There are old, curious records of willow ptarmigan in northern WI and MN, and I’ve always wondered what the story was there. I found a paper (reference at the bottom of the post) describing the biome changes that accompanied the transition from the Pleistocene to the Holocene.
“From the west to the east, a new forest zone, soon to be a boreal forest biome, cut the tundra-steppe zone apart. The southern belt of the grasslands transformed into the steppe and forest-steppe biomes. Many species of the tundra-steppe formed disjunct ranges on both the southern and northern parts of the boreal forest, while others survived either on the northern or southern side of the boreal forest…For Willow Ptarmigan, this process forced the formation of four types of habitat adaptations for raised bogs, moors and river valleys, forest-steppe, and mountainous habitats.” (Potapov 2011)
So basically, willow ptarmigan adapted to live in bogs, and we’ve probably driven them out with…as usual, land cover conversion and climate change. Related changes affecting community interactions may also be at play. The current range is nothing short of mid-Canada, though it’s interesting that they move south in the winter. Also, like many Arctic dwellers, it appears they have population cycles.
POTAPOV, R., AND E. POTAPOV. 2011. Willow and Rock Ptarmigan monitoring in Russia: An historic overview. Pages 213–228 in R. T. Watson, T. J. Cade, M. Fuller, G. Hunt, and E. Potapov (Eds.). Gyrfalcons and Ptarmigan in a Changing World, Volume II. The Peregrine Fund, Boise, Idaho, USA. http://dx.doi.org/ 10.4080/gpcw.2011.0221