California Partners in Flight Riparian Bird Conservation Plan

Swainson's Thrush (Catharus ustulatus)

Photo by James Gallagher, Sea and Sage Audubon

Prepared by: Stacy L. Small (

University of Missouri, Columbia


Small, S. 1998. Swainson's Thrush (Catharus ustulataus). In The Riparian Bird Conservation Plan:a strategy for reversing the decline of riparian-associated birds in California. California Partners in Flight.


range map


SUBSPECIES STATUS: 2 subspecies, Russet-backed and Olive-backed, described by Grinnell and Miller (1944) as breeding in California.

Phillips (1991) describes 3 forms of the Olive-backed, only one of which occurs in CA --C. u. almae. He describes 3 forms of the Russet-backed, 2 of which occur in CA --C. u. ustulatus, breeding SE Alaska south to Humboldt Co. and C. u. oedicus, breeding from Lake, Nevada, and (probably) Siskiyou Counties south coastwise to San Diego County. Phillips disputes Grinnell’s claim of intergradation of Pacific and interior races on E. slope of Sierria Nevada. He claims that "all specimens seen are of one or the other type" and points out the fact that the Pacific and interior races maintain distinct, widely separated winter ranges, with no records of individuals in between. Bent (1949) makes a point regarding subspecies of Swainson’s Thrush that is most relevant to the creation of a riparian habitat conservation plan. He describes a contrast in habitat preference between the two subspecies, stating that the Russet-backed most typically nests in willow-alder thickets, while the Olive-backed prefers young conifers. Phillips emphasizes this point when he directly quotes Bent (1949) in his statement that "haunts of Western [Russet-backed] races are in marked contrast to those of most Eastern and Northern [Olive-backed] populations" and that "further studies are needed."


North American historic breeding range of Russet-backed subspecies: West of the Cascades and the Sierra Nevada from Juneau, AK, south to San Diego Co., CA (Bent, 1949). North American historic breeding range of Olive-backed subspecies: From forested regions of Alaska south to Northern California east of Cascades and Sierra Nevada, across Canada, Nevada, Utah, Colorado, northern Michigan, northern New England south to mountains of West Virginia and Pennsylvania (Bent, 1949). C.u.almae described as "fairly common summer resident," breeding above 4800 feet, in Warner Mountains of Modoc Co. and locally along E. slope of Sierra Nevada south through Mono Co., by Grinnell and Miller (1944).


Klamath Bioregion: CURRENTLY, ONLY KNOWN TO BREED ALONG COAST IN THIS BIOREGION. Present on 2 sites in Humboldt County -- Lost Man Creek and Mad River; absent on 13 sites, including Shasta region and Upper Sacramento River, as well as sites in Trinity, Glenn and Colusa Counties. Modoc Bioregion: NO CURRENT BREEDING RECORDS FOR THIS BIOREGION. Absent from all 5 sites in Lassen National Forest. Absent from one site in Modoc County. Sacramento Valley Bioregion: NO CURRENT BREEDING RECORDS FOR THIS BIOREGION. Absent from all 10 sites along Sacramento River. Sierra Bioregion: PRESENT DURING BREEDING SEASON AT SOME SITES, BUT FEW RECORDS OF HATCH YEAR BIRDS. Described as "presently rare and local" in Western Sierra Nevada, reason for decline unknown (Verner and Boss, 1980). Declining for at least 50 years in the Sierra; BBS data shows decrease of -2.6% per year (DeSante 1995). DeSante (1995) reports that no young have been captured in mist-nets in the 1990’s at any of the three Sierran MAPS stations where they occur, although on 2 sites, Zumwaldt Meadow in Kings Canyon National Park and the Sierra Nevada Field Campus in the Tahoe National Forest, males and females have occured simultaneously in some years. He states that "Sierran MAPS data on this rare species is too sparse to produce reliable estimates of adult survival rates for Sierran birds." Territorial behavior recorded in Plumas National Forest by Stefani in 1991-93 and on return visits in 1996. Swainson’s Thrush absent from historic range in Yosemite Valley since 1928 (Gaines 1992) and Eshom Creek, Whitaker’s Forest in southern Sierra Nevada in 1980’s, where they had been abundant in the 1930’s (Marshall 1988). No evidence of breeding on 15 of 16 Sierra study sites currently monitored throughout historic range (see map).  

