The Mountain Bluebird (Sialia currucoides) is a striking migratory thrush of western North America, easily recognized by the male’s brilliant turquoise-blue plumage. These birds inhabit open country, including mountain meadows, grasslands, and high-elevation sagebrush flats across the Rocky Mountains and Sierra Cascades. The bird’s diet is highly flexible, shifting dramatically to maximize nutrient intake based on seasonal availability.
The Primary Diet of Insects and Arthropods
During the spring and summer breeding season, the Mountain Bluebird’s diet is heavily focused on animal matter, which can make up over 90% of their total food intake. This carnivorous focus provides the high protein and energy necessary for reproductive success and feeding nestlings. Adults and their young consume a wide array of invertebrates, primarily captured on the ground in open areas.
Specific prey items frequently documented in their diet include grasshoppers, beetles, spiders, caterpillars, flies, and ants. Nestlings are fed a steady stream of soft-bodied insects like caterpillars and beetles, ensuring the rapid growth and development required for young birds to fledge quickly.
Because they primarily forage in exposed, short-grass fields, they favor ground-dwelling invertebrates. This specialized insect diet distinguishes them as more insectivorous than their Eastern and Western Bluebird relatives.
Seasonal Shifts to Berries and Seeds
As summer ends and the breeding season concludes, the Mountain Bluebird’s diet undergoes a significant shift to plant matter to sustain them through the colder months. When insect availability plummets, the birds rely on small fruits and seeds that persist on plants through the fall and winter. This plant-based food provides the necessary carbohydrates and fats for maintaining body temperature and energy stores.
Specific winter staples include the berries of juniper, particularly the One-seed Juniper, along with mistletoe, hackberry, and sumac seeds. Juniper berries are often an important resource, remaining on the branches and offering a calorie-rich food source when other options are scarce. This seasonal flexibility allows the birds to remain in or near their breeding range longer.
The birds do not typically consume large, hard seeds, instead focusing on small fruits and the seeds contained within them. This dietary adaptation is a survival mechanism that minimizes the energetic cost of foraging in cold weather.
Unique Foraging and Hunting Methods
To acquire their predominantly insect-based diet, Mountain Bluebirds employ specialized and energetically demanding hunting techniques that set them apart from other bluebird species. Their most characteristic method is a distinctive behavior often referred to as “hover-hunting” or “kiting.” The bird hovers in place, sometimes for several seconds, over an open field while scanning the ground for prey before dropping down to make a capture.
This hovering behavior is more common in Mountain Bluebirds compared to other bluebirds because of their lower body-weight to wing-area ratio, which makes the technique more efficient for them. They also frequently use a perching method, where they scan from an elevated spot, such as a fence post or low branch, before dropping down to the ground to snatch an insect. When perches are unavailable, they rely more heavily on the costly but effective hovering method.
In addition to these aerial maneuvers, the Mountain Bluebird also engages in simple ground-foraging, hopping along the soil to pick up accessible insects. They will also occasionally dart out from a perch to catch a passing insect in mid-air, a technique known as flycatching. The combination of these methods ensures they can exploit insect resources both on the ground and in the air.