3i6 THE BIOLOGY OF BIRDS 



nest in a tree — a huge structure which looks from a distance 

 like a Zulu kraal. 



Perhaps the climax is to be found in the most familiar 

 case, that of the rooks and their rookery, a case the more 

 remarkable since, apart from the jackdaws, the other members 

 of the crow tribe are solitaries and individualists. There 

 may be far over a thousand nests in a rookery ; the same 

 site may be used for more than a century ; there is a sort of 

 objective enregistration since the old nests remain for a 

 long time in evidence and may be used over again ; there 

 seems to be some utilisation of sentinels ; there is some 

 indication of social conventions, for after a nest has reached 

 a certain stage its materials are not stolen. As every one 

 knows there is much talking and clear evidence of several 

 different " words." There are gambols and sham-fights, 

 frolics and wild chases — in other words, a good deal of play. 



Finally, as we have already noticed, there is often a very 

 distinct social note in mtgratioti. No one dare say much 

 about " social tradition," but it is noteworthy that many 

 birds which insist strongly on privacy and rights of " terri- 

 tory " in their summer quarters are very social when they 

 undertake their journey southwards. This is seen in the 

 preliminary massing, in the excitement of the crowd, in the 

 trial flights, in the gregarious journeying, and in the 

 utilisation of leaders. 



§ 9. Retrospect on the Evolution of Parental Care 



The organic starting-point is carrying the eggs before 

 birth. In many cases, e.g. crabs, spiders, some fishes and 

 amphibians, but not in birds, of course, the eggs are carried 

 after they are laid. 



On another line is viviparity, which occurs at many 

 levels, where the eggs are hatched, as it were, vdthin the 

 body, and where there is sometimes a more or less intimate 

 partnership between the unborn young and the mother. 

 Some reptiles show this, but this path was not followed by 

 birds. 



