Mountain Bluebirds are a beautiful species of thrush that breed in the mountainous regions of western North America. Come wintertime, most Mountain Bluebirds migrate south to escape the cold temperatures and snowfall. Their winter range extends from southern British Columbia all the way down to central Mexico.
Do Mountain Bluebirds migrate for the winter?
Yes, the majority of Mountain Bluebirds are migratory and head south for the winter months. They breed during the summer across mountainous terrain in western North America from Alaska down to northern Mexico. But as winter approaches, most individuals migrate various distances south to reach warmer climates with more abundant food.
What triggers Mountain Bluebird migration?
The urge to migrate in Mountain Bluebirds is triggered by changes in day length as winter approaches. The decreasing daylight hours cause hormonal changes that spur the birds to begin migrating south. The cooler temperatures and dwindling food supplies provide additional environmental cues that it’s time to move on.
How far south do they migrate?
The distance Mountain Bluebirds migrate for the winter depends on how far north they breed in the summer. Those that nest in Alaska may migrate as far south as central Mexico. Birds that breed in the northwestern U.S. and southwestern Canada will migrate shorter distances, to the southern U.S. or northern Mexico.
What is the winter range of Mountain Bluebirds?
The winter range of the Mountain Bluebird stretches along the Pacific Coast and throughout the southern Rockies, from southern British Columbia down to the Transverse and Peninsular Ranges of southern California, southern Arizona, and south to the Sierra Madre Oriental of northeastern Mexico.
Western North America
Many Mountain Bluebirds that breed in Alaska, western Canada, Washington, Oregon, Idaho, Montana and Wyoming migrate south and west to spend the winter in coastal and interior regions of California, Oregon, and Washington. Some reach as far south as Baja California in Mexico.
Southwestern U.S.
Mountain Bluebirds that breed in inland regions of the western U.S., such as Utah, Colorado, Nevada, and New Mexico, tend to migrate south to winter in Arizona, New Mexico, and west Texas. Some continue into central Mexico.
Southeastern U.S.
A smaller number of Mountain Bluebirds winter in southeastern Arizona, southern New Mexico and southwest Texas after breeding farther north and east in the Midwest and Great Plains.
What habitat do they use in winter?
During the winter months, Mountain Bluebirds use a variety of open and semi-open habitats at lower elevations, including:
- Fields
- Pastures
- Grasslands
- Savannas
- Open woodlands
- Brushy areas
- Mountain foothills
- Forest edges
- Suburban parks and yards
They prefer habitats with scattered trees and shrubs, fence posts, or other perches where they can watch for insects or wait to ambush prey.
Do some Mountain Bluebirds stay year-round?
While most Mountain Bluebirds migrate south for the winter, some may stubbornly hold out further north if food is abundant. Small numbers regularly overwinter in southern British Columbia. Irruptive winter movements sometimes lead greater numbers to attempt overwintering across southern Canada and the northern U.S. when food is plentiful.
British Columbia
In coastal and interior southwestern British Columbia, small numbers of Mountain Bluebirds regularly overwinter rather than migrating farther south. These individuals stick to low elevations near the Pacific coast where the climate is milder.
Western and Midwestern U.S.
During irruptive winters, variable numbers of Mountain Bluebirds may overwinter in Washington, Oregon, Idaho, Montana, Wyoming, Colorado, Nebraska, and the Dakotas. These wintering birds depend on abundant food supplies to make it through the colder months so far north.
Do Mountain Bluebirds migrate alone or in flocks?
Mountain Bluebirds may migrate either alone or in loose flocks. Males and females, and young-of-the-year and adults often migrate separately. Larger flocks occasionally form where birds concentrate along migration routes and stopover sites. But they generally travel in smaller, loose aggregations of a dozen or fewer individuals.
Why do they migrate separately?
There are a few reasons why Mountain Bluebirds often migrate independently or in separated groups rather than as family units:
- Males migrate earlier than females in order to claim winter territories first.
