AREIVAL OF THE SPRING-MIGRANTS. 9 



quite as strictly migratory in their habits. Thus there is 

 the case of birds whose summer and winter limits may be 

 said to overlap. Such birds are, of course, found per- 

 manently within the boundaries of the overlapij'mg zone, 

 as shown in the rough diagram annexed : — 



3000 



The upper oblong represents the habitat of any given 

 species — say the Golden Plover— during summer ; the 

 lower oblong its habitat during wilder. Assuming that the 

 annual range of each individual bird is approximately equal, 

 those breeding in summer at a would winter at b — the 

 two most northerly points of the respective areas. The 

 intermediate birds summering at b would likely winter at 

 c ; while those at the latter point would move on in winter 

 to D — the two latter being the most southerly points in the 

 two areas. In the overlapping zone (the doubly crossed 

 portion from b to c) there will obviously be found Plovers 

 at all seasons of the year. But they are clearly not the 

 same individual birds. Those individuals which occupied 

 this area in summer will be wintering — say 1,000 miles 

 south — at E : while their vacated places are re-occupied 

 by others which had passed the summer 1,000 miles north, 

 at F. 



Now such birds are clearly quite as much migrants as are 

 the Swallow and the Cuckoo. If one happened to live in 

 Siberia or in Africa, there w^ould be no difficulty in re- 

 cognizing the fact : but to those happening to live as we do. 

 in the central area, where the summer and winter ranges 

 overlap, it is less patent, and easily overlooked. It is a 

 prevalent mistake to regard birds of this class as resident 

 (or non-migratory), and this is a point I wish to elucidate. 