Bay Delta Bioregion: BREEDING ONLY ALONG COAST IN THIS BIOREGION. Present and breeding in 14 drainages in Marin County. Present at one site in San Mateo County. Not breeding at one site in Alameda County. Possibly breeding in Santa Clara County. Absent at 4 sites on Cosumnes River Preserve. San Joaquin Bioregion: NO BREEDING RECORDS FOR THIS BIOREGION, VERY LITTLE DATA. Absent on all 5 sites on San Luis NWR. Central Coast Bioregion: Present on 2 sites in Monterey County and 7 sites in Santa Barbara County. Absent from 15 sites, 5 in Ventura, 10 in Santa Barbara. South Coast Bioregion: Present on 7 sites -- 2 in San Diego, 3 in Los Angeles, 1 in Orange, and 1 in San Bernadino. Absent on 42 sites.

Russet-backed Swainson’s Thrush (Catharus u. ustulatus and oedicus): NO EVIDENCE EXISTS THAT OLIVE-BACKED SUBSPECIES CURRENTLY BREEDS IN CALIFORNIA. Ten current study sites are in historic range of this subspecies.

ustulatus: Pacific coast region, west of Cascade-Sierran axis, south to Humboldt County.

oedicus: west of Sierra, from Nevada, Lake, and Siskyou counties south coastwise to San Diego County. Marin Co. birds approach ustulatus. 

Russet-backed subpopulations breed sea level to 7300 feet, but not above 4000 feet in southern CA.


Russet-backed Swainson’s Thrush
Catharus ustulatus ustulatus: Winters central western Mexico (northwestern Nayarit) south to Costa Rica -- both coastal slopes (rarely and locally on interior slopes) -- and mountains of western Panama. Also, Tres Marias Islands.  

Catharus ustulatus oedicus: Tres Marias Islands and mainland Nayarit south and east along Pacific coast to Nicaragua, north to central and eastern Guatemala, possibly Northeastern Chiapas.  

Olive-backed Swainson’s Thrush 

Catharus ustulatus almae: Central and northwestern South America, mainly from Northwestern Argentina and Bolivia north and west, possibly to eastern Panama.

The distinct habitat requirements of the subspecies should be a major theme in conservation planning for the Swainson’s Thrush. However, data on breeding requirements of the western Russet-backed Swainson’s Thrush , published or unpublished, is extremely sparse. Although the majority of the Swainson’s Thrush range in CA is occupied by the Russet-backed subspecies, no distinctions are made in published CA wildlife habitat manuals between habitat requirements this and the more Northern and Eastern Olive-backed subspecies. 

MANAGEMENT STATUS: Stefani (1996) reports that Swainson’s Thrush has been proposed as a Species of Special Concern by Laymon (pending Fish and Game approval) and that the USDA Forest Service identified the Swainson’s Thrush as one of two priority landbird species for monitoring in the Sierra Nevada.


Bay Delta Bioregion: 1.9 hectares, in alder/willow riparian habitat (PRBO, 1997 Muddy Hollow data). South Coast Bioregion: 2.9 hectares, on riparian woodland plot in San Diego Co. (Weaver 1989). Swainson’s Thrush density decreased yearly on this plot after 1989, disappearing by 1992 and absent at least through 1994. Weaver reports drought in 1990 and spring floodwaters which scoured the plot in the years 1991-1993. But by 1997, Swainson’s Thrushes had returned to the plot, with 3 males detected countersinging. Sierra Bioregion: Stefani reports 1.8 Swainson’s Thrushes/ha in Plumas National Forest, 1996.  


Arrival date on breeding grounds: Arrives late March/early April in Coast and Mountain regions, remaining throughout breeding season (McCaskie 1988). PRBO banding data for Marin County (1996 and 1997) -- earliest capture dates of adult April 29, 1996 and April 17, 1997. Earliest full brood patches May 20, 1996 and May 28, 1997.