- Adults migrate before young birds are fully ready to make the long journey.
- The dispersal of individuals reduces intraspecific competition for resources.
What are the largest flocks seen?
While loose flocks of 6-12 Mountain Bluebirds are most common, large concentrations of 30-70+ birds will assemble where food is particularly abundant at migration hotspots. Prominent wintering sites in California, Arizona, and Mexico may draw flocks of 50+ individuals.
How do they keep warm in winter?
Mountain Bluebirds have several adaptations that help them retain body heat and survive cold winters across parts of their range:
- Feather insulation – Their dense plumage traps air against the skin to retain heat.
- Roost cavities – They roost in enclosed spaces like tree cavities that conserve warmth.
- Caching food – Storing food allows them to reduce foraging time.
- Puffed feathers – Fluffing up feathers creates more insulating air pockets.
- Shivering – Shivering warms the muscles when extremely cold.
Their high metabolism and need to eat frequently makes it more challenging to endure frigid weather. Access to winter food sources is critical for individuals that overwinter farther north.
What do they eat in winter?
The diet of Mountain Bluebirds shifts during the winter months but continues to be focused on protein-rich insects and other invertebrates. Some key winter food sources include:
- Beetles
- Grasshoppers
- Crickets
- Caterpillars
- Spiders
- Ants
- Wasps
- Bees
- Small snails
They use exposed perches to watch for prey moving on the ground and fly down to capture it. On cold days they may eat more berries and fruits including juniper, mistletoe, and hackberry.
Do Mountain Bluebirds compete with other species in winter?
Mountain Bluebirds overlap and potentially compete with a few ecologically similar bird species on the wintering grounds, including:
Western Bluebird
There is substantial overlap between overwintering Western and Mountain Bluebirds in California. The two congeners occupy similar habitats and likely compete for insect prey to some extent. Mountain Bluebirds are dominant over the smaller Western Bluebird in winter conflicts.
Eastern Bluebird
Eastern and Mountain Bluebirds co-occur in parts of Texas and Mexico during winter. Again, the Mountain Bluebird asserts dominance in any direct competition over food or perches.
American Robin
Mountain and American Robins frequent many of the same open winter habitats across North America. Both are primarily ground-foraging insectivores. While they may compete locally, their vast winter ranges generally limit conflict.
When do Mountain Bluebirds migrate north in spring?
The timing of northbound spring migration in Mountain Bluebirds depends on an individual’s wintering latitude:
- Southern winter range – Early February to early March
- Southwestern U.S. – Mid-February to late March
- California and Pacific Northwest – Late February to mid-April
Males migrate north first to claim nesting sites, followed by females 1-2 weeks later. Adults precede young birds during spring migration as well.
What triggers spring migration?
Increasing daylight, warming temperatures, and urges to pair up and breed prompt Mountain Bluebirds to migrate north in the spring. The age and sex of individual birds influences their response to these cues and departure timing.
How do they navigate migration?
Mountain Bluebirds are believed to use a combination of innate and learned navigational mechanisms to accomplish their seasonal migrations, including:
- Genetic orientation and timing based on responses to changing day length.
- Magnetic field detection based on an internal compass.
- Landmark recognition and cognitive mapping.
- Following leading lines of mountain ranges, coastlines, or rivers.
Younger birds may imprint on migration routes and stopover sites learned from their parents. Experienced adults likely have stronger mental maps guiding their journeys.
Conclusion
Mountain Bluebirds are skilled long-distance migrants that head south from their mountain breeding grounds each winter to escape harsh conditions. The majority overwinter across the southern U.S. and Mexico, while some hold out further north. A variety of behavioral and physiological adaptations help Mountain Bluebirds survive cold winters and make their epic seasonal journeys guided by genetic programming and learned mental maps. Their migration is a testament to the complex navigational capacities of birds.
