Survival, family breakups, and dispersal of yearling and subadult black bears in western Virginia.
Lee, Daniel James
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Reported survival rates, dates of family breakup, and dispersal patterns for yearling and subadult bears in hunted black bear (Ursus americanus) populations in North America are scarce. We estimated survival rates of yearling and subadult black bears from a hunted population in western Virginia during 1999 - 2002. We captured and marked 307 different individual yearling and subadult bears on 2 study sites, and attached ear tag transmitters or radio collars to 54 (34M : 20F) 1-year-old, 52 (23M : 29F) 2-year-old, and 35 (8M : 27F) 3-year-old black bears. We used the known fate model in program MARK to estimate annual, non-hunting, and hunting season survival for radio-marked bears of each age and sex class. Additionally, we used mark-recapture data in the recaptures only, dead recoveries, and BurnhamÂ¡Â¯s combined models within program MARK to estimate annual survival for each age and sex class. One-, 2-, and 3-year-old female survivorship was 0.87 (95% C.I. 0.78 - 0.92), while 1-year male survivorship was 0.32 (95% C.I. 0.20 - 0.47), and 2- and 3-year-old male survivorship was 0.59 (95% C.I. 0.47 - 0.71) from the Burnham's combined model. Survival rates for 1-year-old females (Â¦Ã 2 = 6.20, P = 0.01) and 2-year-old females (Â¦Ã 2 = 7.74, P = 0.01) were higher than males in each age category, respectively. However, we detected no difference between 3-year-old females and 3-year-old males (Â¦Ã 2 = 2.61, P=0.11), likely due to small sample size of males (n = 4). Low yearling and subadult survival is not likely a cause for alarm due to the importance of adult female survival to population growth and the promiscuous mating system in black bear populations. Family breakup is an important event in the life history of black bears, marking the initial dispersal and home range construction of yearling bears, and perhaps marking the timing of estrus and breeding opportunities for adult females. We monitored 6 black bear family groups with 12 yearlings (6M : 6F) to determine the timing of family breakup; we intensely monitored 3 of the family groups to document home range establishment and movements by 5 subadult bears (2M : 3F) following separation from their mothers. Estimated dates of family breakup were 28 May and 2 June. Family breakups occurred before peak dates of estrus on our 2 western Virginia study areas. We detected 2 reassociations between a mother and her yearling offspring. Following family breakup, female yearlings (n = 3) remained within or partially on their mothers' home range while subadult males (n = 2) left their mothers' home ranges. All yearlings (n = 5) shared Â¡Ã 50% of post-breakup home range with their mothers. We studied the movements of 31 (11M : 20F) subadult black bears born on our 2 study areas in western Virginia and 70 (44M : 26F) subadult bears captured during the summer on the study areas for dispersal. No radio-marked, resident, subadult female bears exhibited dispersal behavior while 3 of 11 (27%) radio-marked, resident, subadult males dispersed (P = 0.04). Resident and summer capture male bears moved greater distances than females from yearling den location (Â¦Ã 2 = 8.54, P = 0.01, df = 2) or summer capture location (Â¦Ã 2 = 22.02, P < 0.01, df = 2); no female moved > 10 km between initial and final locations (x = 2.7 km, range 0.2 - 9.0 km). The greatest subadult male movement was 80 km (x = 13.4 km, range 0.6 -0.80 km), and dispersal movements primarily occurred within the 1 and 2-year-old age classes. Direction of movement between initial and final locations for dispersing bears was not random (Rayleigh's r = 0.56, P = 0.02); bears appeared to follow the orientation of the predominant ridgelines and avoided leaving the national forest.
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