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Varied Thrush - Ixoreus naevius
Species of Concern Native Species Global Rank :
G5
State Rank :
S3B
(see State Rank Reason below)
Agency Status USFWS :
MBTA
USFS :
BLM :
FWP SWAP :
SGCN3
PIF :
3
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Copyright by: The Macaulay Library at the Cornell Lab of Ornithology, Ithaca, NY, all rights reserved.
State Rank Reason (see State Rank above)
The Varied Thrush has undergone recent population declines in Montana and across the Northern Rockies and where timber harvest, insect outbreak, and fire result in a loss of suitable breeding habitat.
Details on Status Ranking and Review
Varied Thrush (Ixoreus naevius ) Conservation Status Review
Population Size
Score G - 100,000-1,000,000 individuals
Comment Recent Intermountain Bird Conservation Region monitoring data for 2010 estimates the statewide population at 390,594 +/- SD of 85,930.
Range Extent
Score F - 20,000-200,000 km squared (about 8,000-80,000 square miles)
Comment 139,977 square kilometers based on Natural Heritage Program range maps that appear on the Montana Field Guide.
Area of Occupancy
Score U - Unknown
Comment Unknown.
Long-term Trend
Score E - Relatively Stable (±25% change)
Comment Species favors mesic conifer forests which, in the Northern Rockies, have remained relatively stable within +/- 25% of pre European levels.
Short-term Trend
Score C - Rapidly Declining. Decline of 30-50% in population, range, area occupied, and/or number or condition of occurrences
Comment Breeding Bird Survey data has moderate credibility in Montana and shows an insignificant decline of -6.7% per year or 51% decline per decade. For the Northern Rockies as a wholethe data has highest credibility and shows a significant decline of -4.3% per year or 46% decline per decade. Idaho, British Columbia, Alberta, Washington, and Oregon all show declines with high credibility.
Threats
Score B - Moderate and imminent threat. Threat is moderate to severe and imminent for a significant proportion (20-60%) of the population or area.
Comment Fire, timber harvest, and insect outbreak related to climate change are the greatest threats to the species due to its reliance on mature mesic conifer forests.
Severity Moderate - Major reduction of species population or long-term degradation or reduction of habitat in Montana, requiring 50-100 years for recovery.
Comment Loss of unbroken stands of mature trees takes a long time to recover.
Scope Moderate - 20-60% of total population or area affected
Comment Fire and beetle kill are drastically changing a large portion (20-60%) of the landscape.
Immediacy Moderate - Threat is likely to be operational within 2-5 years.
Comment Ongoing but could accelerate.
Intrinsic Vulnerability
Score C - Not Intrinsically Vulnerable. Species matures quickly, reproduces frequently, and/or has high fecundity such that populations recover quickly (< 5 years or 2 generations) from decreases in abundance; or species has high dispersal capability such that extirpated populations soon become reestablished through natural recolonization (unaided by humans).
Comment Not Intrinsically Vulnerable. Species matures quickly, reproduces frequently, and/or has a high fecundity such that populations recover quickly (< 5 years or 2 generations) from decreases in abundance. Species has good dispersal capabilities such that extirpated populations generally become reestablished through natural recolonization.
Environmental Specificity
Score B - Narrow. Specialist. Specific habitat(s) or other abiotic and/or biotic factors (see above) are used or required by the Element, but these key requirements are common and within the generalized range of the species within the area of interest.
Comment Narrow specialist. Relies on mature mesic forest types and riparian forests and often most abundant in areas with more closed canopy.
Raw Conservation Status Score
Score
3.5 + 0.25 (population size) + 0.0 (geographic distribution) - 0.5 (short-term trend) - 0.75 (threats) = 2.5
General Description
The Varied Thrush is unmistakable with its black and orange plumage and ethereal song. Yet, its shy behavior and tendency to nest in dense mature and old-growth forests have made study of this the breeding biology of this species difficult.
For a comprehensive review of the conservation status, habitat use, and ecology of this and other Montana bird species, please see
Marks et al. 2016, Birds of Montana. Phenology
In Montana, active Varied Thrush nests have been observed beginning mid- to late-April. Nestlings and fledglings have been observed as early as mid-May. Fledglings from likely second broods observed mid- to late-August (Montana Natural Heritage Program Point Observation Database 2014).
