Graduate Thesis Or Dissertation
 

Juvenile survival and birth-site selection of Rocky Mountain elk in northeastern Oregon

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https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/j38608843

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  • With declining populations and low calf recruitment in northeastern Oregon, much interest has been generated to study the survival rates and causes of mortality of Rocky Mountain elk (Cervus elaphus nelsoni). I investigated the causes of elk calf mortality and the effects of predation risk on birth-site selection by cow elk. Cow elk were captured in March of 2002-2004 to determine their pregnancy status and condition (percent fat, mass, and age). Pregnant cows had temperature-sensitive vaginal implant transmitters inserted that were expelled at parturition in order to locate newborn calves and birth sites later during the calving season. Elk calves were captured in two adjacent study areas, the Wenaha and Sled Springs, with the use of vaginal implant transmitters, a helicopter and net gun, and ground searching. A total of 222 calves were captured during the spring and summer of 2002-2004. Calves were sexed, aged, weighed, and fitted with radio-transmitting collars that had mortality sensors integrated into the circuitry. Monitoring of the radio-collared calves was done from fixed-wing aircraft. Calf collars in mortality mode were located on foot to investigate and determine cause of death. Predator-related mortalities were marked with a global positioning unit and the most likely spot where the calf was killed was flagged. I took microhabitat measurements that described horizontal and vertical cover at each birth and predation site and 2 paired random sites. During the summers of 2003-2004, I measured 49 birth sites and 62 predation sites along with 2 paired random sites for each. Using a geographic information system (GIS) I obtained slope and aspect for each site. I also used GIS to obtain macrohabitat data on birth, predation, and 80 random sites (for comparison). For each site, I measured the percent canopy cover and the amount of forest edge within a 250, 500, and 1,000 m radius circle. In addition, I also obtained the distance from each site to the nearest edge. I found differences in annual survival rates between the two adjacent study areas, with calves in Wenaha having lower survival (0.26; 95% CI = 0.15 to 0.42) rates than calves in Sled Springs (0.52; 95% CI = 0.37 to 0.56). Overall, predation was the main proximate cause of death. In 2003-2004, cougars (Felis concolor) killed 54% and 35% of radio-collared calves in Wenaha and Sled Springs, respectively. Annual calf survival was influenced by birth date (13 -0.35; 95% CI = -0.64 to -0.06). Both birth date (13 = -0.25 3; 95% CI = -0.502 to -.0003) and birth weight (13 = 0.13; 95% CI = -0.14 to 0.39) influenced summer calf survival, with earlier and heavier-born calves having higher survival rates than late and light-born calves. I found no differences in annual survival rates between the sexes in this study. However, I did find differences in summer survival rates between the sexes depending on the area. In Wenaha, females (0.57; 95% CI = 0.42 to 0.72) had higher survival than males (0.42; 95% CI 0.27 to 0.59), and in Sled Springs, males (0.82; 95% CI = 0.66 to 0.91) had higher survival than females (0.46; 95% CI = 0.31 to 0.61). Preliminary analyses suggested that cow condition did not influence calf survival, nor was cow condition correlated with the estimated birth date and birth weight of calves. However, the analyses that examined the effects of cow condition on calf survival were preliminary and inconclusive due to small sample sizes. I found that birth-site selection of cow elk was influenced by predation risk at the microhabitat scale. Cow elk selected birth sites that had less horizontal cover (0.97 times the odds; 95% CI = 0.94 to 0.99) and more overhead density of vegetation (1.022 times the odds; 95% CI = 1.005 to 1.040) than paired-random sites. Although predation sites also had less horizontal cover (0.985 times the odds; 95% CI = 0.973 to 0.998) than random sites, birth sites had less horizontal cover (0.983 times the odds; 95% CI = 0.967 to 1.001) and more overhead density of vegetation (1.02 times the odds, 95% CI = 1.01 to 1.04) than predation sites. I found no evidence that cow elk were influenced by predation risk at the macrohabitat scale. Cow elk chose birth sites with less canopy cover than random sites within a 500 m-radius circle. Cow elk were likely more influenced by forage availability rather than predation risk when selecting a birth site at the macrohabitat scale. When birth sites were compared to predation sites at this scale, I found no differences, further suggesting that birth-site selection was not influenced by predation risk. Cow elk were likely selecting broad areas for parturition that had sufficient forage to meet high nutritional demands due to lactation. Within these areas, cow elk selected birth sites that had high visibility at the microhabitat scale, presumably to detect predators visually and to avoid predation on calves.
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