EVOLUTION IN PROGRESS
By Steve Gniadek
The Timberline Sparrow (Spizella breweri taverneri) is a subspecies of the Brewer’s Sparrow, but it’s in the process of becoming a separate species. Some scientists have argued it already is a species. As the name implies, the Timberline Sparrow is found near tree line, where it nests in stunted conifers. In contrast, sagebrush habitat is preferred by the Sagebrush Brewer’s Sparrow (S. b. breweri). The two are very similar in appearance – small (5.5 inches in length), slim, long-tailed sparrows with subtle brown and gray plumage with a finely-streaked brown crown and a complete white eye-ring.
The Sagebrush Brewer’s Sparrow is a common breeding bird wherever large tracts of sagebrush are found in the state. Specimens were collected from Montana as early as the 1870s, but it was not reported in Montana west of the divide until the 1950s, mostly near Niarada and in the Blackfoot Valley near Ovando. The Timberline Sparrow was first described as a separate species in 1925 from northwestern British Columbia, but the American Ornithologists Union has never recognized it as a full species. More recently, ornithologists using genetic information have revived the argument that the Timberline Sparrow is a full species, although others using similar methods disagree.
Until recently, the Timberline Sparrow was not considered to occur south of the Canadian border. However, there have been sight records in Glacier NP for decades. Lloyd Parratt’s book The Birds of Glacier National Park from 1964 included the Brewer’s Sparrow, and Dave Shea’s first checklist of the birds of Glacier NP in 1971 included it. These were likely Timberline Sparrows, although Brewer’s Sparrows from Big Prairie on the west side of the park may be Sagebrush Brewer’s Sparrows. Beginning in 1988, I documented Timberline Sparrows at several locations on the east side of the park. During 1998 to 2000, more intensive surveys found over 200 territorial males in Glacier NP and extended the known breeding range south along the Rocky Mountain Front.
Male Timberline Sparrows arrive on breeding territories in late May, 3-4 weeks later than Brewer’s Sparrows return to sagebrush habitats in eastern Montana. Timberline Sparrows typically leave by late August. Among the most accessible places to find them are the krummholz clumps of subalpine fir along the trail switchbacks to Scenic Point in the Two Medicine Valley, or the cirque basin east of Firebrand Pass, in Glacier NP.
Typical windy conditions along the east slope can make it difficult to hear them, but listen for the buzzy yet musical trills, ascending and descending, from the territorial males. Early in the nesting season, males sing long songs that can last up to 30 seconds or more. Later in the season they sing shorter songs. They sing from perches at the tops of the stunted conifers, but they forage and nest deep within the foliage. Look for the birds flying low over the krummholz.
Range-wide surveys since the 1960s indicate the Sagebrush Brewer’s Sparrow is declining because of the loss of sagebrush habitat. Nothing is known about the status of the Timberline Sparrow, owing to the lack of long-term monitoring efforts. Uncertain impacts from climate change may imperil Timberline Sparrows if their tree-line habitats decline. Unless climate-induced changes bring Timberline and Brewer’s sparrows closer together, continued geographic separation may lead to greater differentiation. There seems to be agreement among ornithologists that, with separation and time, the Timberline Sparrow will eventually become a separate species. Stay tuned.