LIFE FORMS AND LIFE HABITS 253 



taxonomic groups, such as the mammals, birds, or insects, exhibit sig- 

 nificant figures for expressing the characteristics of a particular biome 

 and permitting comparison with others. 



These data are based on lists by Gary (1917) for Colorado grass- 

 land and Howell (1921) for deciduous forest in Alabama. The two 

 sets of data were gathered for the U. S. Biological Survey at the same 

 period. Comparison can be made only within a natural group, for 

 example grassland birds may be compared with forest birds, mammals 

 with mammals, etc. 



In the case of grassland birds the majority breed in nests built on 

 or near the ground, 53 per cent contrasting with 20 per cent for the 

 forest. On the other hand, fallen or standing trees are used by 68 per 

 cent of the forest birds. A great preponderance of subterranean spe- 

 cies characterizes the grassland mammals, exclusive of bats. There 

 are other life-habit characteristics which cannot be expressed in tables. 

 For example, Craig (1908) points out that forest birds rarely sing on 

 the wing, but eight species of common North Dakota birds do so, 

 namely: horned lark, bobolink, Smith's longspur, chestnut-collared 

 longspur, lark sparrow, lark bunting, purple martin, and Sprague's 

 pipit. 



Visibility in grassland is high, and animal habits are adjusted in 

 accordance with it. Eyesight is keen in prairie species, and observa- 

 tion from vantage points takes the place of secretive retreat (Bailey, 

 1931:307; Seton, 1929:443), which characterizes similar animals in the 

 forest. The prairie dog sitting up on its burrow mound exhibits a 

 habit shared by its ecological equivalents, the Richardson ground 

 squirrel and picket-pin gopher. This outlook habit also characterizes 

 the behavior of one or more burrowing forms of other grasslands, e.g., 

 the viscacha in southern South America; the bobac or tarbagan in 

 central Asia, and the meerkat, a carnivore, in Africa. 



Several of these live in ''towns" or are aggregated into groups, this 

 particular mode of life evidently fitting well into the grassland com- 

 munity. The large prairie-dog towns, large herds of antelope (400 

 animals or more reported by early explorers), and the enormous herds 

 of bison (100,000 to 2,000,000; Seton, 1929) bespeak this habit, which 

 is evident also in the grasslands of Eurasia and Africa. In southern 

 South America, where larger game is scarce, the pampas deer is not 

 particularly gregarious, but the weasel or tayra and the ostrichlike rhea 

 assume this habit (Hudson, 1892). As to smaller birds, Hudson also 

 reports the carancho as hunting in bands, and, according to Brehm 

 (1896), flocks of the lesser kestrel and the redfooted falcon seek in- 

 sects on the Asiatic grasslands. Craig (1908) points out the gregari- 



