Breeding

Pair Formation

Arrival varies by location and elevation, with males preceding females: mid- to late Apr in coastal California (147 Johnson, M. D. and G. R. Geupel. (1996). The importance of productivity to the dynamics of a Swainson's Thrush population. Condor 98 (1):133-141. Close ); mid-May at a midelevation Idaho site (DEM) and in se. Alaska (148 Rogers, C. M. (1994a). Avian nest success, brood parasitism and edge-independent reproduction in an Alaskan wetland. Journal of Field Ornithology 65:433-586. Close ); and about mid-May in Massachusetts (56 Veit, R. R., and W. R. Petersen (1993). The Birds of Massachusetts. Massachussetts Audubon Society, Lincoln, Massachusetts, USA. Close ). Only Nearctic-Neotropical migratory thrush species whose second-year and older males may begin releasing sperm into cloaca spontaneously and continuously prior to arrival on some breeding grounds (documented 800 km south of nesting range), possibly as mechanism for preparation for appropriately timed mating (119 Quay, W. B. (1986b). Timing and location of spring sperm release in northern thrushes. Wilson Bulletin 98:526-534. Close ).

Nest-Building

Figure 3 . Mean date of nests observed under construction (not backdated from hatch) 18 Jun ± 7.5 d SD in Idaho (range 9 Jun–5 Jul, n = 21; DEM, J. Bednarz, J. Hovis, and C. Davis unpubl.); individuals with nesting material observed 3–16 May in Monterey, CA (83 Roberson, D. (1993g). "Swainson's thrush." In Atlas of the breeding birds of Monterey County., edited by D. Roberson and C. Tenney, 286-287. Carmel, CA: Monterey Peninsula Audubon Soc. Close ). Limited data indicate nest built at least 2–3 wk after arrival at breeding sites: e.g., approximately 20 d in Idaho (DEM) and Maine (10 Stanwood, C. J. (1913). The olive-backed thrush (Hylocichla ustulata swainsoni) at his summer home. Wilson Bulletin 25:118-137. Close ).

First/Only Brood Per Season

Across the range, eggs laid mid-Apr–late Jun, although later dates recorded for what are presumed to be renests from early failures. Mean dates (± SD) of eggs in nests (generally the incubation period, not necessarily first day of incubation) from combined sources (Western Foundation of Vertebrate Zoology [WFVZ], Cornell nest record program [CNRP], and 96 Evans, D. M., J. C. Bednarz, C. R. Davis and J. C. Hovis. (1998). The effects of forest landscape modification and management on Neotropical migratory songbird populations in west-central Idaho. State University: Arkansas State Univ. Close : 3 Jun ± 16 d (range 18 Apr–25 Jul, n = 345) in California; 18 Jun ± 14 d (range 29 May–12 Jul, n = 20) in Alaska; 21 Jun ± 11 d (range 28 May–25 Jul, n = 83) in the Pacific Northwest; 20 Jun ± 9 d (range 21 May–14 Jul, n = 90) in the Northeast, and 23 Jun ± 9 d (range 17 May–19 Jul, n = 87) in Inland North-west. In British Columbia, 51% of 328 nests had eggs present 14–30 Jun (42 Campbell, R. W., N. K. Dawe, I. McTaggart-Cowan, J. M. Cooper, G. W. Kaiser, M. C. E. McNall, and G. E. J. Smith (1997). The Birds of British Columbia. Volume 3. Passerines: Flycatchers Through Vireos. University of British Columbia Press, Vancouver, BC, Canada. Close ). Mean date of first egg 10 Jun ± 2.4 d SE for Golden Gate National Recreation Area, CA (range 11 May–Jul 6, n = 33; 149 Gardali, T., S. E. Scoggin and G. R. Geupel. (1999). Songbird use of Redwood and Lagunitas creeks: management and restoration recommendations. Stinson Beach, CA: Point Reyes Bird Observatory. Close ); mean date of nest initiation 27 Jun ± 7 d (range 6 Jun–5 Jul, n = 25) in Green Mtns., VT (K. McFarland pers. comm.).

