AMERICAN BATS — HANDLE Y 169 



a collector may utilize the night-time roost as an effective trap. 

 Two small buildings at Lebec, Calif., served as bat traps for Dalquest 

 on 4 June 1945. At midnight the traps held a Plecotus tovmsendii, 

 6 Myotis yumanensis, and 2 Myotis volans. Earlier in the night 

 these same traps caught 11 Myotis yumanensis, 3 Myotis volans, 

 1 Myotis calif ornicus , and 4 Antrozous pallidus. Three Myotis 

 thysanodes were taken later in the traps. Often the night-time 

 retreats are uninhabited by bats during daylight hours. 



There is much variation in the selection of specific microhabitats 

 within the roosting structures chosen by various kinds of bats. Many 

 authors have noted the preference of P. tovmsendii for an open ceil- 

 ing, wall, or beam rather than a crack or crevice as a roosting site. 

 Apparently this species seldom crawls away from the spot where 

 it first lands, and thus normally is found hanging in places open 

 enough to be reached by flying. In Kansas caverns Twente (1955b, 

 p. 721) observed that this bat characteristically roosted on very rough 

 walls. Wind currents and consequently temperature fluctuations were 

 often great in such places. However, the bats usually rested in slight 

 concavities or behind projections on the rough walls. In these pockets, 

 air movement was slight and the temperatm-e relatively constant in 

 contrast to the gross conditions prevailing a few inches away. 



The intensity of light in roosting sites of P. townsendii varies 

 from the total darlvness of deep caverns or tunnels, to semidarkness 

 in houses, small caves, and near the entrances of deeper structures. 

 With regard to Calif ornian populations, Dalquest (1947, p. 24) re- 

 ported that all retreats of Plecotus that he had examined were dark or 

 but dimly lighted. In the best lighted roosts the bats could be seen 

 without the use of a flashlight once the observer's eyes had become 

 accustomed to the darkness. Lights were necessary for observation 

 in the other retreats. 



Likewise, humidity seems to be usually of secondary importance in 

 roost-site selection. Twente (1955b, p. 716) noted an apparent indif- 

 ference of the big-eared bat to varying degrees of humidity (90 to 21 

 per cent in hibernating sites). Beer and Richards (1956, p. 34) re- 

 ported a similar variation (100 to 38 per cent) in the relative humidity 

 of Eptesicus hibernating sites. 



Temperature appears to be the critical factor. Twente (1955b, p. 

 706) observed that bats must rest in habitats whose temperatures are 

 suited to their metabolic needs. Thus, there may be variation in 

 choice of roost according to season, time of day, weather, sex of bat, 

 etc. For example, hibernating bats must roost in cool places to keep 

 metabolism low so that energy reserves will be conserved. During the 

 feeding season warm roosting sites permit digestion and assimilation 

 of food to proceed. In general, P. tovmsendii roosts in warm places 



