In spring, northern range wildlife grows strong after a long, cold winter as melting snow and rain turn the landscape green and lush. Bears emerge hungry from their winter dens to feed on roots and grasses, winter-killed carcasses, and sometimes elk or bison calves. Summer is short, and by August the grass is dry and brown; bison and a few elk graze in the meadows, while most elk are higher in the mountains where plants remain green. Coyotes often hunt small mammals, and birds are everywhere, singing and soaring on thermal winds. In fall, aspens turn golden and snow falls on the mountain peaks as bull elk bugle and herds return to the valleys for their mating season. Animals feed heavily and store fat for the lean winter days ahead. In winter, large herds of elk and bison make their home in these spacious valleys, where snow is not quite as deep as in the rest of the park, and for many months they dig through the snow for hidden forage while wolves hunt among them, moving easily on top of the snow because of their large paws.