April 18, 2005 was a typical spring day in the Upper Salmon – blustery and cold with the occasional horizontal-blowing snow flurry; certainly not the best day to be out surveying for peregrine falcons. It was about 3:00 p.m. when I turned up the East Fork of the Salmon River heading to the Greenfire peregrine falcon territory. At the first right jog in the road where the Greenfire Preserve begins, I noticed a small flock of brilliant azure mountain bluebirds perched on the jackfence rails along the road. I stopped to view them through binoculars, noting about a dozen birds, both male and female. As I continued driving, I started counting dozens and dozens of mountain bluebirds on the fence rails, marveling at the collective jolt of the males’ brilliant blue plumage on such a dreary day. It was imperative to the safety of oncoming traffic that I pull over! I glassed with binoculars further up the fence line and could easily count another 100 bluebirds. I then scanned the upper fields of the ranch adjacent to the fence and saw what amounted to hundreds more scattered out among the grass-forb uplands at the northwest end of the preserve. By the time I finished counting from my vantage point, 450 bluebirds were tallied. Doubtless, there were more birds out of my field of view, so this count represented a conservative estimate.

The presence of a mountain bluebird flock of this size was perplexing, particularly in mid-April. Mountain bluebirds are one of the earliest migratory birds to arrive in the Upper Salmon Basin, having been sighted as early as February 28. By mid-April, mountain bluebirds have typically established breeding territories and initiated nests. Perhaps these birds, grounded by inclement weather, were late spring migrants heading to more distant breeding grounds. My hunch was supported by a wildlife database report from Alberta, Canada, citing a heavy snowstorm in mid-April 2000 near Calgary that grounded up to 300 migrating mountain bluebirds.

If the huge flock at Greenfire Preserve was grounded, the birds were not sitting idle. The few hundred birds in the upland fields were actively foraging, though I couldn’t identify what food items they were taking. Mountain bluebirds feed mainly on insects, spiders, and other invertebrates, which they glean from short ground vegetation. I assumed the bluebirds were taking advantage of an insect hatch underway in the uplands, which could have been prompted by a warm spell the previous weekend. After I returned to Salmon, I urged a colleague in Challis to head to Greenfire to witness this bluebird spectacle. She visited the site 2 days later and reported seeing only a few pairs of breeding bluebirds near the nest boxes. Apparently the large flock had moved on.

This unique migratory phenomenon is certainly worth watching for in the future. That this huge flock selected Greenfire Preserve as a resting and refueling stop suggests that the uplands restoration projects being implemented are providing important, quality habitat for spring migrant bluebirds. One mystery worth solving would be the identification of the invertebrate(s) the migrating bluebirds were feeding on. Certainly, this happenstance observation is all the more reason to witness and appreciate the remarkable assemblage of native plants and animals on the Greenfire Preserve.

Beth Waterbury is the Salmon Region Nongame Biologist for the Idaho Department of Fish and Game.