The geographical distribution of animals, with a study of the relations ...

;chap. 11.] BIRDS. 19immediately connected with the process of reproduction.as the salmon, enter rivers ;approach the coast in the breeding season ;Some,others, as the herring and mackerel,but the exact courseof their migrations is unknown, and owing to our completeignorance of the area each species occupies in the ocean, and theabsence of such barriers and of such physical diversities as occuron the land, they are of far less interest and less connected withour present study than the movements of birds, to which weshall now confine ourselves.Migrations of Birds.—In all the temperate parts of the globethere are a considerable number of birds which reside only apart of the year, regularly arriving and leaving at tolerably fixedepochs. In our own country many northern birds visit us inwinter, such as the fieldfare, redwing, snow-bunting, turnstone,and numerous ducks and waders ;with a few, like the black redstart,and (according to Rev. C. A. Johns) some of the woodcocksfrom the south. In the summer a host of birds appear—thecuckoo, the swifts and swallows, and numerous warblers, beingthe most familiar,—which stay to build their nests and rear theiryoung, and then leave us again. These are true migrants ; buta number of other birds visit us occasionally, like the waxwing,the oriole, and the bee-eater, and can only be classed asstragglers, which, perhaps from too rapid multiplication one yearand want of food the next, are driven to extend their ordinaryrange of migration to an unusual degree.We will now endeavourto sketch the chief phenomena of migration in differentcountries.Europe.—It is well ascertained that most of the birds thatspend their spring and summer in the temperate parts of Europepass the winter in North Africa and Western Asia. The wintervisitants, on the other hand, pass the summer in the extremenorthof Europe and Asia, many of them having been found tobreed in Lapland. The arrival of migratory birds from thesouth is very constant as to date, seldom varying more than aweek or two, without any regard to the weather at the timebut the departure is less constant, and more dependent on theweather.Thus the swallow always comes to us about the middle