Near Yellowstone National Park, Bison Migration Heralds the Return of Spring
By Tom Torma
Broadcast 4.2012 and 3.23 & 3.26.2022
There were bison in my driveway this morning. In most northerly places, the return of spring is marked with the splendor of bird song returning from warmer, southern climates. However, on Horse Butte, just a few miles north of the town of West Yellowstone, bird song won’t start for a few more weeks. Instead, it is the arrival of bison, migrating from inside Yellowstone, that heralds the return of spring.
For most of the year, Yellowstone’s bison live in the interior of the park. However, come spring they move west and north, out of the park and into Montana. Birds and bats can just fly south, often for thousands of miles, and spend winter in warmer places. For large mammals like bison and elk, such large scale migrations are simply not possible. Instead, in mountainous regions, animals go down. By moving to lower altitudes, animals can gain some of the benefits of a southern migration. Indeed, for every thousand feet dropped, an animal gains the equivalent of going three hundred miles south.
However, when bison move out of the park in the spring, it is not simply to get warm. They can easily tolerate temperatures as low as thirty degrees below zero. By late March, temperatures in the greater Yellowstone area rarely get that cold.
Instead, what drives the bison into Montana is food. For most of the winter, they spend up to one third of their time digging through the snow to get at the dry brown grass underneath. Their very design speaks to their need to do this. The distinctive bison hump is made of thoracic vertebrae which can be almost two feet long. The part that sticks out is called the spinus process – the same part of the vertebra that makes the bumps along a person’s neck and backbone. For the bison, these bones make attachments for the muscles that move the bison’s head – which can be as heavy as two hundred pounds in a large bull. With the power this structure gives them, bison can dig face first through up to four feet of snow.
The digging itself has a high energy cost, and the reward is often poor. Brown winter grass lacks the nutrition and energy of the green grass found in the spring and early summer. By moving to lower grounds in the spring, the bison get to take advantage of the earlier melting of the snow, and the subsequent green up. But this vertical migration does not come without a cost. To get to the lower elevations, bison need to travel miles, often through deep snow. Indeed, many do not survive the journey. Instead, they contribute to the winter kill that sustains the park’s carnivore population.
At a time of the year when nutrition and food energy is scarce, this annual journey to lower elevations is driven by desperation. Recent studies show that deeper snowpack and higher animal populations are both significant predictors of increased bison movement into Montana. Both of these factors lower the amount of available feed to each individual bison. By moving into the Gardiner Basin and Horse Butte, Yellowstone bison are positioned for high quality feed earlier in the season, and their calving season is timed for it.
While there is still snow on the ground in early April, it is starting to melt. The days often get above freezing. In short, spring is beginning. For bison it means softer, thinner snow to be followed by earlier green grass. For the people who live here, bison in the yard is a signal that warmer times are on the way.
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