Ixoreus naeviusvaried thrush

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Geographic Range

Western North America. The Varied Thrush breeds from Northern California north to the extent of the boreal forests in Alaska and the Yukon Territory. They range east into Idaho and western Montana and Alberta (Finley, 1936). Varied Thrushes are known to overwinter as far south as Southern California. These birds occasionally stray to eastern North America and have been recorded in every Canadian Province except Newfoundland and in every U.S. state except Arkansas, Mississippi, Alabama, North Carolina and Hawaii (George, 2000).

There may be four distinct subspecies of Varied Thrushes:

Ixoreus naevius naevius, a subspecies that breeds in southeast Alaska and along the coastline south to northern California,

Ixoreus naevius meruloides, a subspecies that breeds from interior Alaska south through interior British Columbia and into northwestern Montana, northern Idaho, eastern Washington and northeastern Oregon,

Ixoreus naevius carlottae, a subspecies that breeds on Queen Charlotte Island, British Columbia and

Ixoreus naevius godfreii, a subspecies that breeds from southern interior British Columbia to eastern Washington and western Montana (George, 2000).

Habitat

Varied Thrushes are most commonly found in dense, moist woodlands and low coniferous old growth forests. In California, Varied Thrushes prefer the forests of coastal redwoods, Sitka spruce and red alder; in Oregon and Washington they prefer wet coastal forests of Sitka spruce, western hemlock and western red cedar and wet montane forests with Douglas fir, western hemlock and western red cedar; in northwestern Montana they prefer forests of western larch and Douglas fir; in coastal British Columbia they prefer forests of Douglas fir, western hemlock, western red cedar and Sitka spruce; in interior British Columbia they prefer montane coniferous and taiga forests; and in Alaska they prefer wet coastal and taiga forests (George, 2000).

Physical Description

Length 19.0-26.5 cm; Body mass 65-100grams

Male: Characteristics of male Varied Thrushes are a slate blue back and nape, an orange face, eyebrow, and breast with a black eye stripe and black necklace or breast band. Wings; Coverts are slate-grey with two orange bars. Secondaries are slate-grey and tipped with orange. Primaries are slate-grey and dappled with orange (Finley, 1936).

Female: Resembles the male but much more dull and with brownish-olive coloration replacing the slate-grey of the male (Finley, 1936).

Juvenile: Like the female but with a whitish belly (Finley, 1936).

There are four listed subspecies for the Varied Thrush based solely on plumage differences in the females (in George, 2000).

  1. Ixoreus naevius naevius (Gmelin, 1789). The females are a deeper brown with fewer white (light) markings and shorter, more rounded wings than I.n. meruloides.
  2. Ixoreus naevius meruloides (Swainson, 1832). The females are grayer and more pale than the nominate subspecies.
  3. Ixoreus naevius carlottae (Phillips, 1991). The females are reddish dorsally.
  4. Ixoreus naevius godfreii (Phillips, 1991). The females are darker dorsally than I.n. meruloides and more reddish in fresh plumage and paler on the back and rump than I.n. naevius.
  • Range mass
    65 to 100 g
    2.29 to 3.52 oz

Reproduction

Varied Thrushes are difficult to study because of their retiring behavior, love of dark, wet forests and low population densities. The males usually choose territories that serve both for breeding and feeding in wet, mature forests. The males establish individual territories by singing. During the breeding season, a male Varied Thrush will chase rival males away from its territory (George, 2000).

Female Varied Thrushes build their nests in low bushes on or near a stream bank (Finley, 1936). Active nests are often found near old nests in the understory. The nests are three layered with a course outer layer of twigs, leaves, lichen and bark, a dense middle layer of rotten wood and moss or sometimes with mud and wet grass, and a fine inner layer of soft grasses, dead leaves and moss (George, 2000). The eggs are greenish-blue with sparse dark umber-brown spots (Finley, 1936). Along the coast, females tend to lay an average of 3 eggs with a range of 1 to 5, while the interior subspecies lay an average of 4 eggs with a range of 2-6. They may raise two broods a year and the hatchlings are altricial. Varied Thrushes are monogamous and both parents help feed the young (George, 2000).
  • Average eggs per season
    3
    AnAge
  • Average time to hatching
    14 days
    AnAge

Lifespan/Longevity

Behavior

Varied Thrushes perch in trees to locate foraging areas and when on the ground they hop as opposed to walk. They gather insect prey by foraging on the ground and digging through the leafy debris with their bills. They also glean fruit from trees. Varied Thrushes clean their bills by wiping them on hard surfaces (George, 2000).

