448 LAND ANIMALS 



vegetation characteristic of early stages in the grassland succession, 

 and hence extensive burrowings alter the appearance of the grassland 

 as well as opening the substrata to ready penetration by air and water. 

 The ground excavated by such burrowing activities spreads out and 

 forms, for example in the Kalahari, a sand layer of 0.5-1.0 cm. in 

 depth. 11 



A large number of birds which nest on the ground, such as larks, 

 bustards, and gallinaceous birds, are present ; if there are no groves or 

 rocks, even large birds of prey like the eagle nest on the level ground. 12 

 The hardships of winter and the drought of summer influence bird 

 migration. The burrowing habits of the mammals are also found 

 among birds, a number of which breed in holes in the ground in open 

 country, though they seldom prepare their burrows by their own efforts, 

 but rather make use of the abandoned burrows of rodents or occupy 

 the same hole with them. The burrowing owl, Speotyto cunicularia, 

 may prepare a nesting hole in the ground; it does this, for example, 

 both in Uruguay and in the savannas of Guiana, 13 but where oppor- 

 tunity presents itself, it enters into a tenant relationship with the 

 rodent and is found as a co-inhabitant in the holes of the prairie dog 

 (Cynomys) on the prairies of North America and in those of the vis- 

 cacha in the pampas. Two small birds live as tenants in the burrows 

 of the South American viscacha, the mud dauber {Geositta cunicu- 

 laria), which builds its own brood nest in the steep entrance of the 

 burrow, and the swallow Attycora cyanoleuca. The burrows of the 

 whistling hare in high Tibet are used by three small species of finches 

 (Montifringilla) , and many other examples could be given. 14 



The cursorial habit. — Tree trunks, underbrush, tangled roots, 

 dead trees, lianas and aerial roots, and all the barriers which oppose 

 free movement of animals in a horizontal direction in the forest, are 

 absent in the open country. For this reason, many animals may here 

 develop to advantage great speed and endurance in their movements, 

 and the more so as they possess fewer hiding places. The stimulus to 

 such development is greater, the more open the countryside, and the 

 greater the temperature variation from summer to winter and the 

 scarcer the watering places. Because of their speed, such animals are 

 able to escape their enemies, to traverse daily the great distance to 

 water, and to migrate from the summer drought and winter frost. This 

 swift-moving element in the animal communities of the plains contrasts 

 strikingly with the burrowers discussed above. 



The frequent occurrence of quick-moving animals in open country 

 is striking among reptiles. The lizards of the steppe regions are for the 

 most part slender, very active, and agile, like the species of Eremias 



