THE SPRING MIGRATION OF BIRDS. 205 



abundant in autumn. The Whimbrel {Nwnenms 

 pJicvopus) is another instance. In spring it passes 

 the British Islands on passage much more 

 abundantly than in autumn, and it has been 

 remarked to fly much higher at the latter season. 

 The intensity of the migration of each species 

 varies considerably. First a few stragglers appear, 

 and then the individuals gradually become more 

 and more numerous until the migration is at its 

 height ; then it slowly dies down again, and a few 

 laggards bring up the rear and conclude the Passage 

 of each particular species for the season. At 

 Gibraltar, for instance, the Turtle Dove begins to 

 pass north on passage about the middle of April, 

 and the flights gradually increase in volume until 

 the first week in May, during which period the 

 migration of this species is at its height, and then 

 the numbers again decrease, until by the middle of 

 the month the spring passage is practically over. 

 At the same locality the Common Sandpiper 

 {Totamis hypoleucus) begins to arrive early in 

 March, and continues to increase in numbers until 

 the middle of April, when it literally swarms there, 

 after which the passage gradually gets less and less, 

 until early in May, w^hen it has ceased for the 

 season. At Hehgoland, at Malta, in Greece, in 

 Asia Minor, at every station where migration has 

 been carefully w^atched, precisely the same con- 

 ditions prevail, and almost every species is alike in 

 this respect. It is also a curious and interesting 

 fact that some species migrate by night in spring, 



