Winter survival and cause-specific mortality of white-tailed deer in northern Minnesota: An update

Ongoing studies that examine the influences of environmental, intrinsic, and demographic factors on survival and cause-specific mortality rates of white-tailed deer (Odocoileus virginianus) have been critical to enhancing our understanding of population performance and to improving management. A recent evaluation report from the Office of the Legislative Auditor recommended that the “…DNR should conduct field research to collect and utilize more information about Minnesota’s deer… and inform the department’s vital rate estimates of deer births and deaths, and better reflect deer population dynamics” to improve our understanding of demographics and habitat requirements. Using cutting-edge global positioning system (GPS) collars, and remote sensing and geographic information system (GIS) technologies, we recently launched a study that will inform a level of understanding of habitat requirements and drivers of population performance required by managers to prescribe forest manipulations that best support population goals. Herein, our objectives are to compare winter survival rates and cause-specific mortality (and influential factors) of adult (≥1.5 yr) female deer residing on study sites in northcentral (Inguadona Lake) and northeastern (Elephant Lake) Minnesota. We predicted that survival, percent winter mortality, and the impact of wolf (Canis lupus) predation would be influenced by winter severity in a way that is consistent with our understanding of this relationship garnered from a previous long-term (1991−2005) study in northcentral Minnesota. The natural mortality rate during the first winter (2017−2018) was high; 6 of 19 (31.6%) GPScollared adult female deer (3 at each site) were all killed by wolves during 10 April to 31 May 2018. Overall survival had decreased to 0.68 (95% confidence interval [CI] 0.50−0.93) by then. But this was a pilot year, so the survival estimate was limited by small sample sizes (10 collared deer per site) and represented only the late–winter season (12 March to 28 May 2018) due to delayed capture operations. However, during the second winter (2018−2019), with more than twice the sample size (n = 51), the natural mortality rate was also high (36.7%); 17 of 49 deer were preyed upon wolves and 1 by bobcat (Felis rufus) between 1 November 2018 and 20 April 2019 (cutoff for analysis for this annual report). Eight mortalities occurred at Inguadona Lake and 9 at Elephant Lake. The overall survival rate was 0.70 (95% CI = 0.57−0.86). The wolf predation rates during the 2 winters (31.6% and 34.7%) notably exceeded what we had expected based on the documented relationship of the previous long-term study. Typically, adult female deer enter winter in better physical condition than fawns and adult males, and thus have the highest probability of surviving winter. Our findings at least suggest that during both winters overall mortality rates at the population level, across sex and age classes, were likely higher than indicated by our adult female data. Ongoing federal protection of wolves in Minnesota limits Minnesota Department of Natural Resources (MNDNR) management options and has at least contributed to the estimated wolf population almost doubling from winter 1988−1989 (1,521 wolves) to the present (~2,900 wolves). Caution may be warranted in interpreting our preliminary findings, but they highlight the need for multi-year continuation of this study to better understand whether deer-habitat-wolf predation relationships have been changing since completion of the MNDNR’s previous long-term study, a potentially significant consideration relative to implementation of the state’s recently developed deer management plan.