NATURAL HISTORY IN ICELAND — HUXLEY 335 



I mentioned that the Lceland redpoll and wren were larger in size 

 than ours. This is an example of an interesting general rule — that, 

 in general, warm-blooded animals are found to be slightly larger the 

 nearer they live to the pole ; further, in mammals, the relative size of 

 ears, tail, and limbs tend to diminish — a phenomenon strikingly illus- 

 trated by the tiny ears of the Arctic fox as compared with the huge 

 flaps of the fennec fox from the scorching deserts. These changes 

 are undoubtedly adaptations, working to reduce heat loss in cold 

 climates and to promote it in over-hot ones. 



Thus some of the special characters of Iceland birds are adaptations 

 to climate while others, like the color of the Iceland wren, seem to be 

 more or less accidental results of isolation. But there is a third class 

 of difference, and perhaps the most interesting — the differences in be- 

 havior and song. Some of these differences, like the harsher song of 

 the Iceland wren, are again aspects of the distinctiveness of the local 

 subspecies. Others seem to be due to the birds being on the margin 

 of their range, in surroundings quite different from the normal. 

 Thus, as already mentioned, the Iceland wren out of the breeding 

 season has to become almost exclusively a shore bird. 



Frequently, however, the reason is more subtle — the absence of com- 

 petition from close relatives which have not reached this part of the 

 species' range. Thus, in Britain, snipe are inhabitants of open coun- 

 try, so that it was surprising to find them quite common in the one of 

 Iceland's two woods that we visited. James Fisher hit on what I am 

 sure is the solution — namely that there are no woodcock in Iceland. 

 With us, woodcock occupy the habitat provided by boggy woods. But 

 where they are absent, the snipe avail themselves of this as well as of 

 their normal open habitat. 



But the absence of close relatives may have another effect. Wlien 

 two closely allied species come into contact in the same area, it is in 

 general a biological advantage for them to proclaim their distinctive- 

 ness by some characteristic difference of plumage or voice. This will 

 help to prevent actual or attempted cross-breeding, trespassing, and 

 other wastes of time and energy. In Britain, the closely related 

 meadow and tree pipits are not only restricted to different habitats, 

 but sing quite distinctive songs. With us, the meadow pipit is ex- 

 clusively a bird of moors and heaths and other open country, and its 

 song is a rather feeble descending scale gradually accelerated into a 

 little trill, given as the bird parachutes down after having flown up 

 from the ground. The tree pipit, on the other hand, demands scat- 

 tered trees, and has a much more striking song ; this is also given in 

 the air while floating down, but the flight starts from (and often ends 

 on) a tree perch. 



Here the need for distinctiveness cannot well be met by coloration, 

 since both species are adapted to concealment by cryptic coloration ; 