Departure date from breeding grounds: According to McCaskie (1988), departs Coast and Mountain regions late August through October. PRBO banding data for Marin County (1996 and 1997) -- latest capture dates October 16, 1996 and October 4, 1997.
Spring and Fall migration period: While SWTH is rare in Interior and Basin regions during the summer months, abundance peaks in Spring (April through May) and Fall (August through September) in these regions (McCaskie 1988) -- possibly indicating peak of Spring and Fall migration period in California.
Winker et al (1992) found that "staging area" concept does not really apply to Swainson’s Thrush in Minnesota. Mean stopover days were less than five at this site. They hypothesize that spring migrant Swainson’s Thrush frequently make one-day stopovers, feeding by day and flying by night. Land on stopover sites in the dark and locate diurnal foraging grounds at dawn (Cochran et al 1967). Nocturnal migrant (Ziener et al. 1990), routes differ seasonally (Winker et al 1992). Rapid northward migration in spring - 480km/day (straight-line distance) from southern Mexico to Minnesota (Winker et al 1992). Need to quickly locate resources for daily binge before setting off again.
Broad habitat use during spring migration, differs seasonally. Stopover sites provide resources to fuel up for migration and to complete feather growth. These two needs are not met simultaneously for all individuals captured at a Minnesota site. Small patches of 1-3 ha were used for stopover; five different habitat types -- floodplain, swamp, willows, upland, and oaks -- were used equally (Winker et al 1992). Fat accumulated in continental U.S.on both spring and fall migration. Difference in seasonal fat deposition patterns detected - "heavier spring, leaner autumn" at Minnesota site, but not at others. Winker points out that fat deposition patterns may vary by races, according to Ramos (1989).



Like other Catharus species, gleans from litter of forest floor. Also gleans foliage and occasionally hawks aerial insects (Verner and Boss 1980).  


Year round, primarily insects such as beetles, ants, caterpillars, Hymentoptera. Secondarily flies, bugs, grasshoppers, spiders, millipedes, snails, sow bugs, earthworms and a variety of berries (Martin et al 1951, Ehrlich et al 1988). Berries make up larger part of diet in fall, winter, and summer than in spring (Martin et al 1951). Young fed mostly insects, supplemented by berries (Ehrlich et al 1988).


Johnson and Geupel (1996) suggest that habitat specificity on their breeding grounds may be the basis for Swainson’s Thrushes being limited in the summer. They outline degree of habitat specificity by season: in breeding season Swainson’s Thrushes are riparian woodland specialists (Grinnell and Miller 1944, Bent 1949, Verner and Boss 1980), on migration they use a range of habitats (Winker et al 1992), and in winter they are possibly nomadic (Ramos and Warner 1980, Rappole and Warner 1980), although there are arguments against this "winter nomad" interpretation (Marshall 1988). They report a high HY return rate of 18.3% at the Palomarin Field Station in Marin County, CA and hypothesize that "scarcity of habitat may limit dispersal possibilities." Bent (1949) describes a contrast in habitat preference between the two subspecies, stating that the western Russet-backed subspecies most typically nests in willow-alder thickets, while the eastern Olive-backed subspecies prefers young conifers. Verner and Boss (1980) state that in the Western Sierra, Swainson’s Thrushes prefer dense thickets near streams or wet meadows. Swainson’s Thrushes were found to be more abundant in riparian habitats than upland habitats on mixed-conifer study sites in the Cascade Mountains, Lane County, Oregon (Anthony et al 1996).  

Marzluff and Lyon (1983) found Swainson’s Thrush abundance correlated with snag density, and 25-50 cm dbh live stems. They state that density of snags > 30 cm dbh reflect shrub cover and the occurrence of large stems. According to Timossi (1990) the Swainson’s Thrush is found in moist ecotones, such as tree/shrub, tree/grass, or shrub/grass, with water and dense understory being necessary elements. She suggests that a dense understory and canopy closure of 40-100% provides both protection from predators and an ample food supply.  

