September is a time of change for wildlife in Grand Teton. Many ungulates (hoofed animals) are mating, birds are abandoning territories and embarking on their fall migrations, and all animals are preparing for the winter months ahead.
• By the end of September, elk, moose, and pronghorn are still breeding but their breeding seasons are winding down. Bighorn sheep and mule deer won’t start breeding until November. September is a good time to listen for elk bugling—in the crisp mornings just before dawn and quiet evenings before dusk. Bull elk use bugling to communicate a variety of things—a single bugle communicates to the cows of his harem that they are straying too far, a scream bugle communicates to other bulls that he is willing and able to defend his cows if they get too close, and the bull-to-bull scream bugle establishes dominance, which can put a subordinate bull in its place without coming to physical combat.
• This time of year the forests, meadows, and shrublands are quieter in the mornings as birds abandon breeding territories and begin migrations to distant wintering areas. Songbirds driven by instinct and leave before the harsh weather starts. Canada geese are different in that they gather into larger flocks and gorge on grain (grass seeds) in preparation—waiting for the weather to turn cold, water to freeze, and food to become scarce. Young geese must learn their migration routes from their parents and other adult geese. As heavy-bodied birds that fly fast, over 30 mph, with strong wingbeats, they avoid flying when there is a headwind. Geese take advantage of weather conditions by flying at night when it is calmer or with a tailwind during the day. Geese fly in a V formation to save energy. It is a form of drafting that helps pull the trailing bird forward and provides lift. Mature adults take turns at the point, shifting as they tire and allowing younger, less strong geese the advantage of flying behind.
• Pronghorn are starting to aggregate at lower elevations as the weather cools. They will wait until the first snows to make their migration trek south. It is a balancing act as their narrow migration path goes up in elevation over mountain passes and they must cross the highest part before snow makes it impassable. Park biologists are seeing increased numbers of pronghorn this fall—an encouraging sign that the herd is recovering from the 2022–2023 winter die-off due to a combination of disease and harsh weather.
• 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. Berry production was a bit spotty this year pushing bears toward other foods. The high-fat content of whitebark pine nuts is an excellent and preferred food source. Whitebark pines are listed as threatened and their seed production is variable. Bears will climb the trees to get the cones and take advantage of red squirrel caches. A raided cache can be a setback for a squirrel that has carefully stored a winter food supply. 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 join 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.
• Fall spawning native mountain whitefish and non-native lake, brook, and brown trout are heading to natal habitats to commence spawning activities. Water levels will drop as fall progresses into winter. As the water recedes large fish seek winter habitat, and cutthroat trout often move downstream to large pools.