Diagnostic Characteristics
The Varied Thrush is a large, brightly colored thrush. Adult male has a burnt-orange breast and throat, gray to gray-blue rump, back, neck, and crown, a black to slate-gray V-shaped breast band, orange-buffy eyebrow and wing bars, and black to slate-gray wing and tail feathers. Female is similar to male but duller overall with brown-olive to brown-gray upperparts, brown wing and tail feathers, and brown to slate-gray breast band. Plumages are similar throughout the year. Immature birds are similar to adults except the head and neck are brown tinged with buff with an indistinct orange eyebrow. Throat and breast feathers are buff instead of orange. The song of this species is distinctive: a long, whistled tone about two seconds in length with a pause of three to 20 seconds between each tone. The song is somewhat ventriloquial. (George 2000).
Species Range
Montana Range
Range Descriptions Western Hemisphere Range
Range Comments
In Montana, the Varied Thrush breeds primarily in mature and old-growth mixed-coniferous forests of western Montana. Highest number of observations during the breeding season are in northwestern Montana. This species can travel widely during migration and winter, with observations as far east as Sheridan County (Montana Natural Heritage Program Point Observation Database 2014).
Observations in Montana Natural Heritage Program Database
Number of Observations: 6213
(Click on the following maps and charts to see full sized version)
Map Help and Descriptions
Relative Density
Recency
SUMMER (Feb 16 - Dec 14)
Direct Evidence of Breeding
Indirect Evidence of Breeding
No Evidence of Breeding
WINTER (Dec 15 - Feb 15)
Regularly Observed
Not Regularly Observed
(Observations spanning multiple months or years are excluded from time charts)
Migration
The Varied Thrush is a short distance, partial migrant (George 2000). In spring, observations of the species in suburban habitats such as golf courses and backyards into mid-April suggest that arrival on breeding grounds may not occur until mid- to late April. Fall movements likely begin around early September based on multiple observations of the species in unsuitable breeding habitats (Montana Natural Heritage Program Point Observation Database 2014).
Habitat
In Montana, the Varied Thrush breeds in mixed-coniferous forests with most observations occurring in western and northwestern Montana (Montana Natural Heritage Program Point Observation Database 2014). Dominant tree species include Douglas-fir and western larch. This species is more abundant in mature and old-growth forest stands than in younger forests (Tobalske et al. 1991). In winter, the Varied Thrush uses a wider variety of habitats, including suburban areas such as bird feeders and areas where fruits and berries are present (George 2000).
Ecological Systems Associated with this Species
Details on Creation and Suggested Uses and Limitations
How Associations Were Made
We associated the use and habitat quality (common or occasional) of each of the 82 ecological systems mapped in Montana for
vertebrate animal species that regularly breed, overwinter, or migrate through the state by:
Using personal observations and reviewing literature that summarize the breeding, overwintering, or migratory habitat requirements of each species (Dobkin 1992, Hart et al. 1998, Hutto and Young 1999, Maxell 2000, Foresman 2012, Adams 2003, and Werner et al. 2004);
Evaluating structural characteristics and distribution of each ecological system relative to the species' range and habitat requirements;
Examining the observation records for each species in the state-wide point observation database associated with each ecological system;
Calculating the percentage of observations associated with each ecological system relative to the percent of Montana covered by each ecological system to get a measure of "observations versus availability of habitat".
Species that breed in Montana were only evaluated for breeding habitat use, species that only overwinter in Montana were only evaluated for overwintering habitat use, and species that only migrate through Montana were only evaluated for migratory habitat use.
In general, species were listed as associated with an ecological system if structural characteristics of used habitat documented in the literature were present in the ecological system or large numbers of point observations were associated with the ecological system.
However, species were not listed as associated with an ecological system if there was no support in the literature for use of structural characteristics in an ecological system,
even if point observations were associated with that system.
Common versus occasional association with an ecological system was assigned based on the degree to which the structural characteristics of an ecological system matched the preferred structural habitat characteristics for each species as represented in scientific literature.
The percentage of observations associated with each ecological system relative to the percent of Montana covered by each ecological system was also used to guide assignment of common versus occasional association.
If you have any questions or comments on species associations with ecological systems, please contact the Montana Natural Heritage Program's Senior Zoologist.
Suggested Uses and Limitations
Species associations with ecological systems should be used to generate potential lists of species that may occupy broader landscapes for the purposes of landscape-level planning.
These potential lists of species should not be used in place of documented occurrences of species (this information can be requested at:
mtnhp.org/requests ) or systematic surveys for species and evaluations of habitat at a local site level by trained biologists.
Users of this information should be aware that the land cover data used to generate species associations is based on imagery from the late 1990s and early 2000s and was only intended to be used at broader landscape scales.
