246 



THE BRITISH NATURALIST. 



[October 



rare for the whole of a flock to alight on the ground, some almost 

 invariably taking up a position in one or more adjacent trees. But 

 there are frequent changes, some flying up from the ground to the tree, 

 and vice versa ; and to call this " posting sentinels " is mere 

 anthropomorphism. 



The transition from mere crowding in a suitable environment to true 

 sociality is well illustrated amongst the tropical weaver-birds. Most 

 of the numerous species of this family build, or rather weave, hanging 

 nests, which are entered either from the side or from below, 

 and afford shelters admirably protected against the attacks of small 

 monkeys or arboreal reptiles. Some of the species seem to prefer 

 nesting in groups, one tree holding several nests, whilst another — 

 apparently equally suitable — has none ; in this, these species resemble 

 our own rooks, nor is it difficult to see that the habit may be beneficial 

 to them in various ways ; but the tendency reaches its climax, and a 

 much higher level of sociality than appears elsewhere in the weaver- 

 bird family is attained by the South African Philhetcerus saptus, sometimes 

 called the Social Grosbeak. A number of these birds unite together, 

 select a tree — usually one with a smooth and not easily climbable stem 

 — collect coarse grass, and with it weave a roof which is supported by 

 the trunk and branches of the tree. Having completed their common 

 protection, under and suspended from this roof each pair builds its own 

 nest, and communities of more than 100 pairs have been found whose 

 social organization is little below that of the human inhabitants of the 

 same region. Joint action for the benefit of the community is also seen 

 amongst the Australian bush-turkeys (Talegallus), which live in small 

 flocks and construct a common mound of earth and decayed leaves in 

 which the hens deposit their eggs. 



Whether the flocking which usually precedes the migrations of birds 

 can be described as due to a social instinct depends on the limitations 

 of meaning assigned to the adjective. If a liberal interpretation be 

 allowed, it may be said that the instinct is at least rudimentarily social ; 

 very few birds migrate except in flocks, when the personal experience of 

 the older probably plays at least as important a part as the hereditary 

 tendencies of the younger birds in directing the courseof the journey ;and 

 that natural selection, by the elimination of those individuals which being 

 either above or below the general level of intelligence become stragglers, 

 tends to preserve the rudimentary and temporary sociality there can be 

 no doubt ; the evidence of numerous observers both at sea and along 

 various coastlines proves that many bird-stragglers meet a watery death, 

 and others fall victims to enemies or to the weather, dangers which they 

 would have escaped had they flocked with their comrades. 



Amongst the Mammals we find that in the most primitive group, 

 whose intelligence as well as the general pitch of their life is of a very 



