ISLAND AVIFAUNAS 199 



Palaearctic islands in the Mediterranean — the Balearic 

 Islands, Corsica, Sardinia, Sicily, and Malta. It may 

 be remarked that all these islands are situated in a 

 comparatively small and entirely land-locked sea, and 

 that all of them formed part of continental land at no 

 very remote era. So far as our knowledge extends at 

 present, only one of this important series of islands 

 contains an endemic species. In Corsica we have a 

 species of Nuthatch {Sitta wJiitcJicadi) peculiar to the 

 island, so far as is known. That endemic species should 

 be so excessively rare in islands apparently suitable in 

 every way for the development of insular forms would 

 appear to be a somewhat anomalous fact, but the 

 explanation is an extremely simple one. Again, geo- 

 graphical position is the cause of the absence of endemic 

 species ; and each one of these islands is situated in the 

 direct path of a strong migration, indeed acts as a sea- 

 bridge to its progress. The resident avifaunas of these 

 several islands are composed of species that range well 

 into Europe, cross them on their way to Africa, or 

 reside in them during the winter months. This inter- 

 mixture of individuals — due to migration — has prevented 

 any local races becoming established in these islands, 

 with the sole exception of the Corsican Nuthatch. This 

 exception, however, is one that proves the rule. The 

 Common Nuthatch appears everywhere to be a seden- 

 tary species, and to be absent from Malta, Sardinia, and 

 Corsica. The descendants of the individuals which in 

 emigrating north across the Mediterranean after the 

 Glacial Epoch became stranded residents in Corsica (and 

 it is probable that S. wJiiteheadi occurs also in Sar- 

 dinia), have remained sufficiently isolated to become 



