In the Since You Asked column in every issue of BirdWatching, editor-in-chief Julie Craves answers readers’ questions about birds and their behavior. Here’s a question from our August 2018 issue.
Is it a Mountain Bluebird? I photographed it at the end of September in Yellowstone National Park. — Scott Yerges, Verdale, Washington
Male mountain bluebirds are entirely bright blue above and duller blue-gray below, but this bird has hints of brown coloration on its underparts, reminiscent of Eastern and Western bluebirds. Its appearance matches descriptions of hybrids between Mountain Bluebirds and Eastern or Western Bluebirds. These mixed couples have been recorded several times. Their offspring are also generally fertile, as evidenced by successful nests of hybrid adults with pure individuals.
Historical relationships of mixed pairs have been more common between Mountain Bluebirds and Eastern Bluebirds, which are more closely related to each other than they have been to Western Bluebirds. Many of these records come from areas where the ranges of Mountain and Eastern Bluebirds overlap, namely in the Prairie Provinces of southern Canada and the Great Plains states of the northern United States. However, mixed pairs have been recorded in Nebraska, eastern Minnesota, and even southern Ontario. , aided by the wanderlust of mountain bluebirds.
The tendency of Mountain Bluebirds to disperse also contributed to their hybridization with Western Bluebirds. When new habitats are created – often by fires that open up forest areas and generate snags that provide nesting cavities – mountain bluebirds often colonize them first. When sites are within range of another bluebird species, the scarcity of potential mates of the “right” species may favor mixed pairs.
The link between ephemeral successional habitats and bluebird hybridization was tested with mountain and western bluebirds in western Montana. The researchers created a “new” habitat by installing nest boxes and followed numerous pairs for many years. They found that Mountain Bluebirds had moved in first and were eventually replaced by more aggressive Western Bluebirds as they arrived in subsequent years. Hybridization was more common early in the experiment, when mountain bluebirds were more common and dominant western bluebirds had begun to use the habitat.
Given the location of the photo, it seems more likely that this bird is the offspring of a Mountain x Western pair. Wildfires in western North America have increased in number and extent in recent years. With so much first succession habitat formed, we may see more of these hybrid bluebirds in the future.