ANIMALS— PHYSICAL ENVIRONMENT 111 



the genus Tarucus may be observed flitting up and 

 down continuously inside a bush of Zizyphus, with- 

 out leaving the middle of the bush, and sometimes 

 continuing in flight for many minutes. The minute 

 Chilades galba, another member of the same family, 

 is able to limit its flight within a plant of Ononis 

 (" Rest Harrow "), which is only a foot in circum- 

 ference, and to remain on the wing in this Uttle bush, 

 when such a wind is raging outside as to prevent 

 the flight of all other butterflies. 



Another interesting modification of habit has been 

 observed by Hartert, Rothschild, and others in the 

 Algerian Sahara. The prevalent wind is from the 

 east, so that the sand and debris which it carries 

 settle on the west side of any bush or stone, because re- 

 lative calm prevails under the lea of the bush. Several 

 species of Passerine birds regularly make their nest 

 against the windward side of small bushes, and so 

 avoid being smothered with sand and debris. They 

 then fortify the outer side of the nest with a mass of 

 pebbles, apparently in order to protect the nest 

 from wind. This seems to be the invariable habit 

 of Clot-bey's Lark {Rhamphocorys clot-bey), which 

 collects pebbles weighing as much as half an ounce. 

 In the Larks Ammomanes phoenicurus arenicohr and 

 Eremophila alpestris hilopha, the habit of placing 

 the nest on the windward side of a smaU bush is 

 apparently universal, but the barricade of stones is 

 occasionally very small or quite absent. The nests 

 of the Trumpeter Bullfinch {Erythrospiza githaginea 

 zedlitzi) are invariably on the windward side of small 

 bushes, and perhaps protected by their position 



