Winter range arrival and departure of white-tailed deer in northeastern Minnesota

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I analyzed 364 spring and 239 fall migrations by 194 white-tailed deer (Odocoileus virginianus) from 1975 to 1993 in northeastern Minnesota to determine the proximate cause of arrivals on and departures from winter ranges. The first autumn temperatures below −7 °C initiated fall migrations for 14% (95% confidence interval (CI) = 0 – 30) of female deer prior to snowfall in three autumns, but only 2% remained on winter ranges. During 14 autumns, the first temperatures below −7 °C coincidental with snowfalls elicited migration in 45% (95% CI = 34 – 57) of females, and 91% remained on winter ranges. Arrival dates failed to correlate with independent variables of temperature and snow depth, precluding predictive modeling of arrival on winter ranges. During 13 years, a mean of 80% of females permanently arrived on winter ranges by 31 December. Mean departure dates from winter ranges varied annually (19 March – 4 May) and between winter ranges (14 days) and according to snow depth (15-cm differences). Only 15 – 41% of deer departed when snow depths were >30 cm but 80% had done so by the time of 10-cm depths. Mean weekly snow depths in March (18 – 85 cm) and mean temperature in April (0.3 – 8.1 °C) explained most of the variation in mean departure dates from two winter ranges (Ely, R2 = 0.87, P < 0.0005, n = 19 springs; Isabella, R2 = 0.85, P = 0.0001, n = 12 springs). Mean differences between observed mean departure dates and mean departure dates predicted from equations ranged from 3 days (predictions within the study area) to 8 days (predictions for winter ranges 100 – 440 km distant).