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.