A long-term assessment of the variability in winter use of dense conifer cover by female white-tailed deer.

BACKGROUND: Long-term studies allow capture of a wide breadth of environmental variability and a broader context within which to maximize our understanding of relationships to specific aspects of wildlife behavior. The goal of our study was to improve our understanding of the biological value of dense conifer cover to deer on winter range relative to snow depth and ambient temperature. METHODOLOGY/PRINCIPAL FINDINGS: We examined variation among deer in their use of dense conifer cover during a 12-year study period as potentially influenced by winter severity and cover availability. Female deer were fitted with a mixture of very high frequency (VHF, n = 267) and Global Positioning System (GPS, n = 24) collars for monitoring use of specific cover types at the population and individual levels, respectively. We developed habitat composites for four study sites. We fit multinomial response models to VHF (daytime) data to describe population-level use patterns as a function of snow depth, ambient temperature, and cover availability. To develop alternative hypotheses regarding expected spatio-temporal patterns in the use of dense conifer cover, we considered two sets of competing sub-hypotheses. The first set addressed whether or not dense conifer cover was limiting on the four study sites. The second set considered four alternative sub-hypotheses regarding the potential influence of snow depth and ambient temperature on space use patterns. Deer use of dense conifer cover increased the most with increasing snow depth and most abruptly on the two sites where it was most available, suggestive of an energy conservation strategy. Deer use of dense cover decreased the most with decreasing temperatures on the sites where it was most available. At all four sites deer made greater daytime use (55 to >80% probability of use) of open vegetation types at the lowest daily minimum temperatures indicating the importance of thermal benefits afforded from increased exposure to solar radiation. Date-time plots of GPS data (24 hr) allowed us to explore individual diurnal and seasonal patterns of habitat use relative to changes in snow depth. There was significant among-animal variability in their propensity to be found in three density classes of conifer cover and other open types, but little difference between diurnal and nocturnal patterns of habitat use. CONCLUSIONS/SIGNIFICANCE: Consistent with our findings reported elsewhere that snow depth has a greater impact on deer survival than ambient temperature, herein our population-level results highlight the importance of dense conifer cover as snow shelter rather than thermal cover. Collectively, our findings suggest that maximizing availability of dense conifer cover in an energetically beneficial arrangement with quality feeding sites should be a prominent component of habitat management for deer.

Winter body condition of moose (Alces alces) in a declining population in northeastern Minnesota.

Assessments of the condition of moose (Alces alces) may be particularly informative to understanding the dynamics of populations and other influential factors. During February-March 2003 to 2005, we assessed the nutritional condition of 79 moose (39 females, 40 males) in northeastern Minnesota by body condition scoring (BCS(F), scale of 0-10); 67 of these by were assessed by ultrasonographic measurements of rump fat (Maxfat), which was used to estimate ingesta-free body fat (IFBF) in all but two of the females. Scores of the BCS(F) were related (r(2)=0.34, P<0.0001) to Maxfat. Body condition scores were not affected by sex × capture-year, capture-year, or age-at-capture, but the mean body condition score of males (6.5 ± 0.2 [SE], n=40) was less (P ≤ 0.009) than that of females (7.4 ± 0.2, n=39). Overall, Maxfat ranged from 0.0 to 4.6 and 0.3 to 2.8 cm in females and males, respectively, and was unaffected by age-at-capture. There was a sex×capture-year effect (P=0.021) on Maxfat; mean values were stable for males during the winters of 2003 to 2005 but in females were lowest during 2003, consistent with the lowest pregnancy rates and lowest winter and spring survival compared to 2004 and 2005. Based on estimates of percent IFBF, late winter-early spring survival in 2003 of at least 11% of the collared animals assessed by Maxfat, 21% of the adult females, specifically, may have been seriously challenged directly by poor condition. Data from this study provide reference values and assessments of body condition of moose that will be an essential component of the additional, comprehensive research needed to better understand the influence of extrinsic and intrinsic factors on the performance of this viable, but declining, population. For future research, we will concentrate on developing a more-reliable BCS which would allow IFBF estimation once rump fat is depleted.