2o6 THE STORY OF THE BIRDS 



marsh vegetation. In the latter case the Bitterns 

 are aberrant in their general colouration^ for the 

 Herons as a group are showy and conspicuous 

 birds — a deviation from the general prevailing 

 colour of the family in obedience to that law 

 of colour which insists upon harmony between 

 bird and haunt. Out on the open moors and 

 heaths a different kind of protective colouration 

 again confronts us in the marvellous tints of 

 the Red Grouse ; whilst higher up the mountain 

 sides the Ptarmigan in its pencilled dress assimi- 

 lates most closely with the stones and lichens, 

 and in winter (when its plumage is white) with 

 the all-pervading snows. This latter modifica- 

 tion, seasonal with the Ptarmigan, becomes con- 

 stant in such birds as the Snowy Owl, the large 

 Arctic Falcons, the Ivory Gull, and the Snowy 

 Petrel, which all live permanently within the 

 regions of almost perpetual snow. We must 

 also call attention to the fact that all these birds 

 which are protectively arrayed are endowed 

 with the faculty of rendering their concealment 

 as effective as possible, either by always alight- 

 ing on spots where their tints harmonise the 

 best with surrounding objects, or by crouching 

 low to the ground, the tree trunk, the sand, 

 and so forth, and there remaining absolutely 