No published nest data exists for Swainson’s Thrush in California, and little exists for the Russet-backed subspecies overall. Data has been reported for seven nests located near Juneau, AK (Rogers 1994). According to Diane Evans, compiler of SWTH BNA species account, very little nest data, published or unpublished, exists for the Pacific Coast region besides PRBO and KRRC nest data collected between 1995 and 1997 in Marin County (pers. comm.).

The following variables were found to significantly influence nest site selection in coastal Marin County in 1997, using logistic regression analysis (PRBO data):  
Univariable Analysis
Factors Influencing Swainson’s Thrush Nest Site Selection 
n=184 (61 nest sites, 123 reference sites) 
correlation coeff.
Salmonberry cover 
Twinberry cover
Red Elderberry cover
Hedge Nettle cover
Artichoke Thistle cover
Canopy cover
Tree stem count, all spp & sizes
Tree stems, all spp.,23-28 cm dbh
Tree stems, all spp.,>38 cm dbh
Red Alder stems, all sizes
Red Alder stems, 8-23 cm dbh
Red Alder stems, >38 cm dbh
Log ground cover
Grass/sedge ground cover
Fern ground cover
Water ground cover

The following variables were found to significantly influence nest success in coastal Marin County in 1997, using logistic regression analysis (PRBO data):  
Univariable analysis
Factors Influencing Swainson’s Thrush Nest Success 
n=61 nests 
correlation coeff.
Concealment below
Total cover
Willow shrub cover
Western Swordfern

When controlling for plot effect, two of the above variables were found to account for within-plot differences in nest success -- willow shrub cover, which negatively correlated with nest success, and Western Swordfern cover, which positively correlated with nest success. It is interesting to note the negative correlation of nest success with willow shrub cover (defined as < 5m high and stem of <8cm DBH), considering that Bent describes Russet-backed Swainson’s Thrush as being attracted to willow-alder thickets. Perhaps at the early stage of willow regeneration, some nest site requirement is not met for the Swainson’s Thrush. Therefore, the simultaneous presence of willows and breeding Swainson’s Thrushes should not necessarily be taken to represent a healthy population.

Using multiple logistic regression, six variables were found to influence nest site selection in Marin County, 1997 (P-value of model < .001):

Multivariable Analysis
Variables Influencing Swainson’s Thrush Nest Site Selection, Marin Co.,1997 
n=184 (61 nest sites, 123 reference sites) 
+ or - correlation 
Hedge Nettle Cover
Water Ground Cover
Canopy Cover
Fern Ground Cover
Log Ground Cover
Grass/Sedge Ground Cover

Using multiple logistic regression, two variables were found to influence nest success in Marin County, 1997 (P-value of model < .001):
Multivariable Analysis
Variables Affecting Swainson’s Thrush Nest Success 
n=61 nests 
+ or - correlation 
Concealment below
Willow shrub cover

Marin Co. 1997 (PRBO data): Native Blackberry - 37%, Hedge Nettle - 14%, Salmonberry - 13%, other shrubs and forbs - 36%. Baicich and Harrison describe nest being located in a small tree, close to trunk, often in conifer, occasionally in deciduous tree or shrub. This pattern not observed for Russet-backed subspecies in coastal Marin Co. This is likely a description for eastern Olive-backed, conifer dwelling populations, rather than Pacific coastal subpopulations. 


Mean 73 cm (± 4.12), range 0.02-2.2 m in Marin Co. 1997 (n=64, PRBO data) compared to a nest site description in Baicich and Harrison (1997) describing nests 0.6-2.15 m high, exceptionally up to 9.1 m. Also contrast to USDA Forest Service manual for Neotropical Migrants in California, which describes nests as 1.2-6.1 m high. The latter two descriptions may apply to eastern, conifer dwelling populations.


1.03 m (± 10.40).


Shrubs and forb species, see "Nest Site"


Mean 56% (± 3.44). 


71% (± 3.67).

9.98 m (± 0.87).


Red Alder. 


41% (± 2.85).


Willow species

41% (± 2.70).