Young observed in nests, from combined sources: 10–26 Jun in Alaska (n = 3), 10 Jun–6 Jul in Northeast (n = 10), 22 Jun–24 Jul in e.-central U.S. (n = 8), 17 Jun–29 Jul in Inland Northwest (n = 56), and 24 Jun–1 Aug in Pacific Northwest (n = 17). (Forward- and backdating from incubation and fledging dates could extend this period for some locations.) Of British Columbia nests, 52% had young between 29 Jun and 15 Jul (42 Campbell, R. W., N. K. Dawe, I. McTaggart-Cowan, J. M. Cooper, G. W. Kaiser, M. C. E. McNall, and G. E. J. Smith (1997). The Birds of British Columbia. Volume 3. Passerines: Flycatchers Through Vireos. University of British Columbia Press, Vancouver, BC, Canada. Close ). Fledglings 23 Jun–3 Aug in Monterey, CA (83 Roberson, D. (1993g). "Swainson's thrush." In Atlas of the breeding birds of Monterey County., edited by D. Roberson and C. Tenney, 286-287. Carmel, CA: Monterey Peninsula Audubon Soc. Close ); mean fledge date for Idaho study 20 Jul ± 9 d (range 8 Jul–13 Aug, n = 42; 96 Evans, D. M., J. C. Bednarz, C. R. Davis and J. C. Hovis. (1998). The effects of forest landscape modification and management on Neotropical migratory songbird populations in west-central Idaho. State University: Arkansas State Univ. Close ).

Second Brood Per Season

Generally 1 brood/season but may renest in some localities if first clutch is lost early (2 egg sets reportedly collected from same pair in California; WFVZ). A slight second peak in number of broods 3 wk after first peak suggested as evidence of double-brooding in British Columbia (42 Campbell, R. W., N. K. Dawe, I. McTaggart-Cowan, J. M. Cooper, G. W. Kaiser, M. C. E. McNall, and G. E. J. Smith (1997). The Birds of British Columbia. Volume 3. Passerines: Flycatchers Through Vireos. University of British Columbia Press, Vancouver, BC, Canada. Close ). One female at a low-elevation coastal site in British Columbia successfully fledged young and then laid a second clutch (which failed immediately), giving the only direct evidence of an attempt at a second brood (J. Smith pers. comm.). At higher elevations, snow pack limits breeding season to 6–9 wk, allowing insufficient time for a second brood.

Selection

No information.

Microhabitat/Site Characteristics

Most frequently in understory, particularly in thickets of deciduous shrubs or conifer saplings; less frequently but consistently found >3 m high on top of a horizontal branch away from the bole of a larger-diameter tree. Of 981 nests from combined sources (42 Campbell, R. W., N. K. Dawe, I. McTaggart-Cowan, J. M. Cooper, G. W. Kaiser, M. C. E. McNall, and G. E. J. Smith (1997). The Birds of British Columbia. Volume 3. Passerines: Flycatchers Through Vireos. University of British Columbia Press, Vancouver, BC, Canada. Close , 96 Evans, D. M., J. C. Bednarz, C. R. Davis and J. C. Hovis. (1998). The effects of forest landscape modification and management on Neotropical migratory songbird populations in west-central Idaho. State University: Arkansas State Univ. Close , WFVZ, CNRP, K. McFarland pers. comm.): height range 0–24 m; 60% between 1.0 and 2.4 m. Higher placement more typical of conifer forest sites. More than 90% of nests (n = 386) in California and Pacific Northwest <3 m, while 53% (n = 109) of Inland Northwest nests ≥3 m; 70% (n = 113) of nests in ne. North America between 1 and 2.5 m. Mean height in Alaska taiga 1.8 m ± 1.0 SD (n = 121; 9 Kessel, B. (1998). Habitat characteristics of some passerine birds in western North American taiga. Fairbanks, AK: Univ. of Alaska Press. Close ). Nests of isolated population in Allegany Hills, NY, characteristic of high nests, averaging 5.8 m high, 3.6 m from bole of large eastern hemlock (Tsuga canadensis) with limited understory (150 Eaton, S. W. (1998a). Swainson's Thrush nests in the Allegany Hills. Kingbird 48:16-18. Close ). In Idaho, 29% nests of this type (96 Evans, D. M., J. C. Bednarz, C. R. Davis and J. C. Hovis. (1998). The effects of forest landscape modification and management on Neotropical migratory songbird populations in west-central Idaho. State University: Arkansas State Univ. Close ).

