The vivid colors of male bluebirds result from structural features in the feather barbs, and males with more vivid colors father more chicks, with their own mates or with other females. (Courtesy Photo / Kerry Howard)

The vivid colors of male bluebirds result from structural features in the feather barbs, and males with more vivid colors father more chicks, with their own mates or with other females. (Courtesy Photo / Kerry Howard)

On the Trails: Tree sparrows and bluebirds

By Mary F. Willson

For the Juneau Empire

In the early part of April, there were sometimes two dozen mallards on my mostly icy home pond. There was one male who was not fully “dressed’” —while all the other males in the group had long since acquired their glossy green heads, rusty chests, and so on, this one was only part-way into the typical breeding dress. Was he just a very late hatchling last fall, lagging in maturation, or did he have a metabolic deficiency of some sort?

By late April, the mallard mob had dispersed, leaving one pair in charge of the now-ice-free pond. The male was seriously aggressive toward a visiting solo male, defending both his female and, perhaps, the seed-fall from the feeder hanging over the water.

[On the Trails: Tracks and king eiders]

Out at Eagle Beach, a mixed crowd of six species of waterfowl moved offshore, as a gamboling puppy approached along the water line. A scattering of American pipits foraged over the tideflats, readily distinguished from other small, brown songbirds by their characteristic gait—they walk and run, rather than hop. At the edge of a mown grassy area, I found a mixed flock of juncos, both Oregon and slate-colored, and American tree sparrows. They were all scouring the turf for food and flitting hastily to the surrounding brush when startled. I hadn’t seen a tree sparrow for ages, so this was fun.

Tree sparrows nest way up north, on open tundra with scattered shrubs and thickets of willow, birch, or alder. The nest is usually on the ground, and the female incubates the eggs for about twelve days. Nestlings are fed by both parents and stay in the nest only about nine days, then hopping out to follow the adults on foot. Tended by the parents, they begin to fly when they are about two weeks old. Tree sparrows feed on the ground and also by hopping up to grab seeds from the seed-heads of low-growing plants. In season, they also feed in trees, to eat small berries. This species has not been very thoroughly studied, so there are lots of details to discover.

Through much of April, there were reports of migrating mountain bluebirds on their way north, seen now here, now there, and in some other place. I tried several times in several places to find them, but they continued to elude me. Then, the day after finding the tree sparrows, I strolled out toward the Boy Scout Beach. In addition to seeing a fox sparrow, two Townsend’s solitaires, some Bonaparte’s gulls, and a flock of snow geese, I encountered two friends who were intently watching a patch of dead cow parsnip stalks and a few little spruces. They had found a male bluebird, which was foraging in classic bluebird style: watching from an elevated perch, just a few meters above the ground, then dropping down to the ground to pick up an insect and flying back up to an observation perch. Later, I spotted a female bluebird in the same area, and she too was foraging in the traditional way. So, at last, I had found them.

Bluebirds have other foraging techniques too. Sometimes they hover a couple of meters above the ground to scan for bugs or occasionally chase bugs in the air. In the non-breeding season, they add seeds and fruits of many species to the diet. To augment calcium in the diet, egg-laying females may eat bits of egg shell or snail shell to help build the shells of developing eggs, and nesting females may feed shell bits to the chicks to help develop good bones.

Mountain bluebirds nest primarily in the mountains of Montana, Wyoming, and so forth, but some of them range more widely, coming in small numbers to northern British Columbia, Yukon, and interior Alaska. They are cavity nesters, loving old woodpecker holes but also nesting in cliff crevices, nest boxes, and sometimes other human structures. They favor habitats with short grass and scattered trees or snags offering nesting cavities. They are reported to arrive early on the nesting ground, probably because suitable cavities are limited in number, so the early arrivals get the best choices.

Male bluebirds are territorial, defending an area around potential nest sites, and females can then choose among them. Nests are built primarily by females, often guarded by their mates against potential predators and intrusions by other males. Females incubate for about two weeks, usually fed by their mates, and they brood the tiny hatchlings. Both parents feed the nestlings, which stay in the nest about three weeks, and the fledglings.

Mountain bluebirds are socially monogamous, meaning that they form pairs but both partners may go philandering—seeking occasional matings outside the pair bond. This arrangement turns out to be rather common among species that were once thought to be strictly monogamous. A bluebird pair bond typically lasts for a season, including replacement nests or second nests, but a pair does not necessarily stay together the following year.

That blue color is not produced by pigments but by the micro-structure of the feather barbs; both UV and blue wavelengths are reflected. Males within a population differ in the amount and reflectance of their plumage. Now things get more intriguing: In two studies, done in two different geographical areas, brighter males were more likely to go philandering, engaging in more extra-pair copulations, than less-brightly colored males. Their brightness was not related to age or body condition. In one of those two studies, the more colorful males sired more of the chicks in their own nests and more extra-pair chicks. In the other study, the more colorful males were also early nesting and achieved more extra-pair copulations, but the paternity of chicks in their own nests was not enhanced, compared to less colorful males nesting later. Thus, in both cases, the advantage of bright coloration was greater numbers of offspring sired, and potentially greater evolutionary fitness, although the size of the payoff differed between the two areas.

• Mary F. Willson is a retired professor of ecology. “On the trails” appears every Wednesday in the Juneau Empire.

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