Avifauna of Central America. 777 



Great Britain. In the autumn our home-bred Woodcocks 

 and Thrushes, to take but two diverse types, leave us for 

 the south, when their places are at once taken by members 

 of the same species bred in more northern areas. That the 

 departing birds would have starved if they had remained is 

 proved to be an unfounded inference from the fact that the 

 incoming birds find a sufficiency. Again, the Redwing and 

 the Fieldfare — among a number of species — come south, to 

 us, for the winter, but depart north in the spring to breed. 

 These movements are clearly determined by temperature. 

 The birds bred in this country are unable to stand the 

 winter and must perforce move south, while those bred 

 further north find our comparatively mild winters tolerable. 

 The return journey is undertaken, in every case, though 

 unconsciously, for the sake of the young, which would be 

 unable to withstand the heat of the winter resort during the 

 summer months. That this is so is shown by the distress 

 which nestlings display during exceptionally hot days. But 

 this distress is shared no less by the sitting and brooding 

 bird. It has been observed alike in the Snipe crouched in 

 the sedge-grown swamp, the Norfolk Plover out on the 

 sandy heath, and the Razorbills and Guillemots on the 

 ledges of cliffs towering above the sea, where one would 

 suppose that a breeze, if only a light one, was always 

 blowing. The signs of suffering are everywhere tlie same 

 — the open beak, dribbling from the mouth, and every 

 feather raised to admit as much air as possible. Thus 

 affected, bow would it go with such as winter in Africa if 

 they stayed there to breed ? 



But, apart from this, it has been overlooked that there are 

 certain types of birds which are unable to migrate owing to 

 the high degree of specialization which they have undergone 

 in regard to their food. The Toucans, Horubills, Touracous, 

 and Parrots afford instances of this. In the event of a 

 shortage of" food-supply, either from over-population or 

 from any material lessening of the normal food-supply, all 

 which left the normal boundaries of the tribe in search of 

 food would perish by starvation, even if they possessed the 



