34 THE MIGRATIONS OF BIRDS 



of a kind, and needs only to become synchronized 

 with seasonal climatic change to assume the usual 

 rhythm of migration with which we are familiar in 

 the northern hemisphere, and which in Australia is 

 pursued regularly by a number of species, notably 

 swallows and cuckoos. 



Migration may also be explained, in part, in 

 terms of what has recently been called "Territory in 

 Bird-life." During much of the year birds have no 

 other responsibilities than search for food, with rea- 

 sonable alertness to avoid death at the claws of pre- 

 dators. They may be social or solitary according to 

 habit, and may wander, or may remain sedentary. 

 Individuals of the same or different species may 

 range in reasonable proximity without undue bicker- 

 ing or quarrelling. With the approach of the breed- 

 ing season all this changes. Each male seeks an area 

 on his breeding-ground within which later will be 

 constructed his nest, which he guards closely against 

 encroachment. This is his particular and private 

 domain, within which he permits no prolonged in- 

 trusion by rival males of his own kind. In the case 

 of such gregarious species as the sooty tern, which 

 nests in tremendous colonies, this bit of territory 

 may be likened to the narrow quarters of the human 

 dweller in city squares, as it is merely sufficient to 

 permit each individual space to stand free from 

 actual physical contact with its neighbors; in the 



