CECOLOGICAL 79 



distribution which they present. The factors which caused the 

 breaks in the continuity are naturally impossible to trace with 

 any certainty, though occasional glimpses may be caught which 

 may some day be turned to account. 



A very potent influence in the formation of isolation areas 

 is undoubtedly to be ascribed to changes in the plant world, 

 caused in turn by river systems, and changes in the level of the 

 earth's surface and consequent alteration in the condition of 

 temperature, moisture and so on. Both insectivorous and 

 vegetivorous birds are naturally vitally affected thereby, more 

 particularly those which perform but limited migrations, or 

 are non-migratory. 



The study of the haunts of birds is the study of geographical 

 distribution on a minute scale. Just as we look for Humming- 

 biirds and Toucans only in America, Secretary-birds and 

 Ostriches in Africa, and Lyre-birds and Emus in Australia, so 

 we look for Petrels and Auks only in the immediate neighbour- 

 hood of the sea. Grebes and Water-hens in fresh waters. Grouse 

 on moors, and so on. And we know that while some birds 

 may vary their haunts according to the season, others appear 

 unable to affect any change whatever, and are consequently the 

 more endangered in times of stress. Little then can with 

 profit be said with regard to the haunts of birds, for the theme 

 is too discursive. But one point is certainly worth considering, 

 and this relates to the bird-life of large woods and forests. 

 One would expect that such areas would teem with bird-life, 

 yet such is not usually the case. Forests of deciduous trees 

 such as are to be met with only in temperate climates are 

 notoriously poor in bird-life, only the outer fringe being at all 

 thickly populated. But here the tree-dwellers are free to scour 

 the surrounding open country in search of food, for within the 

 gloomy interior of such forests insect-life is scarce, and edible 

 fruits are rare. Coniferous forests harbour a few highly special- 

 ised species, such as Nutcrackers, Cross-bills and Wood-peckers. 

 Tropical forests, from their more luxuriant growth, and the 

 abundance of trailing, flower-bearing creepers, support a far 

 larger and more varied avian population ; but these dwell on 

 the forest, rather than in it, as we shall show, 