Wide variety of plant species used as substrate; most frequently reported (sources listed above except 42 Campbell, R. W., N. K. Dawe, I. McTaggart-Cowan, J. M. Cooper, G. W. Kaiser, M. C. E. McNall, and G. E. J. Smith (1997). The Birds of British Columbia. Volume 3. Passerines: Flycatchers Through Vireos. University of British Columbia Press, Vancouver, BC, Canada. Close ) include willow (17%), fir (15%), spruce (14%), blackberry (11%), and unidentified bush (11%). Also used other deciduous trees, including alder, aspen, birch (Betula sp.), maple (Acer sp.), and oak; deciduous shrubs, such as briers, gooseberry (Ribes sp.), rose (Rosa sp.), sumac, and vinemaple (Acer sp.); and coniferous trees, including cedar (Thuja sp.), hemlock, and lodgepole pine (Pinus contorta). Unusual nests observed on a piece of driftwood, on the ground in grass, in a rootwad, and on a rock base. One nest found in small snag within dense thicket of pole-sized trees. Active nest (full clutch) reported from Vermont built atop artificial nest with quail eggs (K. McFarland pers. comm.). Stumps and posts (6 records) used when enhanced by vine cover.

In mixed-conifer forest in Idaho, nests placed nearer water in areas with denser shrubs and saplings, more trees 8–23 cm diameter, more large trees (>38 cm dbh), and greater canopy cover (85% ± 13.5 SD) than random sites (96 Evans, D. M., J. C. Bednarz, C. R. Davis and J. C. Hovis. (1998). The effects of forest landscape modification and management on Neotropical migratory songbird populations in west-central Idaho. State University: Arkansas State Univ. Close ). Nest sites had more grand fir trees (Abies grandis), and this substrate supported 59% of 117 nests. No attributes measured were significantly associated with nest fate. Similar characteristics reported from Vermont: 86% ± 11.35 SD canopy cover, higher density of small-, medium-, and large-diameter balsam fir (Abies balsamea) than other tree species, and 72% of 25 nests placed in fir (K. McFarland pers. comm.). Nest-plot attributes selected were large birch trees and large and small conifer trees. Microsite attributes for nests in western coniferous and eastern mixed coniferous-deciduous forests compared in Appendix 1 .

From riparian sites in central coastal California, nest sites associated most strongly with canopy cover; red alder (Alnus rubra) density; larger tree density; and fern, salmonberry (Rubus sp.), and hedge nettle (Stachys sp.) cover. Negatively associated with artichoke thistle (Cynara cardunculus) cover, grass/sedge cover, and logs (S. Small, G. R. Geupel, N. Nur, A. L. Holmes, and T. Gardali unpubl.). More than 30% of nests placed in California blackberry (Rubus ursinus); hedge nettle, salmonberry, and lady fern (Athyrium filix-femina) also selected. Nest success positively influenced by total canopy cover, red alder tree density, western swordfern (Polystichum munitum) cover, and nest concealment from below; willow-shrub cover, snag density, forb cover, and shrub species richness had negative associations with success (149 Gardali, T., S. E. Scoggin and G. R. Geupel. (1999). Songbird use of Redwood and Lagunitas creeks: management and restoration recommendations. Stinson Beach, CA: Point Reyes Bird Observatory. Close ; S. Small, G. R. Geupel, N. Nur, A. L. Holmes, and T. Gardali unpubl.).

Construction Process

Constructed in about 4 d by female (1 Bent, A. C. (1949). Life histories of North American thrushes, kinglets, and their allies. U. S. National Museum Bulletin 196. Close , 3 Dilger, W. C. (1956a). Adaptive modifications and ecological isolating mechanisms in the thrush genera Catharus and Hylocichla. Wilson Bulletin 68:171-199. Close ). No information on methods or time of day. Process of shaping nest cup by snuggling low into cup and thrusting with breast and feet inferred from other thrushes (e.g., Wood Thrush, 35 Roth, R.R., M. S. Johnson, and T. J. Underwood. 1996. Wood Thrush (Hylocichla mustelina). In The Birds of North America, No. 246 (A. Poole and F. Gill, eds.). The Academy of Natural Sciences, Philadelphia, PA, and The American Ornithologists' Union, Washington D.C. Close ).

