Movements, habitat associations, and survival of Columbian white-tailed deer in western Oregon

Abstract

Columbian white-tailed deer (Odocoileus virginianus leucurus, CWTD) are a geographically isolated and federally endangered sub-species for which there is a paucity of recent ecological information. I described and examined sources of variation in spatial use patterns (i.e. home range, areas of concentrated use, and movements), habitat associations, and survival for adult and fawn CWTD in Douglas County, Oregon. I radio-collared and monitored 64 adult CWTD and 36 newborn fawn CWTD from ca. December 1995 - September 1998. Locations for adults were obtained from September 1996 - December 1998. Locations for fawns were obtained from June - September, 1997 - 1998. Spatial use estimates for adults tended to be variable among deer. Mean 95% fixed kernel home range size was 74.5 ha (CV = 83%), while areas of concentrated use averaged 8.5 ha (CV = 93%). The sexes appeared to be partitioning space because males had larger home ranges, areas of concentrated use (which are analogous to core areas), and movements than females. Deer inhabiting human-influenced areas (suburban deer) consistently exhibited smaller movements and used less space than those away from human influence (wild deer). Mean home range size, area of concentrated use size, and distance between successive locations was generally greatest in fall and lowest in winter. Site fidelity to seasonal home ranges and areas of concentrated use was lowest between fall-winter and summer-fall. Unexpectedly, size of home ranges and areas of concentrated use were positively correlated with cover type heterogeneity. Annual adult survival rates averaged 0.73. Neither annual survival rates nor functions differed by sex or type. Survival over the entire 3 year study was low (0.39). Most deer died in winter from a combination of emaciation and disease and generally were in poor body condition. Oak-hardwood woodland, riparian, and oak-hardwood savanna shrub were the most frequently used cover types. On an annual basis, the majority of deer (31%) selected riparian areas. The frequency of selection of all non-riparian cover types was < 13%, as most deer either exhibited avoidance or 'neutral selection-avoidance' of these cover types. The probability of use for a particular patch was positively associated with proximity to a stream for 56% of all deer. However, the relative amount of edge within a particular patch had little effect on the odds of use. Patterns of use and selection tended to be similar between the sexes, but suburban deer used conifer and yard cover types more frequently than wild deer. Use and selection of cover types by deer did not significantly differ among seasons. CWTD also demonstrated higher use for more open cover types during crepuscular and nocturnal periods. Areas of concentrated use were random with respect to cover type composition, but were significantly associated with proximity ([less than or equal to] 200 m) to streams. Home ranges, areas of concentrated use, and movements of fawns were variable but tended to reflect their sedentary nature. Home range and area of concentrated use size were not correlated with percent coverage of oak-hardwood woodland, riparian, or shrub dominated cover types. Habitat use patterns were characterized by frequent use of oak-hardwood woodland and riparian cover types, and areas within 200 m of streams. These habitat use patterns may represent some degree of rigidity in habitat composition within fawning areas. However, there was no apparent selection of cover types or distance to stream classes within areas of concentrated use. Fawn survival to 6 months was low (0.15, 95% CI = 0.009 - 0.308) and was most similar to estimates from previous CWTD studies and unhunted white-tailed deer populations. Mortality was highest during approximately the first 1 - 1.5 months of life. There were no differences in survival to 6 months by sex, but females had significantly higher survival during the neonatal period than males. Survival time was unrelated to both movements and use of oak-hardwood woodland and riparian cover types. Predation was the most frequent (n = 8) cause of death, followed by abandonment (n = 5). Overall, adult CWTD were sedentary, dispersal movements were not readily apparent, and survival rates were within the range of rates reported for other white-tailed deer populations. Adult CWTD also exhibited a degree of specialization for riparian areas, and generalized use patterns for other cover types. CWTD fawns tended to use small areas, exhibited frequent use of oak-hardwood woodlands and areas near streams. Survival rates of adults and fawns may be suggestive of density dependent population responses.