COLORADO PARKS & WILDLIFE

Brett Walker, in collaboration with Dr. Andrew Jones, Cleveland Museum of Natural History

Western Colorado

Field data collection is complete. Data analysis is ongoing.

Discoveries of small, local populations continue to expand our knowledge of the breeding distribution, ecology, and conservation status of North American birds. The Brewer’s Sparrow (Spizella breweri) is a migratory songbird with two known subspecies. The nominate subspecies, or “sagebrush” Brewer’s Sparrow (S. b. breweri), commonly nests in sagebrush-dominated shrublands across the western U.S. and Canada. This subspecies is currently considered a Tier 2 conservation priority in Colorado’s State Wildlife Action Plan due to historical and ongoing loss and degradation of sagebrush habitat. The other subspecies, the Timberline Sparrow (S. b. taverneri), is less well known but has been confirmed nesting in alpine willows and stunted conifers (or krummholz) at treeline from eastern Alaska through the Canadian Rockies to northwestern Montana.

There have also been numerous historical observations of Brewer’s Sparrows in high-elevation alpine areas in western Colorado in June-August (typically in willow carrs mixed with krummholz at or near treeline from 11,000-13,000 feet elevation), but the breeding status and taxonomic identity of these birds is unknown. There are several plausible explanations for Brewer’s Sparrows being found in alpine areas of Colorado, each with different implications for our understanding of the species’ ecology and conservation status. First, they could be unconfirmed breeding populations of Timberline Sparrow. If so, it would expand that subspecies’ known breeding population more than 800 miles to the south. Second, these birds could also be sagebrush Brewer’s Sparrows breeding in atypical shrub habitats. Third, they could be itinerant breeders, meaning sagebrush Brewer’s Sparrows that first nested in low-elevation sagebrush in spring, then moved upslope to breed again in alpine willows in summer. Fourth, Colorado could be a previously unknown zone of genetic introgression between breeding the two subspecies. Fifth, although unlikely, it’s possible that alpine birds could represent a third, previously undiscovered subspecies. Or finally, they could simply be dispersing sagebrush Brewer’s Sparrows, migrating Timberline Sparrows, or non-breeding birds of either subspecies.

The goal of this project is to determine the taxonomic identity and breeding status of Brewer’s Sparrows reported from high-elevation, alpine areas across Colorado. To do this, we will first compile and review historical records of Brewer’s Sparrow observations (for example, survey data from Bird Conservancy of the Rockies, records in verified eBird checklists, observations reported on birding listservers, museum records, and Colorado Breeding Bird Atlas records). We will then visit and survey for Brewer’s Sparrows at alpine sites with historical observations to look for evidence of nesting and if found, document habitat features. At a subset of high-elevation sites with alpine willow/krummholz and at a comparable number of nearby lower-elevation sites with sagebrush, we will record songs of territorial singing males, capture and band up to 40 males, and collect morphological data, photos, and genetic samples. We will then compare acoustic, morphological, plumage, and genetic data between alpine and sagebrush sites and against published reference data to determine the taxonomic identity of alpine birds.

This research will answer long-standing, unresolved questions about the taxonomic identity and breeding status of Brewer’s Sparrows in alpine habitats in Colorado. This in turn, will inform whether additional survey or research efforts are needed and allow CPW to update information on the species’ conservation status, breeding distribution, and habitat associations in the species conservation assessment and State Wildlife Action Plan. Regardless of which subspecies they are, confirmation of breeding in alpine areas will substantially increase the species’ estimates of population size, known breeding distribution, and habitat associations in the state. The range-wide conservation status of Timberline Sparrow is unknown due to a lack of adequate survey data from remote portions of the Canadian Rockies and a lack of data on wintering distribution and density. But if a large breeding population of Timberline Sparrows is present in Colorado, it would substantially expand the subspecies’ known breeding range and increase estimates of overall population size. This project also complements research efforts on other high-priority alpine species in Colorado, including American pika (Ochotona princeps), brown-capped rosy-finch (Leucosticte australis), and white-tailed ptarmigan (Lagopus leucura).