FLEAS 87 



United States we find one geographical race and in Europe another, 

 no matter what birds the flea happens to infest. 



So far we have considered certain bird fleas which parallel the 

 host's range, and others which fall short of it. There are a few cases, 

 probably, where a bird flea has extended its range beyond that of the 

 true host, but these are, for obvious reasons, rather difficult to detect. 

 C. gallinae is possibly an example. There is some reason to think that 

 originally it was a tit flea, but on the domestic fowl it has invaded 

 remote islands where the tits are absent. 



There are other aspects of this problem which appear still more 

 complicated and are more difficult to understand. We have for example 

 the two very closely related species of bird fleas C. garei and C. borealis, 

 which both favour ground nesting birds with a preference for wet or 

 swampy nesting sites. The former is distributed throughout the main- 

 land of Britain. The latter is confined to the outer western isles such 

 as St. Kilda, Inishtrahull and the Scillies where C. garei is absent 

 (Map 2). The explanation of such a distribution is obscure and one 

 can but hazard a guess. Maybe the factors are inter-specific competition 

 and C. garei^ a late comer, may have ousted C. borealis throughout the 

 mainland of Britain. The latter has only survived on outlying 

 islands, and on the continent, in the Alpine fastnesses of Central 

 Europe. 



The distribution of C. vagabunda is also interesting although in view 

 of the collecting lacunae little can be offered beyond a few tentative 

 suggestions and speculations. This flea is rather rare and is apparently 

 an ancient species, and it has broken up into geographical races one of 

 which is pecuHar to Britain. It is chiefly an inhabitant of nests of rock 

 dwelling sea birds such as the herring-gull {Larus argentatus) and the 

 shag {Phalacrocorax aristotelis) . It has a Northern or Boreal distribu- 

 tion, and specimens are known chiefly from Spitsbergen, the Shetland 

 Isles and Outer Hebrides, Northern Turkestan, Northern Siberia and 

 Alaska. It is also found in the Alps of Central Europe. This is quite a 

 well known type of distribution and is believed to be the result of the 

 advance and subsequent retreat of arctic conditions during one of the 

 glacial periods or ice ages. There are similar examples among many 

 British plants and animals such as the small gentian {Gentiana nivalis), 

 the star saxifrage {Saxifraga stellaris), a butterfly, the mountain ringlet 

 {Erebia epiphron) and the alpine hare {Lepus timidus) and the ptarmigan 

 {Lagopus mutus). 



