422 K. C. PARKES 



American origin, (2) those of probable South American origin, and 

 (3) a Pan-American element now so widespread in both continents 

 as to be of uncertain continental origin (see Darlington, 1957, pp. 

 280-286, for a critique of Mayr's theories concerning these New 

 World groups). 



Finally, there are the families that are generally acknowledged to 

 be of Old World origin, but that now form a component of the 

 avifauna of the Americas. This is the group which will be reviewed 

 here. 



As Mayr pointed out, birds apparently crossed the Bering Strait 

 connection between Asia and North America more or less con- 

 tinuously through most of the Tertiary. This continuity of origin 

 is reflected in the complete spectrum of zoogeographic and taxon- 

 omic status of the Old World groups in the Americas. At one end of 

 the spectrum are families the representatives of which arrived so 

 early that there has been time for a major secondary radiation in 

 the Americas. Such a history is exemplified by the quails, derived 

 from the Old World pheasant and partridge family, and by the 

 jays, which are much more diversified in the New World than in the 

 Old. It should be acknowledged that factors other than the mere 

 time element are involved in the extent to which these secondary 

 radiations have developed; evolutionary opportunity in the form 

 of available ecological niches and comparative rates of evolution 

 must also be considered. But it seems safe to assume that a group 

 that has reached approximately the subfamily level of difi^erentia- 

 tion must have derived from a rather early invasion. 



At the opposite end of the spectrum Mayr mentions species 

 which have only a slight foothold on this continent and are insep- 

 arable even subspecifically from their Old World progenitors. 

 These include two Alaskan birds, the Yellow Wagtail {Motacilla 

 flava tschiitschensis) and the Red-spotted Blue-throat {Luscinia 

 svecica svecica), and one species, the Wheatear, which has entered 

 Alaska from the west (as Oenanthe oenanthe oenanthe) and the east- 

 em Canadian Arctic from Greenland (as 0. o. lencorhoa). All these 

 species are obviously recent immigrants but belong to two families 

 (Motacillidae and Turdidae) with wide distribution in the New 

 World from earlier invasions. A category even beyond this can be 

 established for those Old World species that do not have a foothold 



