SWAINSON’S THRUSH
Catharus ustulatus
By Sharon L. Moore
As one of seven thrush species in North America, the Swainson’s has four subspecies, one of which is our regional Pacific Russet-backed Swainson’s Thrush. An arboreal forager, it favors tall, thick, mixed conifer forests, hovering on the edges of the tree line as it feeds and protects its nesting territory. Unlike most songbirds, it arrives later in summer in order to find the ripe fruit and insects it needs to mate and successfully raise its young. Since the Swainson’s prefers the shady understory, it can be difficult to see it there, particularly on the ground or in the deep shrub growth.
So how will we find these reclusive birds? The answer is that by carefully listening in forested areas, we may hear its lovely song. As one of the most melodious of all songbirds, it renders an incredibly rich and complex song that you will soon learn to recognize. While it often sings earlier in the morning and later in the evening than other songbirds, its music may reverberate as a solo performance or it may add depth and resonance to the summer songbird chorus. Now that our family has learned to recognize the Swainson’s song, we happily take more morning and evening summer walks.
How do the Swainson’s produce their splendid songs? Though the “how” remained a mystery far back in time, researchers now understand that songbirds have an unusual physiology. Instead of a larynx, as we Homo sapiens have, avian species create sound through a syrinx which has two parts located in the respiratory tract where the right and left lungs form the trachea. At that spot two tiny sets of muscles independently guide the airflow through the two sides of the syrinx. This lets the bird produce two different sounds at the same time. While these may be the same note in many songbird species, in thrushes the two sides of the syrinx create two different sounds – two different notes.
That the male Swainson’s is able to harmonize with himself is quite a discovery. His song spirals upward, flutelike with an echo quality. At times he sings loudly, at other times more softly, leading the listener to think the bird is farther away than he actually is. Thrush songs have long been studied for their varying complexity and it’s well established that males sing to signal their territorial boundaries. Beyond those standard songs, however, surprising newer evidence points to thrush songs being composed of both innate and learned vocal patterns. Currently, researchers strongly suspect parts of males’ songs are individually invented. Are the male Swainson’s and other thrush species capable of composing original music? It appears that they well may be. Both males and females also use one-note calls to communicate with each other during breeding season.
The Pacific Russet-backed Swainson’s Thrush winters in Central America and, traveling only at night, completes a long migration to the Pacific West Coast of Oregon, Washington and Northern California in summer. In their wintering grounds they live in dense forests feeding on army-ant swarms and flycatching the clouds of mosquitoes that breed there. During migration they prefer to feed on tree fruits such as Flowering Dogwood (Cornus nuttallii), as they are rich in lipids that are fatty organic compounds and therefore the most efficient fuel for their 2,000 mile flight. Once here, these secretive birds avoid urban areas altogether and instead gravitate to leafy, dense understories where they forage for insects, tree fruits and berries. Salmon berries, red elderberries, and blackberries are among their favorites.
At only 7 inches long, with a wingspan of 12 inches and weighing only approximately 1.1 ounces, these robin-sized birds are a small, yet robust species. The adult’s back and wings are a warm, russet brown, the belly is off-white, often smudged with light russet and the flanks are tinged also with russet. The bird has a tawny face, a thin bill and light eye rings, similar to white-rimmed spectacles. Their unusually large eyes help them function in their shady habitats and also to forage when light levels are low.
As breeding season begins the female locates a shady spot in the dense forest understory only 3 to 10 feet off the ground. Her preferred nesting plants include Douglas fir and the other true firs, blackberry, rose, alder, willow, spruce, maple or birch. She especially likes to nest in large blackberry shrubs or dense ferns. She builds the nest alone, forming an open cup of 3.5 inches in diameter and 1.5 to 3 inches high. For the exterior construction she uses grasses, moss, stems, tiny twigs and shreds of bark. She lines the interior with moss, lichens and decomposed leaves. While the female is working, the male is guarding his territory with song, sometimes rearranging his territorial melody to spiral downwards. This is a warning to an interloper and may result in a chase and possible attack. When the nest is complete the female lays 1 to 5 blue-green eggs that she incubates for 10 to 14 days. Once the hatchlings emerge blind and covered in down, she and her mate feed them for 10 to 14 days before they fledge. The longest-lived Swainson’s Thrush recorded thus far clocked in at 12 years, 1 month when recaptured in a banding operation in Montana in 2006.
As to the future of Swainson’s Thrushes, their global population of 120 million is estimated to be relatively stable. Their short breeding season, however, may render them vulnerable to disturbances on their breeding grounds. That includes human interference through increased development, invasion by non-native plants, hooved animal grazing, and global warming temperature rise with its attendant increase in the incidence of fire. Though there is no actual data on Swainson’s population stress, a Breeding Bird Survey in 2019 documented stress on both Wood Thrushes and Veery in their winter feeding grounds in Central America and their nesting/breeding environments in North America from recent and ongoing deforestation. Equally as alarming is that Swainson’s Thrushes are the most common migrants killed in collisions with windows, radio and cell-phone towers and buildings. In order to protect all birds, including thrushes, we must increase our activism. Let’s bird-proof all glass in our homes and other buildings. Let’s advocate for reducing clear-cutting of our dense forests that provide habitat for many migratory and non-migratory bird species as well as other wildlife. Let’s make every personal effort we can manage to halt global temperature rise. Finally, let’s “go high” by advocating for the adoption of mandatory, bird friendly standards for all new construction in Washington State.
Photo credit: Pacific Russet-backed Swainson’s Thrush at Nisqually, by Rachel Hudson
