As a consequence, many animal populations are restricted to patchy and isolated mountain-top distributions. In the absence of gene flow, these isolated groups begin to evolve independently, developing morphological and genetic characteristics that differ among populations. Given enough time, these diverging forms may become new species. This process is well documented in terrestrial organisms with limited dispersal ability (like small mammals and amphibians) where closely related—but different—species occupy different mountain ranges. 

But what about birds? Do they simply fly over these otherwise uninhabitable inter-mountain regions, or do they choose not to attempt such journeys, and, like small mammals, their isolated populations are evolving independently of one another on different mountain ranges? These are the questions that Dr. John Klicka, Curator of Birds at the Burke Museum, and Dr. Garth Spellman, Curator of Birds at the Denver Museum of Natural History, sought out to answer with museum collections and field research.

Klicka and Spellman sampled populations of 15 common western montane bird species using genetic data (sequenced DNA) to look for evidence of genetic isolation and to measure relative amounts of gene flow among populations that occupy different mountain ranges.

The species examined include Steller’s Jay, Mountain Chickadee, Brown Creeper, Spotted Towhee, Hairy Woodpecker, and Black-headed Grosbeak. Most of these birds are wide-ranging, meaning they are found on most mountains of western North America, including the Rocky Mountains, the Sierra Nevada and Cascade Ranges, and the Sierra Madre Occidental, and Oriental of Mexico (many of these species are actually common here in Seattle, where the climatic conditions allow conifer forests to thrive, even at sea level).

Some species contain populations that are morphologically distinct from one another, suggesting long periods of isolation have occurred. For example, Hairy Woodpeckers from southern Mexico are much smaller and darker than their northern counterparts, and those on either side of the Cascade Range in Washington have long been known to possess distinctive coloration.

On the other hand, many of the species studied have little morphological variation from one population to another, suggestive of continuous gene flow between mountain ranges.

The genetic data gathered and analyzed for each of these bird species provided no one simple answer. For a few species (e.g. Red-breasted Nuthatch, Western Wood Pewee), it was clear that birds do regularly (and in large numbers) move across the landscape between mountain ranges, as no evidence of genetic isolation was recovered.

However, other species like the Brown Creeper, Mountain Chickadee, and White-breasted Nuthatch, displayed evidence of being “structured genetically” into two (or more!) isolated, independently evolving lineages that correspond with particular mountain ranges or regions.

Article Source: Burke Museum