BIRD SPECIES AND DISTRIBUTIONS IN YUKON-CHARLEY RIVERS NATIONAL PRESERVE
Varied Thrush (Ixoreus naevius)
The family Turdidae is a diverse group of birds including thrushes, Townsend's Solitaires, Northern Wheatears, bluebirds, and robins. Members of this family are eloquent songsters and may be found in various habitats from woodlands to open areas. All have narrow, notched bills used to feed on insects and fruits and the young have spotted breast plumage. The word thrush may be derived from the Greek verb "to twitter," reflecting the active nature of these birds.
Descriptions: The Varied Thrush occurs in dark, dense and moist boreal coniferous woodlands, particularly deep within mature old-growth forest. A large-sized thrush (24cm) similar in shape and habit to the American Robin, the Varied Thrush is distinguished by its orange supercilia (eyebrow) and wing bars and the prominent band across its deep orange breast. Both sexes share this distinct plumage pattern. The male bird has a grayish blue nape and back and a broad, black breast band. Female birds are drabber, with a deep brown back and nape and a fainter, dusky breast band.
Vocalizations: The song of the Varied Thrush may be heard on the breeding grounds continuously during the evening and occasionally throughout the day. Males sing for 10-15 minutes from a high perch in a live conifer before moving to another site. Much like the sound of a referee's whistle or a space ship of an early SciFi movie, the song is a prolonged, whistled note of a single pitch. The softly whistled call note is a descending, low "tshook."
Nests: Varied Thrushes require large stands of forest with dense canopy to conceal nests and minimize predation. Increased nest predation in forest fragments, small stands or along forest edges may explain the scarcity of the Varied Thrush in these areas. The female builds the cup-shaped nest deep within the forest canopy, on low branches of small conifers (spruce) near the tree trunks. Occasionally, new nests are constructed directly on top of old ones or near old nest sites. The outer layer of the nest is composed of loosely woven twigs, leaves, lichen and bark, which supports a middle layer of mud and grasses. Nests are lined with fine grasses, mosses and soft dead leaves. The clutch consists of 2-6, 21mm, sky blue eggs, unmarked or sparingly spotted in browns. Although females alone incubate the clutch for about 12 days, both parents assist in feeding the young. Young birds fledge in another 13-15 days post-hatching.
Diet: Varied Thrushes forage on the ground for insects and spiders during the breeding season. Their diet primarily includes fruits, berries, and acorns during winter and fall. They form loose flocks around winter food sources, which they aggressively defend. These birds hunt by gleaning insects from the ground as they walk along the forest floor.
Distribution: A short-distance, partial migrant, the Varied Thrush may winter throughout much of its breeding range. Most individuals spend the winter along the Pacific Coast from Alaska to southern California, but occasional birds may be seen in the western states. The breeding range spans throughout interior and southeastern Alaska, most of Yukon and British Columbia and parts of Montana. In summer, early migrants reach interior Alaska by late April and leave breeding grounds by early September. Within Yukon-Charley Rivers National Preserve, these birds were found in fairly high abundance in almost all ecological units during the Yukon-Charley Rivers National Preserve Bird Inventory, June 1999 and 2000. They occurred in highest density in the coniferous forests of the Ogilvie Foothills (OF), Ogilvie Lime/dolostone Mountains (OM), Charley Foothills (CF), Upper Charley Valleys (UV) and the Three Fingers Subalpine basin (TF) ecological units.
|Density of Varied Thrushes by detailed ecological unit in Yukon-Charley Rivers National Preserve, Alaska, Avian Inventory, June 1999 and 2000.|