Implications
Our work adds to a growing body of literature on the importance of snow for mediating survival and population persistence of winter-adapted species in northern ecosystems (Petty et al. 2015, Zuckerberg and Pauli 2018, Smith et al. 2022). Phenotypic mismatch with snow presence or absence appears to impact ruffed grouse winter survival and provides a possible explanation for the proposed geographic structuring of morph ratios across the ruffed grouse’s range. It has long been speculated that morph ratios in grouse covary with latitude and snow season length, with higher proportions of gray birds in northern areas and more red individuals in southerly and coastal regions (Bump et al. 1947, Rusch et al. 2020), but there has been no previous work to formally describe the potential mechanisms of these patterns.
Investigating drivers of morph-specific survival differences in color polymorphic species is critical for understanding how species might adapt to novel environmental conditions such as reduced snow cover in a rapidly warming climate. Modern climate change has created and will continue to create novel selective pressures and environmental conditions for species (Parmesan 2006). For many high-latitude ecosystems, and specifically the Upper Midwest, climate change has resulted in earlier snow melt, a reduction in the number of days with persistent snow cover (Choi et al. 2010), and an overall decrease of snow cover extent across the Upper Midwest (Andresen et al. 2014). Climate predictions for the Upper Midwest and Northeastern United States project these trends will continue in future decades (Notaro et al. 2014, Demaria et al. 2016), and pose a significant threat for winter-adapted species (Williams et al. 2015). Understanding how snow cover affects survival for winter-adapted species is crucial to informing conservation decisions as climate change continues to alter snow cover duration and extent (Sultaire et al. 2016, Zuckerberg and Pauli 2018). This is especially true for ruffed grouse, a species experiencing population declines across the Upper Midwest, specifically along its southern range boundary in Wisconsin. Our findings may help inform future conservation actions such as selecting individuals best-suited for translocations or identifying regions in a species’ distribution where they may be most vulnerable or resilient to shifting winter conditions (Roulin 2014, Baling et al. 2016, Mills et al. 2018, Smith et al. 2022).