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PROCEEDINGS OF THE NATIONAL MUSEUM 



in adults). Basing their figures on these facts and on recovery of 

 banded individuals, Pearson, et al., reasoned that a colony of 100 fe- 

 males should produce about 45 young females in a season. Of this 

 number, 20 would siu-vive to join the colony as breeding adults the 

 following season. Assmning an average survival of 80 per cent of these 

 bats each year after their first and an equal rate of mortality for adults 

 of all ages, the colony would, as they found through three years of 

 observations, remain static at about 100 breeding females each season. 

 Age composition of such a colony would be as in table 6, with the me- 

 dian age 3.1 and the average age 5.0 years. Figures beyond the 3-year- 

 old age class are theoretical. 



Table 6. — Theoretical age composition of a nursery colony of 100 female Plecotus 

 townsendii. {Modified from Pearson, et al., 1952, p. 315.) 



Pearson, et al. (1952, p. 316) estimated a population density of one 

 P. townsendii per 310 acres on 62,000-acre Santa Cruz Island off the 

 coast of California. The island was shared with at least an equal 

 number of Antrozous pallidus and an undetermined number of Myotis 

 calif ornicus. Basing their estimate on the spacing of known colonies, 

 these authors supposed that on the mainland of California the popula- 

 tion density might be as small as one Plecotus per 419 acres, but 

 supposed that in actuality each Plecotus probably had available con- 

 siderably less than 419 acres. This would be especially true if un- 

 known food habits restrict the hunting of P. townsendii. By doubling 

 the estimated total of individuals in nursery colonies, Twente (1955a, 

 p. 387) determined that there might be 300 to 500 P. townsendii in 

 the gypsum caves of extreme northern Oklahoma and adjacent parts 

 of Kansas. For the same area he estimated 25,000 to 50,000 Tadarida 

 brasiliensis, 15,000 to 20,000 Myotis velifer, and 200 to 400 Antrozous 

 pallidus. No estimate was made for Eptesicus fuscus and Pipistrellus 

 suhjlavus which were present in smaller numbers. 



How populations of P. townsendii are limited is not clear. Probably 

 the same factors do not operate throughout the bat's range. Twente 

 (1955a, p. 388) calculated that the yearly mortality could not exceed 

 33.3 per cent in animals with a reproductive potential of only one 

 young per year if the population was to maintain a constant level. 

 On this basis the daily population decrease could not be more than 

 1.1 per thousand. Twente suggested that in Kansas and Oklahoma 

 the observed predation on bats by rat snakes, raccoons, hawks, owls, 



