OF BRITISH BIRDS. 37 



three choose a subtropic climate — 



Charadrius asiaticus, 

 Larus ichthyaetus, 

 Otis macqueeni ; 



one may be regarded as subtropic and tropic — 



Caprimulgus (Egyptius. 



The breeding-range of one species extends from the valley of the Elbe to 

 that of the Yenesei and may be regarded as subarctic and subtropic — 



Corvus cor one comix. _ 



The breeding-range of four species extends from South-east E-ussia to some 

 part of Central Asia — 



Saxicola isabellina. 

 Alaucla sibirica. 

 Vanellus gregarius. 

 Pasto?' roseus. 



They are all accidental visitors^ breeding in a subtropic climate. 



Climatic Distriliution of British Birds during the 

 Breeding-season. 



Very few birds have a very wide climatic distribution during the 

 breeding-season. They pay little or no attention to parallels of latitude, 

 but they appear to be very particular to choose breeding-grounds with a 

 mean temperature during the breeding-season to suit their individual 

 requirements. The isothermal lines of July may be regarded as forming 

 natural boundaries of the latitudinal ranee of most of our birds durins: 

 the breeding-season, if due importance be attached to two factors which 

 play a very important part in modifying it. In our maps the isothermal 

 lines are corrected to sea-level. The corrections must of course be entirely 

 disregarded in considering the breeding-ranges of birds, which are governed 

 by the actual mean temperature, irrespective of cause. The climate may 

 be produced by a low elevation at a high latitude, or by a high elevation at 

 a comparatively low latitude. For example : the climate of the Dovre 

 Fjeld, 6000 feet above the level of the sea in latitude 62°, is very similar 

 to that of Lapland, 60 feet above the level of the sea in latitude 70°, and 

 many purely arctic birds and plants are found in both localities. 



The other factor which appears to play an important part in deciding the 