Structure And Composition Matter

Outer layer of nest composed of grasses, plant stems, moss, small pieces of bark, small twigs; inner lining of skeletonized leaves, rootlets, lichens, or moss (10 Stanwood, C. J. (1913). The olive-backed thrush (Hylocichla ustulata swainsoni) at his summer home. Wilson Bulletin 25:118-137. Close , 78 Jewett, S. G., W. P. Taylor, and J. W. Aldrich (1953). Birds of Washington State. University of Washington Press, Seattle, WA, USA. Close , 151 Reed, C. A. (1965). North American birds eggs. Rev. ed. New York: Dover Publ., Inc. Close ). Mud often mentioned in literature, but not observed in Idaho or Vermont nests (K. McFarland pers. comm., DEM), and grass and moss predominated in nests from British Columbia (42 Campbell, R. W., N. K. Dawe, I. McTaggart-Cowan, J. M. Cooper, G. W. Kaiser, M. C. E. McNall, and G. E. J. Smith (1997). The Birds of British Columbia. Volume 3. Passerines: Flycatchers Through Vireos. University of British Columbia Press, Vancouver, BC, Canada. Close ). External appearance varies. Described at times as conspicuous (10 Stanwood, C. J. (1913). The olive-backed thrush (Hylocichla ustulata swainsoni) at his summer home. Wilson Bulletin 25:118-137. Close ), but in general, nests difficult to find, as evidenced by few nest discoveries relative to other species in Breeding Bird Atlas efforts. Characteristic nests in Idaho are small to bulky, light brown–colored grassy cups, and new nests identified by pieces of newly woven grass protruding out and hanging down from nest cup (DEM). In coastal Pacific Northwest, green moss replaces grass and has a draped effect. Other nests predominantly of small twigs (1 Bent, A. C. (1949). Life histories of North American thrushes, kinglets, and their allies. U. S. National Museum Bulletin 196. Close ). Rhizomorphs of horsehair fungus (Marasmius androsaceus) discovered as common nest-lining in spruce-fir forests in New England (152 McFarland, K. P. and C. C. Rimmer. (1996). Horsehair fungus (Marasmius androsaceus) used as nest lining by birds of the subalpine spruce-fir community in the northeastern United States. Canadian Field-Naturalist 110:541-543. Close ); these authors suggested previous descriptions of “rootlets” in nest-linings may be this material. Fungus known to have antibiotic capabilities (i.e., could function in deterring nest pathogens).

Dimensions

Outside diameter 9.4–15.4 cm (10.6 ± 1.32 SD), height 3.8–10.5 cm (8.2 ± 1.7 SD); inside diameter 5.0–8.5 cm (6.0 ± 0.85 SD), depth 2.5–6.0 cm (3.8 ± 0.98 SD) from 19 nests in Green Mtns., VT (K. McFarland et al. unpubl.). Some nests in Idaho outside these reported ranges: outside diameter 16.5 cm, depth 1.75 cm (DEM, J. Bednarz, J. Hovis, and C. Davis unpubl.).

Microclimate

Usually placed in shady location. No information on temperature or moisture at nests.

Maintenance Or Reuse Of Nests, Alternate Nests

New nest built each season, and also if renest attempted after failure within a season (DEM). Nest structures occasionally persist through winter in Idaho, but no reuse documented.

Nonbreeding Nests

Not known to be used. May build and then abandon a nest before eggs laid, possibly due to disturbance (2 nests built near frequently used hiking trails abandoned; 96 Evans, D. M., J. C. Bednarz, C. R. Davis and J. C. Hovis. (1998). The effects of forest landscape modification and management on Neotropical migratory songbird populations in west-central Idaho. State University: Arkansas State Univ. Close ).

Shape

Ovate, varying to short and long ovate (1 Bent, A. C. (1949). Life histories of North American thrushes, kinglets, and their allies. U. S. National Museum Bulletin 196. Close ).

Size

Length 20.2–25.7 mm (mean 23.19 mm, calculated from subspecies means); breadth 15.27–18.07 mm (mean of subspecies means 16.66 mm, n = 238 eggs, 66 clutches in WFVZ collection). C. u. swainsoni and ustulatus similar; clarescens (now swainsoni) on average 1.5 mm longer.

Mass

No information.

