Willow ptarmigan complicated changes in plumage several times a year and the differing colour and pattern of the summer plumage: The willow ptarmigan often hybridises with the black grouse ("Tetrao tetrix") and the hazel grouse ("Tetrastes bonasia") and occasionally with the western capercaillie ("Tetrao urogallus"), the spruce grouse ("Falcipennis canadensis") and the rock ptarmigan ("Lagopus muta"). During the Pleistocene, the willow ptarmigan widely occurred in continental Europe. Authors who recognize paleosubspecies have named the Pleistocene willow ptarmigan "L. l. noaillensis" (though the older name "medius" might be the correct one). These marginally different birds are said to have gradually changed from the
Ocean acidification in the Arctic Ocean has also allowed more Pacific water to flow into in the Arctic ocean during the winter, this is called Pacific winter water. The Pacific water flows into the Arctic ocean carrying additional amounts of carbon dioxide by being exposed to the atmosphere and absorbing carbon dioxide from decaying organic matter and from sediments. The Arctic ocean pH levels are rapidly decreasing because not only is the ocean water absorbing more carbon dioxide due to increased surface area exposure as a result of a decrease in sea ice. It also has large amounts of carbon dioxide being transferred to the Arctic