222 RENNELL - AN OUT OF THE WAY CORAL ISLAND 



The Whitney Expedition found 38 species of birds on Rennell Island, 

 to which we were able to add four more. Of these, 35 species breed 

 on the island. It has been shown that in the time they have lived in isola- 

 tion no fewer than 23 of the 35 have become so differentiated that they 

 must now be classed as separate species or subspecies endemic to Rennell 

 Island. This is an extremely high proportion. 



Of these 23 endemic birds, eight are most closely related to near species 

 in the Solomon Islands, while eight others have their nearest relatives on 

 the Santa Cruz and New Hebrides islands. It is a remarkable fact, how- 

 ever, that, whereas the nearest large island in the Solomon group to the 

 north is only 150 kilometres away, the distance to the Santa Cruz Islands 

 to the east is as much as 600 kilometres. There are two possible ex- 

 planations of the mystery. First, the prevailing winds in this region are 

 easterly and south-easterly (56 per cent.), only one-sixth being northerly. 

 Secondly, the geography of some of the Santa Cruz Islands is not very 

 different from that of Rennell, whereas the Solomons are mountainous 

 with a substratum of granite or lava. 



For a species to spread from one region to another there must be at 

 least one specimen of either sex. A close study of the habits of the Rennell 

 birds and their nearest relatives shows that nearly all the small birds on 

 Rennell Island are gregarious. The inference is that the bird invasion chiefly 

 took the form of small wind-driven or storm-driven flocks. 



It has already been noted that most of the birds have lived long enough 

 on Rennell to have become differentiated into separate species or sub- 

 species (as indicated by differences in size, length and thickness of beak, 

 colour of coat, habits, food, and so on). Now, when we compare the 

 endemic Rennell birds whose nearest relatives live on the Santa Cruz Is- 

 lands (and New Hebrides) with the endemic Rennell birds which have 

 their nearest relatives in the Solomon Islands, we find that the former 

 show greater differences from their ancestors (the Santa Cruz birds) than 

 the latter do from theirs (the Solomon Island birds). In fact, the Rennell 

 birds related to the Santa Cruz birds have in most cases split off into 

 separate genera and species, while those related to the Solomon Island 

 birds usually differ only to the extent of forming seperate subspecies. 



The explanation may lie in the fact that the distance from Rennell to 

 the Santa Cruz Islands is so much greater than from Rennell to the nearest 

 islands of the Solomon group that the proportion of Santa Cruz birds, in 

 spite of the prevailing winds, is much smaller than that of birds which 

 have come — though at long intervals — from the Solomons. The Santa 

 Cruz birds would thus have a greater tendency to split off into separate 