Color

Blue to greenish blue with reddish or brown speckles (151 Reed, C. A. (1965). North American birds eggs. Rev. ed. New York: Dover Publ., Inc. Close ). Density and size of spots variable, from uniformly covering egg to concentration toward larger end (10 Stanwood, C. J. (1913). The olive-backed thrush (Hylocichla ustulata swainsoni) at his summer home. Wilson Bulletin 25:118-137. Close , 1 Bent, A. C. (1949). Life histories of North American thrushes, kinglets, and their allies. U. S. National Museum Bulletin 196. Close ). Brown-on-blue color combination common among species that nest in fork of bush or tree and that often have nests with dark linings; egg placement on dark lining could reduce visibility to predators (153 Lack, D. (1958). The significance of the colour of turdine eggs. Ibis 100:145-166. Close ).

Surface Texture

Smooth.

Eggshell Thickness

Shell thickness not available. Empty shell weight: 0.140–0.218 g, mean of sub-species means 0.182 g (WFVZ).

Clutch Size

One to 5 eggs; 4 eggs most common (61% of 911 nests; 42 Campbell, R. W., N. K. Dawe, I. McTaggart-Cowan, J. M. Cooper, G. W. Kaiser, M. C. E. McNall, and G. E. J. Smith (1997). The Birds of British Columbia. Volume 3. Passerines: Flycatchers Through Vireos. University of British Columbia Press, Vancouver, BC, Canada. Close , 96 Evans, D. M., J. C. Bednarz, C. R. Davis and J. C. Hovis. (1998). The effects of forest landscape modification and management on Neotropical migratory songbird populations in west-central Idaho. State University: Arkansas State Univ. Close , CNRP, WFVZ).

Egg-Laying

Begins after nest completion. Number of days not well quantified; within 5 d at nests in Idaho (DEM). One egg/d laid in morning (10 Stanwood, C. J. (1913). The olive-backed thrush (Hylocichla ustulata swainsoni) at his summer home. Wilson Bulletin 25:118-137. Close ). No information on parental behavior during this period or egg replacement. No evidence of intraspecific egg-dumping.

Onset Of Broodiness And Incubation In Relation To Laying

Female begins incubating with last egg laid, but young still hatch asynchronously (10 Stanwood, C. J. (1913). The olive-backed thrush (Hylocichla ustulata swainsoni) at his summer home. Wilson Bulletin 25:118-137. Close ).

Incubation Patch

Of Swainson's Thrushes captured on breeding sites, females had visible brood patch, no brood patch observed on males in the hand (J. Bednarz pers. comm.).

Incubation Period

Ranges 10–14 d from last egg laid (42 Campbell, R. W., N. K. Dawe, I. McTaggart-Cowan, J. M. Cooper, G. W. Kaiser, M. C. E. McNall, and G. E. J. Smith (1997). The Birds of British Columbia. Volume 3. Passerines: Flycatchers Through Vireos. University of British Columbia Press, Vancouver, BC, Canada. Close , 96 Evans, D. M., J. C. Bednarz, C. R. Davis and J. C. Hovis. (1998). The effects of forest landscape modification and management on Neotropical migratory songbird populations in west-central Idaho. State University: Arkansas State Univ. Close ). Young found on days 10, 11, 12, and 13 from onset of incubation of Maine nests (10 Stanwood, C. J. (1913). The olive-backed thrush (Hylocichla ustulata swainsoni) at his summer home. Wilson Bulletin 25:118-137. Close ).

Parental Behavior

Only female incubates (10 Stanwood, C. J. (1913). The olive-backed thrush (Hylocichla ustulata swainsoni) at his summer home. Wilson Bulletin 25:118-137. Close ). Female cautious while approaching nest and generally will not go if she detects an observer or is emitting a Whit Call. Approaches nest from near ground, moves up to nest limb, then quickly goes to nest; departs by flying down and away. Individuals vary in response to humans by flushing or sitting tight when incubating, but tend to sit very tight when eggs are hatching or nestlings are 1–2 d old (DEM). Female generally does not vocalize while on nest (D. Masingale pers. comm.). Male delivers food to female at nest (DEM), although frequency and extent to which female relies on male for food not reported. Male sings nearby throughout incubation period (10 Stanwood, C. J. (1913). The olive-backed thrush (Hylocichla ustulata swainsoni) at his summer home. Wilson Bulletin 25:118-137. Close , DEM). No information on rhythm or duration of female attentive periods.

