water turnover in the vole is about tlie same as in 

 the mouse, but water loss through the lungs and skin 

 is mucli lower, indicating acclimatization to a drier 

 habitat (Chew 1931). 



Diiirnalion and aspccliim 



When two or more species are competing for 

 the same resources of a single habitat, this competi- 

 tion is reduced or eliminated if one species makes use 

 of these resources at a different time of day, or in a 

 different season, than the other. 



The white crappie and black crappie are very sim- 

 ilar in habits, food requirements, and local distribu- 

 tion, except that the white crappie is more often found 

 in small rivers and creeks than the black crappie, 

 which prefers hard-bottom lakes. Both species feed 

 most extensively at dusk ; there is a smaller feeding 

 activity peak at dawn. Both species feed to some ex- 

 tent during the night. However, the white crappie 

 feeds considerably more during the daylight period 

 than does its more aggressive black cousin, and this 

 slight difference in timing may be sufficient to permit 

 it to occur in the same areas as the black crappie 

 (Childers and Shoemaker 1953). 



During a winter in England, when birds were com- 

 ing to banding traps for food, it was noted that the 

 European robin did so most frequently just after sun- 

 rise and just before sunset, the European blackbird 

 just before and after midday, while the blue tit had 

 peaks of feeding between the feeding times of the 

 other two species (Lees 1948). 



The females of the butterfly Colias citrythcme 

 may be either orange or white : all males are orange. 

 There is partial separation of the two color phases 

 in that the white females are relatively more active 

 in the early morning and the yellow females later in 

 the day (Hovanitz 1948). 



The two grasshoppers Arphia sulphur ea and A. 

 .vanthoptcra occupy similar niches except that A. sul- 

 phurea overwinters in the nymph stage, reaching ma- 

 turity from April to late July, while A. xanthoptera 

 overwinters in the egg stage and hence requires a 

 longer time to mature in the spring. The adults of the 

 latter species occur from late July to early November 

 (Blatchley 1920). 



Three kinds of sockeye salmon occur in Cultus 

 Lake, British Columbia (Ricker 1938) : (1) the nor- 

 mal anadromous stock, whose ofTspring may either 

 migrate out to sea or remain as (2) residuals, and 

 (3) the land-locked salmon, which remain continu- 

 ously in the lake. The anadromous and residual popu- 

 lations differ in breeding coloration, but both spawn 

 from October to December. The land-locked forms, 

 which closely resemble anadromous sockeye in breed- 

 ing coloration, spawn only in August and September. 



The tern Sterna virgala nests on the Kerguelen 

 Islands in October and November ; S. viltata uses the 

 same nesting area in January and February. The 

 niche of each species differs somewhat further in that 

 ^. viryata feeds to some extent in inland waters, but 

 S. vittata is strictly marine (Murphy 1938). 



Trojiical species of dragonflies and damselflies 

 which have recently extended their range northward 

 confine their main periods of flight to the warmest 

 months and to the middle of the day. Native species 

 that are better adajited to colder climates are active 

 in early spring and autumn and in the twilight hours 

 of the day (Kennedy 1927). Times of emergence of 

 stream insects are also correlated with their an- 

 cestral places of origin and the extent of their acclima- 

 tization to temperature. 



Shelu 



'gctalion 



Animals require shelter or cover as a protection 

 against unfavorable weather and enemies. Caves, 

 overhanging ledges, deep valleys or canyons, or bur- 

 rows in the ground may serve as shelter for terrestrial 

 animals. The darkness of night is a protection against 

 diurnal predators; daylight is a protection against 

 nocturnal ones. 



Vegetation is an important source of shelter for 

 animals. Some animals cannot tolerate too much solar 

 insolation, hence seek shade. In arid habitats, jack 

 rabbits shift the location of their forms on the ground 

 at different times of the day to stay in the shade of 

 bushes, and it is a common sight in prairie or desert 

 regions to see horned larks, meadowlarks, or other 

 birds lined up in the narrow shadow cast by telephone 

 poles or fence posts. Burrows of all sorts, whether 

 in the ground or in trees, give the animal good insula- 

 tion against both winter cold and summer heat. The 

 foliage of trees, shrubs, and even grasses and vines 

 reduces the amount of heat radiated from the bodies 

 of animals, especially warm-blooded ones, on cool 

 clear nights, and vegetation in general serves as a 

 windbreak. Birds keep to the lee side of exposed 

 patches of woods during cold windy weather. 



By staying under cover, prey animals may escape 

 notice of passing predators, or if detected, may more 

 easily avoid capture. Dense vegetation, thorny thick- 

 ets, burrows, and other situations impenetrable to 

 predators are sometimes called escape cover. We also 

 speak of nesting cover, zvintcr cover, and roosting 

 cover, depending on the particular purpose which the 

 cover serves. Animals are often protectively colored 

 to conceal themselves better from enemies in partic- 

 ular kinds of cover. 



WHnen the beetles Tribolium confusum and Ory- 

 zaephilus surinamensis are introduced experimentally 

 into a flour medium, Tribolium is ordinarily success- 



Niche segregation 247 