Varied Thrushes are known to give a variety of agonistic displays when feeding with other Varied Thrushes or with other species of birds. Martin (1970) described three different agonistic displays of Varied Thrushes at birdfeeders during the winter. When a feeding thrush comes too near another feeding thrush or other bird, the thrush will lift its tail, a display thought to help maintain spacing between the birds. If the other bird does not back off, the thrush will put its head forward, at which point the subordinate bird often retreats. Finally, if both birds maintain their respective positions and after a prolonged head forward display, the dominant bird usually attacks the other bird (Martin, 1970). Varied Thrushes do not usually flock with other species of birds (George, 2000).

Communication and Perception

Food Habits

During migration they eat fruits, berries and acorns. During the summer they eat arthropods, fruits and berries (George, 2000). During the winter, Varied Thrushes eat arthropods, fruits and acorns and can be lured to backyard feeding stations and are fond of apples (Finley, 1936 and George, 2000). Wells et al. (1996) suggest that the population cycling of the Varied Thrush is tied to the fruiting cycle of oak trees in the thrush's habitat.

Economic Importance for Humans: Negative

Varied Thrushes may eat orchard fruit (Finley, 1936)

Conservation Status

Varied Thrushes are currently neither endangered nor threatened in any portion of their range. Due to the fact that Varied Thrushes live in western old growth forests, deforestation and clear cutting pose potential threats to some population of Varied Thrushes. Flying into windows is a major anthropocentric cause of mortality (George, 2000).

Wells, et al. (1996) found that Varied Thrush populations undergo 2-3 year cycles of abundance and decline in most of the normal winter ranges.

Contributors

Daniel Gaebel (author), University of Michigan-Ann Arbor, Robert Payne (editor), University of Michigan-Ann Arbor.

Glossary

Nearctic

living in the Nearctic biogeographic province, the northern part of the New World. This includes Greenland, the Canadian Arctic islands, and all of the North American as far south as the highlands of central Mexico.

World Map

acoustic

uses sound to communicate

bilateral symmetry

having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.

chemical

uses smells or other chemicals to communicate

endothermic

animals that use metabolically generated heat to regulate body temperature independently of ambient temperature. Endothermy is a synapomorphy of the Mammalia, although it may have arisen in a (now extinct) synapsid ancestor; the fossil record does not distinguish these possibilities. Convergent in birds.

forest

forest biomes are dominated by trees, otherwise forest biomes can vary widely in amount of precipitation and seasonality.

iteroparous

offspring are produced in more than one group (litters, clutches, etc.) and across multiple seasons (or other periods hospitable to reproduction). Iteroparous animals must, by definition, survive over multiple seasons (or periodic condition changes).

motile

having the capacity to move from one place to another.

mountains

This terrestrial biome includes summits of high mountains, either without vegetation or covered by low, tundra-like vegetation.

native range

the area in which the animal is naturally found, the region in which it is endemic.

oviparous

reproduction in which eggs are released by the female; development of offspring occurs outside the mother's body.

sexual

reproduction that includes combining the genetic contribution of two individuals, a male and a female

tactile

uses touch to communicate

taiga

Coniferous or boreal forest, located in a band across northern North America, Europe, and Asia. This terrestrial biome also occurs at high elevations. Long, cold winters and short, wet summers. Few species of trees are present; these are primarily conifers that grow in dense stands with little undergrowth. Some deciduous trees also may be present.

visual

uses sight to communicate

References

Finley, W. 1936. Varied Thrush. Pp. 239-241(Part III) in T Pearson, ed. Birds of America. Garden City, NY: Garden City Publishing Company, Inc.

George, T. 2000. Varied Thrush: *Ixoreus naevius* No. 541. Pp. 1-20 in A Poole, F Gill, eds. The Birds of North America. Philadelphia, PA.: The Birds of North America, Inc..

Martin, S. 1970. The agonistic behavior of varied thrushes (*Ixoreus naevius*) in winter assemblages. The Condor, 72: 452-459.

Wells, J., K. Rosenberg, D. Tessaglia, A. Dhondt. 1996. Population cycles in the Varied Thrush (*Ixoreus naevius*). Canadian Journal of Zoology, 74: 2062-2069.