Grand Teton National Park Foundation

Wildlife Whereabouts: Autumn Transition

Monday, September 30th, 2019

September is a time of change for wildlife in Grand Teton. Many ungulates (hoofed mammals) are mating, birds are abandoning territories and embarking on their fall migration, and all animals are preparing for the winter months ahead.

A healthy bull moose in fall. Photo: Ryan Sheets.

• By the end of September, elk, moose, and pronghorn are still breeding but won’t be for much longer. Bighorn sheep won’t start breeding until November.

• This time of year the forests, meadows, and shrub lands are quiet in the mornings as breeding birds abandon territories and begin migrations to distant wintering areas.

• Summer rainfall is a good predictor of available fall forage for elk, bison, and other herbivores and can influence fall migration timing, with poor production generally favoring earlier migrations. Abundant early snow in the high country can also favor early migrations.

• Black and grizzly bears are now in a phase of their annual life stage known as "hyperphagia," an intense period of foraging that will help individual animals put on fat needed for the long winters spent in their dens.

• By the end of October, most black bears will have entered dens, even if little snow has fallen. Most grizzly bears will wait until November to begin denning. Some adult male grizzly bears and others that have learned to find food late in the fall may not enter dens until December. Bear dens can be found at any elevation and aspect, but higher elevation sites on north aspects are preferred.

• Wolf pups will now be joining their pack members on most foraging adventures, having graduated from rendezvous sites in August or early September, and getting to know their territories in the process.

• Hunting season for the park’s elk reduction program (which starts in November) and those on surrounding national forest lands influence animal’s movements. Wolves, bears, coyotes, eagles, and magpies will all seek animal remains wherever they occur. This is a tremendous energy boon for scavengers, but it can also put some, particularly bears, at higher risk of conflicts with hunters.

• Fall spawning has begun for native mountain whitefish and non-native lake, brook, and brown trout and they are heading to natal habitats to commence spawning activities.

• Water levels will recede as fall progresses into winter. As the water drops, large fish seek winter habitat and cutthroat trout often move downstream to large pools.

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