south. It is highly adaptable, as shown by its Old World 

 range, which extends from northern Scandinavia to India and 

 Arabia. Therefore, with the retreat of the glacier, populations of 

 this shrike were able to remain as far south as southern Mexico, 

 while northern populations reoccupied an area extending to central 

 Canada. At some uncertain time the species again crossed the 

 Bering Strait and quickly spread through the boreal portion of 

 North America. This second wave is currently regarded as con- 

 specific with and closely related to the Old World races through 

 the Siberian population, while the descendants of the first invasion 

 are given specific rank (L. liidovicianus) . Certain well-marked 

 differences that have evolved in these earlier and more southern 

 populations are invoked as specific characters. However, members 

 of this so-called species, the Loggerhead Shrike, resemble some of 

 the ecologically parallel populations of the Old World species 

 (Olivier, 1944, p. 43), Judging from published maps and range 

 descriptions, the Northern and Loggerhead shrikes nowhere come 

 into actual contact during the breeding season in North America. 

 There is thus no available natural testing site for the criterion 

 of reproductive isolation. We have here an interesting problem of 

 deciding what to call the two North American shrikes; although 

 they have almost always been listed as full species, there are argu- 

 ments and precedents for the opposite view. The problem they 

 illustrate is an interesting, although perhaps uncommon one: 

 the question of the taxonomic treatment of obvious derivatives, 

 from two well-separated invasions of the same stock, that are now 

 separated spatially, so that no evidence as to reproductive iso- 

 lation is forthcoming. 



Perhaps the most intriguing problem connected with the palae- 

 arctic element in the New World avifauna, and the one least likely 

 to be solved, deals with the related questions "which?" and "why?" 

 — which groups made the trip, and why these and not others? 

 Undoubtedly the most striking aspect of this problem is the apparent 

 one-way traffic between the two continents; Darlington (1957, 