1. Logs: 1.4% (± 0.34).

2. Grass/sedge: 4.3% (± 1.1).

3. Water: 7% (± 1.42).

4. Leaf litter: 33% (± 3.23).

5. Rock: N/A

6. Bare ground: 14% (± 1.86).

7. Fern: 9% (± 1.65)


8% (± 1.96).

78% (± 13.98).

2.6 in 11 m radius (± 0.61).
Nests near water according to Grinnell and Miller (1944) and Bent (1949). Optimal habitat contains moisture from a river, stream, lake, creek, slough, marsh, swamp, or bog (Thomas 1979). Possibly attracted to a site by availability of mud for nest lining (Bent 1949, PRBO data).


Open cup -- from Baicich and Harrison (1997): "compact cup of fine twigs, mosses, lichens, weed stems, decayed leaves, and mud, and bark shred, with inner lining of decayed leaves, rootlets, grasses, and other materials. Built by female over 4 days."



Marin Co. (PRBO data) -- May 16, 1997. Mean nest initiation date, 1997 -- June 5 (± 1.46). Latest nest initiation date, 1997 -- July 5. Latest fledge date, 1997 -- August 2.  


Monogamous (Ehrlich 1988). 


3-5 (Ehrlich 1988).  


Female, both adults tend young (Ehrlich 1988). 


10-13 (Ehrlich 1988).   


10-12 days (Ehrlich 1988).  


Altrical (Ehrlich 1988).


Both adults (Ehrlich 1988). Alarm calls at nest -- parents countercall at nest, single "water-drop" calls rapidly repeated. Adults also make long distraction flights away from nest, as well as bouncing from branch to branch within a few meters of nest, often with food in bill (Small, unpublished field notes).


Data from banding sites in NY and Ontario demonstrate pre-molt and early molt movement of hatch year birds from breeding grounds (Cherry 1985).


Conflicting reports: Single brooded in Marin Co., no instances of double brooding reported in 1996 or 1997 (PRBO and KRRC nest data). A study in coastal Alaska (Rogers 1994) also reports single brooding. Baicich and Harrison (1997) report double brooding for the species in general, but do not indicate subspecies, bioregions, or habitat. Therefore, it may be a mistake to apply this claim to California populations, Russet-backed subspecies in particular. Ehrlich claims number of broods unknown (1988).

No data to demonstrate that SWTH is a common Brown-headed Cowbird host (Friedmann 1929, Garrett 1997, pers. comm., unpubl. PRBO and KRRC nest data). Only 2 of 64 nests in coastal Marin Co. were parasitized in 1997 (PRBO data).



On historic California breeding range (Grinnell and Miller 1944):

Western (Russet-backed) subspecies-sea level to 2225 meters, but not above 1220 meters in southern CA; Eastern (Olive-backed) subspecies-above 1460 meters.
Nests located within 150 meters of an edge were found to have higher nest predation rates than those located > 150 meters from edge, which had predation rates more similar to those of a continuous forest (Andren and Angelstam 1988).
In a study of avian abundance in riparian habitat of the Cascade Mountains, Lane County, Oregon, breeding densities differed by seral stage. Plots were characterized as old-growth (400-450 yrs), mature (130-200 yrs), and young (25-35 yrs). Young plots had been clearcut; mature, even-aged plots had burned in a fire in the 1850s; and old-growth were natural, unlogged forests. Swainson’s Thrush was least abundant in old-growth (6.50/40 ha, ± 3.90) mixed coniferous woodland, dominated by Douglas fir, Western hemlock, and Western red cedar, more abundant in mature even-aged stands (14.30,± 9.70), and most abundant in young stands (25.5/40 ha, ± 3.00), regenerating from clearcutting practices. Young stands had a "conspicuous component of red alder, which was co-dominant with big-leaf maple and willow." Young stands had lowest shrub cover, because of dense young Douglas fir and Western red cedar, but early seral plants such as willow and snowbrush ceanothus were common. This study does not address productivity, only abundance. It is unknown whether requirements for successful breeding were met on these young clearcut sites.
Marshall (1988) suggests that the disappearance of the Swainson’s Thrush from the Sierran west slope Ponderosa pine forests surrounding Whitaker’s Forest may be attributed to drastic logging in Sequoia National Forest. He argues that declines in the Sierras may also be attributed to destruction of Latin American wintering grounds. He cites Ramos and Warner (1980) and Rappole et al. (1983) with the statement "the decrease results from yearlong behavioral continuity, including fidelity to winter territory structurally similar to that of summer. Nearctic migrants are as vulnerable in winter as in summer to loss of forest." This view contrasts with Johnson and Geupel’s (1996) interpretation of these same studies, regarding winter site fidelity. Based on these studies, Marshall describes as vulnerable those thrushes "condemned, through loss of habitat, to become nomads and floaters," while Johnson and Geupel suggest that nomadic tendencies documented among Swainson’s Thrushes imply lower habitat specificity on their wintering grounds than their breeding grounds. DeSante (1995) agrees to some extent with Marshall that declines may be due to a loss of wintering habitat, but points out that subpopulations on both the east and west slopes of the Sierra seem to be declining, although they winter separately; the east slope subpopulations winter in South America, and the west slope population winters in Mexico and Central America.

