EXPERIMENTAL TESTS OF NEST-SITE COMPETITION IN 2 PEROMYSCUS SPECIES

The importance of interspecific competition for nest sites between the white-footed mouse (Peromyscus leucopus noveboracensis) and the cloud land deermouse (P. maniculatus nubiterrae) were investigated in the mo ntane forests of southwestern Virginia over 3 years. Trials were condu cted for both species using large, outdoor enclosures in order to exam ine: (i) nest site preference in isolation and (ii) nest site selectio n made in the presence of potential competitors. Both species demonstr ated a strong preference for arboreal nest sites when tested without c ompetitors present. After the introduction of a heterospecific intrude r, Fl leucopus often shifted to a non-arboreal nest while P. maniculat us continued to use arboreal nests. Intruding P. maniculatus displaced resident P. leucopus from P. leucopus' preferred nest sites in all 3 years while intruding P. leucopus never displaced P. maniculatus. Neit her species was routinely displaced in conspecific trials. Resident P. maniculatus also excluded P. leucopus from access to preferred nestin g sites in all three years while P. leucopus were only able to exclude P. maniculatus in the 3rd year. Both species exhibite relatively low frequencies of exclusion in conspecific trials with the exception of P . maniculatus which excluded high proportions of conspecific intruders in the second year. We suggest that the asymmetrical advantage enjoye d by P. maniculatus in nest site selection may result from both site-s pecific effects and a species-specific influence on P. leucopus. Nonet heless, the intensity of such influence varied between years, perhaps as a function of population density, and did not appear to drastically influence continued coexistence of these congeners.