Mountain Bluebird is a vivid azure-colored summer denizen of mountain meadows in the Rocky Mountains and Sierra Nevada, high Great Basin desert, and western Great Plains. In fall and winter they move downslope or southward, and often gather in large flocks, sometimes numbering in the hundreds. Like many western mountain birds, there are irruptive tendencies to their movements.
This animated occurrence map shows their annual cycle. The areas of summer occurrence outline the mountain ranges, high deserts, and plains where Mountain Bluebirds breed. Detectability wanes a bit as summer progresses, but it is not until October that they start moving southward in numbers. Certain areas of Colorado, Utah, and Nevada, may harbor Mountain Bluebirds year-round, but many of the wintering birds may be different individuals than those that breed. The species winters well south into Mexico, and in exceptional years occurs south along the Central Mexican Plateau. In March and April they move northward and upslope, with most birds back on the breeding grounds before May. Often, this is well before snow melts and the males are nothing short of stunning against the bright snow.
Notice how their distribution in October bleeds eastward onto the plains of Nebraska and Kansas. With some breeding so far north into Canada, the fall migration brings them a bit further east as they move towards wintering grounds in Texas and Mexico. Notice also how the breeding population in the Sierra Nevada is bright in summer, but fades in winter as the species moves to lower elevations during the colder months.
A somewhat nomadic species in winter, the apparent shifts in winter range probably relate more to slight changes in detectability (partly driven by birder activity) than to bluebird movements. A slight downward shift in occurrence can make the species seem to “disappear”, when in fact it is just an issue of scale. With more data and more refinement of these models, we may be able to learn something more about where and how this species moves within a given winter.