Swainson’s Thrushes declined and eventually disappeared over a 5 year period (1989-1994) from a 11.7 ha BBC plot in San Diego County, after heavy spring flooding scoured the understory in the years 1991-93 (Weaver 1989-1994). They have since returned, with three males detected countersinging in 1997 (Weaver, pers. comm).


Disturbance by recreational activities on waterways or by vehicles using roads along waterways may be detrimental to reproductive success (USDA).

Timing of riparian restoration activites

On one site in the Sierra region, disturbance to riparian habitat during breeding season may have caused disruption or displacement of Swainson’s Thrushes in 1995 and 1996. Stefani (1996) discovered that Swainson’s Thrush productivity declined between the period 1989-91 and 1996 at a site where a waterfowl habitat creation project had commenced in Fall 1995. Restoration activities continued through the breeding season of 1996, possibly contributing to the population decline on the site. She recommends avoiding such activities during future breeding seasons. Also, two active Swainson’s Thrrush nests were destroyed in Marin Co. in 1997, as a direct result of summer fish habitat restoration activities; the nest was inadvertantly knocked from its supporting shrub during fence-building (PRBO data 1997). These examples serve merely to illustrate that timing of riparian restoration activites is critical, particularly when considering productivity of a species with a very short breeding season.

In Marin Co., predation is the most common nest mortality factor, surpassing other potential mortality factors, such as parasitism, weather, and plant structural failure. Depredation accounted for 66% of nest mortality, followed by nest desertion/abandonment at 11%, and structural failure of supporting plant species 6%. Presumed victim of avian, snake and mammalian predators (Verner and Boss 1980, PRBO data).



According to BBS data for 1966-1996, declining, but not quite significantly (P <0.20). PRBO banding data for 1980-1992 at Palomarin Field Station, Marin Co. also shows insignificant decline. Manly and Davidson (1993) report Swainson’s Thrush declining in California but not in North America, from 26 years of BBS data.  


A very short breeding season, documented by Bent (1949), PRBO nest and banding data (1996-97), and Rogers (1994), means a very narrow window of opportunity for reproduction, and possibly a high sensitivity to disturbance of breeding grounds. Therefore, Swainson’s Thrushes are limited by production of young on the breeding grounds (Johnson and Guepel 1995). Prolonged negative impacts on nest survivorship could be catastrophic to the Swainson’s Thrush. Unfortunately, we have a profound lack of knowledge of nest survivorship and nesting requirements of the Swainson’s Thrushes breeding in California. This gap in knowledge could be distastrous for the conservation of the species, which appears to be extirpated from a large portion of its historic range.   


Monitoring of nests and collection of vegetation data according to BBIRD protocol should take place on all sites currently known to support breeding populations, particularly Russet-backed subpopulations.
Aggressive efforts should be made to locate remaining breeding populations throughout historic range. Nest monitoring and spot mapping efforts should be established at sites representing all bioregions which still support breeding populations.


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