Hardiness Of Eggs Against Temperature Stress; Effect Of Egg Neglect

No information.

Very little information. From 2 4-egg nests, 7 of 8 young were newly hatched in mid- to late morning (09:30–11:55; 10 Stanwood, C. J. (1913). The olive-backed thrush (Hylocichla ustulata swainsoni) at his summer home. Wilson Bulletin 25:118-137. Close ). In each nest, first 2 young hatched together, suggesting partial synchrony, but 3 of remaining 4 were approximately 24 h apart (48 h between first and last hatch). Egg-shells removed shortly after hatch, presumably by female, but no documentation.

Condition At Hatching

From Stanwood (10 Stanwood, C. J. (1913). The olive-backed thrush (Hylocichla ustulata swainsoni) at his summer home. Wilson Bulletin 25:118-137. Close ). Altricial, partially covered with natal down, eyes closed. Body length approximately 4 cm, mass 3.9 g, wing length 0.3 cm. Natal down dark, burnt amber, 1 cm long; beak, legs, and feet tinged burnt orange. Young lay limp in nest cup, ≥2 curled together “vibrating as one.”

Growth And Development

Few data. Observations of nests in Maine (10 Stanwood, C. J. (1913). The olive-backed thrush (Hylocichla ustulata swainsoni) at his summer home. Wilson Bulletin 25:118-137. Close ) found eyes beginning to open 2 d after hatching; feather tracts obvious by end of day 3; quills and pin-feathers evident by day 5; pin-feathers beginning to show along feather tracts; heads speckled by day 7; well feathered by end of day 10. Body length more than doubled in 10 d, increasing to 9.5 cm; wing length increased to 5.6 cm, mass increased 6-fold to >25 g. Moving in nest, panting, and soft vocalizations in response to food by day 4; preening by day 7; frequent movement in and on nest, including wing flaps, by end of day 10. Degree to which nestlings beg for food variable; noticeably audible in Maine and Vermont (10 Stanwood, C. J. (1913). The olive-backed thrush (Hylocichla ustulata swainsoni) at his summer home. Wilson Bulletin 25:118-137. Close , K. McFarland pers. comm.), but not recorded in Idaho (J. Bednarz pers. comm., DEM).

Brooding

Female broods closely for first 3–4 d, frequently grooming young (10 Stanwood, C. J. (1913). The olive-backed thrush (Hylocichla ustulata swainsoni) at his summer home. Wilson Bulletin 25:118-137. Close ). At approach of human observer, sits in low incubating position, entirely still but alert, and may remain even when observer brushes past nest (DEM). As young develop, female is off nest more frequently and for longer periods of time (10 Stanwood, C. J. (1913). The olive-backed thrush (Hylocichla ustulata swainsoni) at his summer home. Wilson Bulletin 25:118-137. Close ); no quantitative information on duration, rhythm, or relationship to weather. Will still brood young 10–11 d old (DEM).

Feeding

Both male and female share feeding responsibilities from hatch (10 Stanwood, C. J. (1913). The olive-backed thrush (Hylocichla ustulata swainsoni) at his summer home. Wilson Bulletin 25:118-137. Close ), although proportion of effort not quantified. Initially feed regurgitated food as well as fresh insects. In 3 h, adults made 20 visits to a brood of 4 young 3–6 d old (10 Stanwood, C. J. (1913). The olive-backed thrush (Hylocichla ustulata swainsoni) at his summer home. Wilson Bulletin 25:118-137. Close ). Daily feeding frequency (n = 2 nests, age unknown) calculated at 72 feedings/adult/d, based on each nestling being fed 48 times in 14 h, equating to 144 feedings to a brood of 3 (6 Beal, F. E. L. (1915a). Food habits of the thrushes of the United States. U.S. Dep. Agric. Bull. 280. Close ). Animal food, primarily caterpillars, beetles, moths, flies, and other bugs, made up higher percent of nestling diet (93%) than adult diet (63%); fruits (raspberries and blackberries) relatively small proportion.

Nest Sanitation

Female appears to eat or remove parasites from nest and bodies of young by grooming with bill. Both adults consume fecal sacs in early nestling period; begin to remove them from nest about 9 d after hatching (10 Stanwood, C. J. (1913). The olive-backed thrush (Hylocichla ustulata swainsoni) at his summer home. Wilson Bulletin 25:118-137. Close ).

