Hybrid bluebirds and why they may become more common

In the column Since You Asked in every issue of BirdWatching, Contributing Editor Julie Craves answers readers’ questions about birds and bird behavior. Here’s a question from our August 2018 issue.

Is this a Mountain Bluebird? I photographed it in late 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 chestnut coloration on his underparts, reminiscent of Eastern and Western Bluebirds. His appearance matches descriptions of hybrids between Mountain Bluebirds and Eastern or Western Bluebirds. These mixed pairs have been recorded multiple times. Their offspring are also usually fertile, evidenced by successful nestings of hybrid adults with pure individuals.

Historical reports of mixed pairs have been most common between Mountain and Eastern Bluebirds, which are more closely related to each other than either is to Western Bluebirds. Many of these reports have come from where the ranges of Mountain and Eastern Bluebirds overlap — in the southern prairie provinces of Canada and the northern Great Plains states of the U.S. 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 has contributed to their hybridizing with Western Bluebirds, too. When new habitats are created — often by fire that opens up forested areas and results in snags that provide nest cavities — Mountain Bluebirds often colonize them first. When the sites are within the range of another bluebird species, the scarcity of potential mates of the “right” species can promote mixed pairs.

The connection between ephemeral successional habitats and bluebird hybridization was tested with Mountain and Western Bluebirds in western Montana. Researchers created “new” habitat by putting up nest boxes and followed numerous pairs over many years. They found Mountain Bluebirds moved in first and were eventually displaced by more aggressive Western Bluebirds as they arrived over 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 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 early successional habitat being formed, we may see more of these hybrid bluebirds in the future.

Originally Published July 13, 2018