Results
Over five winter seasons (November through March; 21 weeks per winter season), we monitored survival of 94 ruffed grouse: 21 in Winter Season 1 (nred = 12, ngray = 9), 15 in Winter Season 2 (nred = 9, ngray = 6), 20 in Winter Season 3 (nred = 12, ngray = 8), 15 in Winter Season 4 (nred = 10, ngray= 5), and 23 in Winter Season 5 (nred = 19, ngray = 4). We detected 34 mortalities (nred = 23, ngray = 11) across all winter seasons. Most mortalities were attributed to predation: 18 by raptors, 5 by carnivorans, and 3 attributed to unspecified species. There were 5 mortalities attributed to exposure and 3 unknown causes.
While snow presence or absence varied spatially across Sandhill, snow was present consistently from weeks 9 through 15 across all five winters, with variable onset and snowmelt dates across winters. Snow onset was earliest (week 5) in Winter Season 2 and snowmelt was latest (week 21) in Winter Season 5 (Fig. S1). The total number of days with snow on the ground varied from 80 days in Season 4, 91 days in Season 1, 101 days in Season 2 and Season 5, and 104 days in Season 3. Average daily minimum temperature did not vary statistically between years (ANOVA, F 4,735 = 2.148, p = 0.08).
The top-ranked survival model from our final set of models indicated that winter survival varied among years (Fig. 3B). The mean derived survival was lowest during Winter Season 2 (S = 0.41; 95% CI: 0.24 – 0.59), and during other seasons, mean survival ranged from 0.55 (95% CI: 0.35 – 0.73) during Winter Season 4 to 0.85 (95% CI: 0.61 – 0.95) during Winter Season 3 (Fig. 3B).
In our first candidate model set testing the effect of phenotypic mismatch and winter weather on grouse survival, the two top-ranked models included a common intercept and phenotypic mismatch for each winter season (Table 1). During Winter Season 2 (the season with lowest overall survival), phenotypic mismatch was associated with lower survival (β = -1.54, SE 0.52), but parameter estimates of mismatch for other years had confidence intervals which overlapped zero. The next top-ranked model contained phenotypic mismatch and minimum temperature, although the 95% confidence interval for minimum temperature overlapped zero (β = 0.018, SE 0.03), and was likely an uninformative parameter (Arnold 2010). Together, these top-ranked models carried 66% of the cumulative model weight (Table 1). Models containing parameters for snow cover or color morph, without accounting for phenotypic mismatch, were not competitive (Table S2).
Table 1. Model selection results for the effects of phenotypic mismatch, snow cover, and minimum temperature on ruffed grouse overwinter survival. Phenotypic mismatch (MM) with snow cover is present in both top-ranked models, supporting our hypothesis that mismatch is an important driver of survival. Variables below include MM1 – MM5 (weekly time-varying individual covariates of mismatch for each winter season, i.e., the proportion of days in a week an individual is phenotypically mismatched with the snow cover), and Min. Temp. (mean daily minimum temperature). All values were averaged spatially over individual winter use areas and time-varying values were averaged temporally by week. Models ranked by AICc (Akaike’s Information Criterion, corrected for small sample size). ΔAICc represents the difference between the top-ranked model in the candidate set, wis the model weight, and k is the number of parameters. Only models within 2 ΔAICc of the top-ranked model and the intercept-only model are shown.