CHAP. II.] BIRDS. 27 



preceding spring, whereas those which went away in autumn 

 were two or three times as numerous. Those young birds that 

 do get back, however, have learnt by experience, and tlie next 

 year they take care to go with tlie old ones. The most striking 

 fact in favour of the " instinct" of migration is the " agitation," 

 or excitement, of confined birds at the time when their wild 

 companions are migrating. It seems probable, however, that 

 this is what may be called a social excitement, due to the 

 anxious cries of the migrating birds ; a view supported by the 

 fact stated by Marcel de Serres, that the black swan of Australia, 

 when domesticated in Europe, sometimes joins wild swans in 

 their northward migration. We must remember too that migra- 

 tion at the proper time is in many cases absolutely essential to 

 the existence of the species ; and it is therefore not improbable 

 that some strong social emotion should have been gradually 

 developed in the race, by the circumstance that all who for 

 want of such emotion did not join their fellows inevitably 

 perished. 



The mode by which a passage originally overland has been 

 converted into one over the sea offers no insuperable difficulties, 

 as has already been pointed out. The long flights of some birds 

 without apparently stopping on the way is thought to be inex- 

 plicable, as well as their finding their nesting-place of the 

 previous year from a distance of many hundreds or even a 

 thousand miles. But the observant powers of animals are very 

 great ; and birds flying high in the air may be guided by the 

 physical features of the country spread out beneath them in a 

 way that would be impracticable to purely terrestrial animals. 



It is assumed by some writers that the breeding-place of a 

 species is to be considered as its true home rather than that to 

 which it retires in winter ; but this can hardly be accepted as a 

 rule of universal application. A bird can only breed success- 

 fully where it can find sufficient food for its young ; and the 

 reason probably Mdiy so many of the smaller birds leave the 

 warm southern regions to breed in temperate or even cold lati- 

 tudes, is because caterpillars and other soft insect larvae are 

 there abundant at the proper time, while in their winter homo the 



