( 17) 



Dr. J. A. Allen supposes that " the spring movement is incited by the periodic 

 activity of the reprodnctive organs, resnlting in the necessity for the retnrn of the 

 sjiecies to the pecnliar conditions and surronudings to which for long ages it has 

 been undergoing special adaptation — in other words, to its home." * On the other 

 hand, Professor W. W. Cooke and Mr. W. Eagle Clarke have demonstrated the 

 imjwrtance of temperature as a factor, the latter even asserting, in regard to the 

 spring emigratory movements from the Continent of Europe to the British Islands, 

 that " it has invariably been found that all such movements, except those performed 

 late in the season, are to be correlated with a rise of temperature in south-western 

 Enrope, and perhaps in northern Africa. That this indnces the birds to embark on 

 their northward journey does not admit of doubt." + No doubt those pronounced 

 movements, sometimes called " rushes," are caused by a decided rise in temperatnre 

 over the areas whence the movements began, but during the interval between suc- 

 cessive " rushes " migration must be supposed to be in progress, even if unobserved. 

 Further, the remarkable uniformity of climatic conditions prevailing in the Tropics 

 makes it clear that we must look elsewhere for an explanation of the departure of 

 migratory species which winter in this zone. 



MIGRATION ROUTES. 



While some ornithologists think that birds migrate with an extended frout, 

 roughly corresponding in width to that of the breeding area, others maintain 

 that they follow geographically defined routes, whose deflexions depend primarily 

 upon topographical features.^: 



Our knowledge of the precise boundaries of the winter' range of many even 

 of the best known summer visitants to Europe is very imperfect. Having reached 

 the winter quarters, there is not the same necessity for birds to occupy a limited 

 area as there is during the breeding season, and doubtless more or less individual 

 wandering takes place, as indeed we see to a pronounced degree in the winter 

 immigrants to our own area. The spring journey, therefore, for the same bird 

 may start <luring successive years from points widely separated. The conclusi(,in 

 which best harmonizes with actual observations is that, in the performance of 

 the journey to and from the breeding quarters, each species traverses a definite 

 route, some sections of which may deviate widely from its general trend. The 

 route may intersect other routes, or may coincide with them for a longer or 

 shorter distance. Where physical features are followed, we may be sure it is 

 not from the guidance they afford, but because they mark out convenient high- 

 ways. There is some indubitable evidence that migration at times proceeds at 

 great heights. Whether on these occasions the routes followed are more direct 

 and wholly independent of the relief of the land is not yet known. 



* "The Geographical Origin and Distribution of North ^imerican Birds, considered in Eelatiou to llie 

 Faunal Areas of Nortli America" (^Auh, x. p. 104). 



t " Digest of the Observations on the lligrations of Birds at Lighthouses and Light-vessels, 1880-1887 " 

 (JHejJort Brit. Asmo., ISDti, p. 474). See also Cooke, " Report on Bird Migration in the Mississippi Valley 

 in the years 1884 and 1885 " (^Bullclm No. 2, Dimsion of A'eonvmie Ornitholoijy [now Biological .Sur\cyJ. 

 U.S. Dcpaftment of Agriculture, p. 1(! et seij.). 



t " Sie vcrfolgen \ielmehr ganz bcstimmtc, geographisch bcgrenzte Strassen, deren Biegungcn 

 vorallem durcli die topographischen Vcrhiiltnisse der Gegenden bedingt werden "(J. A. I'almfn, Hefcrat 

 ilber den Stand iter Kenntniss den Voi/clzuges, p. 3). Professor I'almen, it should be mentioned, is 

 speaking of certain Arctic-breeding birds, but he appears to think the statement may be taken 

 generally. 



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