Mountain Bluebirds are in trouble. While in some of their range, populations are stable after years of decline, in other areas the Mountain Bluebird numbers are still declining. One of the reasons for the decline is that bluebirds, which nest in cavities, are being out-competed by Eurasian Starlings, an invasive species.
A classic scene: a female Mountain Bluebird working on her nest in a cavity created earlier by a woodpecker. The female builds the nest, starting with sticks and dried grasses and then lining the nest with feathers.
Males sometimes engage in symbolic nest-building, in which the guys go through the motor patterns of bringing material to the nest cavity, but they either carries no material or drop it en route.
There are predators, and the bluebirds go to some trouble to make certain that no one sees where the nest is located. While WC was watching, the female ducked back in to the cavity when an Osprey flew over. Not a predator, but a predator shape, you see.
It may be that the male, who sits and watches the nest-building process at a distance, serves as a distraction for predators, drawing attention to himself. He’s certainly conspicuous enough.
In Idaho, at least, there has been considerable effort to help Mountain Bluebirds by constructing thousands of nest boxes. But many times, the boxes are occupied by other species. There was a nest box about five meters from the cavity the bluebirds were working on.
A House Wren is noisily proclaiming a Mountain Bluebird nest box is his.
In fact, as WC watched, the wren removed portions of the nesting materials already in the box. Here he carries out a feather, one of perhaps a dozen the wren extracted as WC watched. House Wrens don’t line their nests with feathers, instead stuffing the box with sticks and creating a tiny nest cavity. The mass of stick deters other birds from trying to use the box. It may be that this wren is dismantling a nest built by the bluebird pair the previous year.
Starlings, Tree Swallows and other species seem to be able to evict or exclude Mountain Bluebirds from nest sites. Even though, in the case of swallows and wrens, the Mountain Bluebirds are larger.
Still, it was a treat to see and have a chance to photograph the bluebirds in action.