52 HOW BIRDS MIGRATE 



The rate at which a bird migrates, however, is of course not to be 

 considered its limit of speed. The migrating bird, like the long-distance 

 runner, must adopt a pace which will enable it to reach the goal without 

 danger of exhaustion by the way. Should necessity arise, it doubtless, 

 for a time, could more than double the speed at which it normally 

 travels. 



The rate of progress of the individual must not be confused with 

 that at which the species advances. Nocturnal migrants probably 

 cover 200 to 400 miles in a single night's journey; but, as Cooke ('03) 

 has shown, "The average speed of migration from New Orleans to south- 

 ern Minnesota for all species is close to 23 miles per day." From this 

 latitude northward, however, in response to the more rapid develop- 

 ment of the season, the speed is constantly accelerated until the breed- 

 ing-place is reached. Thus, the same author remarks, "Sixteen species 

 maintain a daily average of forty miles from southern Minnesota to 

 southern Manitoba, and from this point twelve species travel to Lake 

 Athabasca at an average speed of seventy-two miles a day, five others 

 to Great Slave Lake at 116 miles a day, and five more to Alaska at 

 150 miles a day." 



The slow rate at which a species moves, when compared with that 

 It which the individuals composing it travel, is evidence that its mi- 

 gration is not performed continuously, night after night, until the haven 

 is reached, but that after a flight birds pause to rest, to await favorable 

 weather conditions, and the further seasonal change which such con- 

 ditions hasten. The observations of Wright ('09) in the Boston Public 

 Garden, where, as in other city parks, local conditions are exceptionally 

 favorable for the correct interpretation of migration phenomena, also 

 confirm this view, if indeed further confirmation be required. 



The impelling motive being more powerful, the object more definite, 

 and the seasonal influences more pronounced, the spring migration 

 of birds is a more orderly and regular movement than the return in 

 the fall. Clear nights and a rising thermometer are most likely to 

 induce birds to travel, a 'wave' of migrants coming often on the crest 

 of a 'wave' of warmer temperature. When such a movement is checked 

 by cold or stormy weather, the result is an overflow of migrants which 

 flood the woods. Ordinarily rare species may then become compara- 

 tively common, and the impression is produced of an actual increase in 

 bird-life. In the fall, when physiological factors incident to reproduc- 

 tion are not potent and seasonal changes are less marked, birds travel 

 more leisurely. Clear nights and a falling thermometer are then most 

 favorable for a general movement. i 



The high mortality in the ranks of migrating birds occasioned by 

 the storms they encounter is evidence of their inability to anticipate 

 changes in the weather. On the coast of Texas I have known birds to 

 migrate northward in great numbers directly into the face of a 'norther/ 

 with evidently no warning of the unfavorable conditions toward which 

 they were hastening. Again, under the influence of exceptionally 