Land cover mapping accuracy is particularly problematic when the systems occur as small patches or where the land cover types have been altered over the past decade.
Thus, particular caution should be used when using the associations in assessments of smaller areas (e.g., evaluations of public land survey sections).
Finally, although a species may be associated with a particular ecological system within its known geographic range, portions of that ecological system may occur outside of the species' known geographic range.
Literature Cited
Adams, R.A. 2003. Bats of the Rocky Mountain West; natural history, ecology, and conservation. Boulder, CO: University Press of Colorado. 289 p.
Dobkin, D. S. 1992. Neotropical migrant land birds in the Northern Rockies and Great Plains. USDA Forest Service, Northern Region. Publication No. R1-93-34. Missoula, MT.
Foresman, K.R. 2012. Mammals of Montana. Second edition. Mountain Press Publishing, Missoula, Montana. 429 pp.
Hart, M.M., W.A. Williams, P.C. Thornton, K.P. McLaughlin, C.M. Tobalske, B.A. Maxell, D.P. Hendricks, C.R. Peterson, and R.L. Redmond. 1998. Montana atlas of terrestrial vertebrates. Montana Cooperative Wildlife Research Unit, University of Montana, Missoula, MT. 1302 p.
Hutto, R.L. and J.S. Young. 1999. Habitat relationships of landbirds in the Northern Region, USDA Forest Service, Rocky Mountain Research Station RMRS-GTR-32. 72 p.
Maxell, B.A. 2000. Management of Montana's amphibians: a review of factors that may present a risk to population viability and accounts on the identification, distribution, taxonomy, habitat use, natural history, and the status and conservation of individual species. Report to U.S. Forest Service Region 1. Missoula, MT: Wildlife Biology Program, University of Montana. 161 p.
Werner, J.K., B.A. Maxell, P. Hendricks, and D. Flath. 2004. Amphibians and reptiles of Montana. Missoula, MT: Mountain Press Publishing Company. 262 p.
Commonly Associated with these Ecological SystemsForest and Woodland Systems
Recently Disturbed or Modified
Shrubland, Steppe and Savanna Systems
Wetland and Riparian Systems
Occasionally Associated with these Ecological SystemsForest and Woodland Systems
Recently Disturbed or Modified
Food Habits
During the breeding season, the Varied Thrush feeds upon ground-dwelling arthropods. Fruits and berries become important later in the breeding season and during migration. Winter food items are similar but can include mast if available. This species forages primarily on the ground for arthropods, unless foraging for fruits and berries (George 2000).
Ecology
The Varied Thrush is a rare host of the Brown-headed Cowbird, which generally avoids dense forested habitats (George 2000).
Reproductive Characteristics
Nests are often poorly concealed and placed on branches close to the trunk of small conifers, on the ground, in shrubs and vines, and near the ends of branches of large conifers; generally 2-4 meters above ground. Nest is an open cup consisting of three layers: an outer layer of loosely woven twigs, leaves, and bark; a middle layer of rotten wood, moss, and mud; and an inner layer of fine grass, soft leaves, and fine mosses. Eggs are light sky blue and slightly paler than the American Robin with spare marks of small, dark-brown spots. Average clutch size is four eggs. Only female incubates; incubation period lasts about 12 days. Both males and females feed nestlings. Young leave nest 13-15 days after hatching (George 2000). Based on the length of breeding observations, this species likely has two broods (George 2000, Montana Natural Heritage Program Point Observation Database 2014).
Management
Logging of mature and old-growth forests reduces the suitability of these habitats for nesting (Tobalske et al. 1991, George 2000). This species requires large, unfragmented patches of suitable habitat for nesting. Additionally, forest management practices that promote even-aged stands may leave large areas unsuitable for nesting (George 2000).
Stewardship Responsibility
Threats or Limiting Factors
Loss of suitable breeding habitat due to forestry practices, insect outbreaks, and wildfires reduce breeding populations. Observations of individuals killed by domestic cats. Dead individuals have also been found after collisions with motor vehicles (Montana Natural Heritage Program Point Observation Database 2014). Collisions with windows also contributes to mortality of the species (George 2000).
References
Literature Cited AboveLegend:
Additional ReferencesLegend: Do you know of a citation we're missing? Conservation Biology. 9 (5): 1041-1058.Effects of recreation on Rocky Mountain wildlife: a review for Montana . [Montana]: Montana Chapter of the Wildlife Society.
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