MALES ARRIVE BEFORE FEMALES 39 



which denotes immaturity, amongst the first 

 batch of arrivals. But though plumage may 

 sometimes be a satisfactory guide, yet to rely 

 upon it alone, or upon a more perfect develop- 

 ment of feather, is to exceed the limits of 

 safety. How, then, can we ascertain whether 

 all the males that arrive before the females have 

 had some previous experience of reproduction ? 

 Well, we take a particular locality and note the 

 migrants that visit it year after year, and we 

 find that the respective numbers of the different 

 species are subject to wide annual fluctuations. 

 Not every species lends itself to an inquiry of 

 this kind : some are always plentiful and fluctua- 

 tion is consequently difficult to discern ; others 

 are scarce and variation is easily determined. 

 Those which are of local distribution but con- 

 spicuous by their plumage, or easily traced by 

 the beauty or the peculiarity of their song, 

 afford the more suitable subjects for investiga- 

 tion. For example, the Grasshopper- Warbler, 

 Marsh- Warbler, Nightingale, Corncrake, Red- 

 backed Shrike, or Whinchat have each some 

 distinctive peculiarity which makes them con- 

 spicuous, and each one is subject to marked 

 fluctuation in numbers. The small plantation or 

 wooded bank may hold a Nightingale one year, 

 but we miss its song there the next ; the 

 osier bed or gorse-covered common which vibrates 

 with the trill of the Grasshopper- Warbler one 

 April is deserted the following season; the 

 plantation which is occupied by a host of 

 common migrants this summer may be enlivened 



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