2010 - Wildlife on the Move

Mammals do it. Birds and fish do it; even insects do it. They migrate as part of their inborn strategy for survival, and the arrival of winter triggers a massive migration of all kinds of wildlife.

They may travel a thousand miles or a few feet. The distance is not what defines migration; it’s that animals move between habitats during the year to survive. They may move for many reasons – to find food, breed or raise their young. Migration is a tool they use when a habitat no longer meets their needs.

Migration patterns and routes are ancient and have been influenced by the natural features of the land, water and air. The same natural features that foster wildlife movement are also attractive to human activities. Roads bisect open spaces. Wind turbines pop up on ridgelines. Dams block rivers. Communication towers light up the night sky. Houses are built on habitat. These human structures frequently create problems for migrating wildlife.


Wildlife and vehicle collisions are the most visible conflict between roads and wildlife on the move. More than 5,000 deer, elk and moose were killed by cars on Idaho’s roads in 2011.

Information is gathered through the Roadkill database. 

In known hotspots around Idaho up to 100 or more animals are killed crossing roads every year. Some of these include:

  • Interstate 15, between Pocatello and Inkom, cuts through a major deer migration corridor.
  • Highway 75 north of Salmon, where 55 bighorn sheep have been killed since 1986.
  • Highway 30, from east of Montpelier to Wyoming, sees up to 6,000 deer and elk crossing the road and has one of the worst wildlife road mortality rates in southeast Idaho.    
  • Highway 20 in the Island Park area is known for collisions with elk and moose.
  • Highway 95 from the Canadian border to just south of Coeur d’Alene had 900 animals, most of them white-tailed deer, hit by cars in 2011.
  • Highway 21 northeast of Boise repeatedly crosses the primary migration route of up to 9,000 deer and elk in the Boise Mountains. Hundreds of deer and elk are hit every winter along this road.

Sometimes money, talent and motivation score a win for wildlife. Increasingly, transportation and fish and wildlife agencies, sportsmen, local governments, and conservation groups are realizing protection of priority wildlife linkage and migration areas. Examples include the wildlife underpass on Highway 21 outside of Boise, another underpass recently built into Highway 95 just north of Coeur d’Alene, and wildlife fencing along Interstate 15 outside of Pocatello. 

Click here for more information about what Idaho Fish and Game and its public and private partners are doing to help migrating wildlife.


One of Idaho’s most dramatic wildlife migrations is its anadromous fish runs.

Salmon and steelhead travel from Idaho’s mountain streams to the ocean as juveniles, then return as adults to their home waters in Idaho to spawn. For sockeye and Chinook salmon this adds up to 1,800 river miles round trip.

Fish biologists have discovered that many other kinds of fish migrate as well, including bull trout, cutthroat and rainbow trout, suckers, to name a few. The three of the biggest obstacles to fish when they migrate are:

  • Culverts that allow a stream to flow under a road. They can become obstacles for fish passage if the water‘s energy lowers the downside river bed, creating an impassable barrier to fish going upstream. In other northwestern states, surveys have documented that the majority of road culverts may be partial or complete fish passage barriers.   
  • Unscreened water diversions. Diversion that direct water into irrigation canals can also direct fish into the canals and onto farmers’ fields. This is called entrainment and in some instances can result in significant losses to native fish populations.
  • Dams installed for irrigation and power production. Many also block fish migrations.

Idaho Fish and Game works with private and public partners to reduce the impacts of these barriers for migrating fish. For water diversions, screens are installed to keep fish out of the canals. Problem culverts can be replaced with newer designs or replaced with bridges to allow fish passage.  

Although difficult and expensive, many dams can be retrofitted with fish ladders to allow safe passage of native fishes to spawning and rearing grounds. Some dams are too high for conventional fish ladders so alternative methods of providing passage must be explored like trap and haul. 


All types of birds migrate. Some travel huge distances and while others simply move up and down a mountainside. In Idaho, we often associate migration with waterfowl. They migrate by the thousands and noisily announce their coming and goings. Idaho is part of the migratory route call the Pacific Flyway.

Structures, such as power lines, wind farms and offshore oil-rigs, have been known to affect migratory birds. Habitat destruction by land use changes is the biggest threat, and shallow wetlands that are stopover and wintering sites for migratory birds are particularly threatened by draining and reclamation for human use.

Additional Resources:

Roadkill Database - http://fishandgame.idaho.gov/roadkill

Wildlife Crossings in Idaho - www.idahowildlifecrossings.com