The Townsend’s Solitaire, not a familiar species to most, is a bird found year- round in the mountains of the western U.S. and Canada during the breeding season. Wintering at 5000 feet elevation in northeastern California at the shores of Eagle Lake, I studied them over four winter seasons. From about New Year’s Day to the end of January, I studied Townsend’s Solitaires on an 80-acre plot of Junipers, Jeffery Pine, and Mountain Mahogeny. Cold from dawn to dusk, dragging myself out of my cozy cabin took at least a whole pot of coffee. Walking through the forest stumbling through a thick undergrowth of gooseberry bushes and several inches of snow kept me warm except when I stopped to measure or take notes. (Hint to budding field ornithologists: use waterproof paper and write in pencil. Melting paper and running ink can ruin your day.)
Too difficult to watch birds, measure territories, and count juniper berries simultaneously, I alternated days until I had all the measurements and berry counting done, about 10 days. At night, around the Franklin stove, I rewrote and organized my data and dried my clothes and boots.
Townsend’s Solitaires eat virtually nothing but juniper berries the entire winter, although they may have eaten some insect larvae and eggs because the berries (actually cones) are low in protein (4%) and high in undigestible fiber (34%). But their 46% carbohydrate and 16% fat composition were enough to get the birds through the winter. I sampled the berries on each juniper tree in their territory but not by counting every berry – that would have been an onerous chore. I used a wooden frame of one-half square meter to sample several random areas on each tree. Every year the crops were different – sometimes the berries were abundant and sometimes they were scarce.
Sitting on the tops of tall trees, the solitaires sing to declare their territory and warn intruders – other solitaires as well as American Robins. It makes sense to guard a dense and easily defensible food supply as a concentrated food supply is easy to defend and the energy expended in defense is more than made up by the easily accessible food supply. The birds spent up to 90 percent of their daytime actively defending their territories by singing from treetops and chasing intruders. When they fed, they did so rapidly by picking berries from the ground under the trees, apparently deriving sufficient nutrition.
Dense food years saw the solitaires defend their territory, but in years of a low concentration of juniper berries, the Townsend’s Solitaires did not defend a territory, they just wandered, finding what berries they could.
A territory low in berry density would have to be much larger to encompass a sufficient number of berries for survival. But a larger territory requires more time in defense, so there comes a point when defense is no longer energy-efficient and random foraging becomes the better strategy. Hummingbirds do the same thing when nectar-filled flowers become scarce. This is called “facultative territoriality” and has been found in other birds whose food supply is unpredictable.
But food supply is not the only factor. The weather, the number of competitors, predators, snow cover, and alternative food sources come into play as well.
Again, the rewarding and even fun part of this kind of research in uncomfortable, sometimes miserable, conditions, is coming back to the main university campus and my warm office to analyze the data and write it for publication. (Facultative Territorialty in Townsend’s Solitaire (Myadestes townsendi). The Southwestern Naturalist, Vol. 25, No. 4 (Feb. 16, 1981), pp. 461-467.