xvii. 4 MIGRATION 493 



contains elaborate social and sexual rituals for many occasions. For 

 example, relief of the one bird by the other at the nest is accompanied 

 by a peculiar wing-flapping ceremony in herons and other birds. 

 Greeting ceremonies are common in many species, and there is a host 

 of sexual recognition and courtship rituals, to be considered later, 

 serving the immediate function of regulating the aggressiveness that 

 otherwise arises when two individuals approach each other closely. 

 Such ceremonies prevent attempted copulation with the same sex and 

 ensure it with a member of the opposite sex, and of the right species. 

 Apart from sexual behaviour birds show many complex mutual and 

 social reactions. Thus in communities of hens or pigeons there is 

 quickly established a rank of 'pecking order', such that each bird is 

 submissive to the one above it, the order changing, however, when age, 

 moulting, or experiment (e.g. sex-hormone injection) alters the state 

 of the birds. More pleasing communal habits are the dances and 

 corporate flights, which are well known in cranes and other species. 



4. Bird migration and homing 



Among the remarkable devices of birds is their habit of seasonal 

 movements to obtain the advantage of the favourable conditions 

 offered in more northerly regions only during the summer. The most 

 familiar migrations are those north and south (sometimes north-east 

 and south-west) over the land masses of the northern hemisphere, but 

 there are similar movements also in the southern hemisphere, though 

 they are of lesser extent. Some tropical birds migrate to breed in the 

 rainy season in the outer tropics, removing to the central tropics in 

 the dry season. Marine birds also may make extensive migrations. 

 Thus the great shearwater (Puffinus) breeds on Tristan da Cunha, but 

 comes as far north as Greenland or Iceland in May, returning again 

 after months of wandering at sea, apparently without making a land- 

 fall. The Arctic tern (Sterna) breeds in the north temperate zone, and 

 migrates to the Antarctic along both sides of the Atlantic. Penguins 

 make migrations by swimming. Distances up to 6,000 miles from 

 Northern Europe to South Africa have been recorded for swallows 

 and storks, and even farther for the Arctic tern. The factors deter- 

 mining the direction and course of migration are beginning to be 

 known. It has been shown that the power to follow a given course 

 depends partly on the ability to navigate by observation of the position 

 of the sun or stars (Matthews, 1955; Sauer, 1957). Birds certainly do 

 not learn the routes from their elders, indeed the young often leave 

 first. However, juveniles return only approximately to their birthplace, 