Not known to occur.

Identity Of The Parasitic Species

Brown-headed Cowbird only documented brood parasite for this species.

Frequency Of Occurrence, Season Or Geographic Variation

No Brown-headed Cowbird eggs in 526 nests (348 of these from California) in the WFVZ collection spanning 1867–1970, and only 6 inci-dences of cowbird eggs from 98 records of CNRP dating from 1894 to 1996. Current parasitism rates vary geographically and may reflect the changing distribution of cowbirds in some parts of U.S. Declines in Swainson's Thrush populations along central and s. California coast (see Distribution: historical changes, above) have been attributed in part to cowbird brood parasitism (45 Garrett, K., and J. Dunn (1981). Birds of Southern California: Status and Distribution. Los Angeles Audubon Society, Los Angeles, CA, USA. Close ; L. Kiff, S. Laymon pers. comm.), although 0 of 39 nests monitored in 1995–1996 at Point Reyes National Seashore, CA, contained cowbird eggs (154 Halterman, M. D. and S. A. Laymon. (1997). The effects of Brown-headed Cowbird parasitism on Neotropical migrants in Point Reyes National Seashore. Weldon, CA: Kern River Res. Center. Close ), and only 2 of 65 nests (3%) were parasitized across central Californian riparian sites in 1997 (S. Small, G. R. Geupel, N. Nur, A. L. Holmes, and T. Gardali unpubl.). Rates were slightly higher (8%; n = 38) at 2 drainages in Golden Gate National Recreation Area (149 Gardali, T., S. E. Scoggin and G. R. Geupel. (1999). Songbird use of Redwood and Lagunitas creeks: management and restoration recommendations. Stinson Beach, CA: Point Reyes Bird Observatory. Close ). Similar rate (8%) reported from a large sample in British Columbia (n = 395), with little difference between inland and coastal regions (42 Campbell, R. W., N. K. Dawe, I. McTaggart-Cowan, J. M. Cooper, G. W. Kaiser, M. C. E. McNall, and G. E. J. Smith (1997). The Birds of British Columbia. Volume 3. Passerines: Flycatchers Through Vireos. University of British Columbia Press, Vancouver, BC, Canada. Close ). Of 110 nests with known outcome monitored in w.-central Idaho from 1994 to 1998, only 1 was parasitized by cowbirds (96 Evans, D. M., J. C. Bednarz, C. R. Davis and J. C. Hovis. (1998). The effects of forest landscape modification and management on Neotropical migratory songbird populations in west-central Idaho. State University: Arkansas State Univ. Close ). No parasitism in a small sample (n = 7) of nests in se. Alaska (and in 42 nests of 3 other species of open-cup nesters in same area; 148 Rogers, C. M. (1994a). Avian nest success, brood parasitism and edge-independent reproduction in an Alaskan wetland. Journal of Field Ornithology 65:433-586. Close ), and no parasitism documented from 25 nests in New York and Vermont forests during 1996–1997 (K. McFarland pers. comm.).

Timing Of Laying In Relation To Host'S Laying

Brown-headed Cowbird egg(s) observed laid after Swainson's Thrush egg 1, 2, and 3 (WFVZ, CNRP).

Response To Parasitic Mother, Eggs, Or Nestlings

No reported interaction with Brown-headed Cowbird female. Limited data provide no pattern of response, which included continued incubation of all eggs but successfully raising only 1 cowbird (n = 3); abandonment after cowbird egg laid (n = 3); and removing the egg and continuing incubation and brooding (n = 1; CNRP, J. Smith pers. comm.). Up to 3 cowbird eggs reported from a single nest.

Effects Of Parasitism On Host

Despite egg loss from individual nests, data do not suggest that parasitism is suppressing populations through decreased reproductive success, even in California; declines in Sierra Nevada preceded cowbird presence there (R. Stefani pers. comm.). Across range, more nests fail from causes other than parasitism: 14% of 36 nests with known outcome in CNRP database failed due to Brown-headed Cowbirds, while 42% failed from predation and 31% successfully fledged at least 1 Swainson's Thrush. Failure rates approached 60% in Idaho, none due to parasitism.

Success Of Parasite With This Host

Limited data suggest that Brown-headed Cowbirds have low success with this species, based on relatively low rates of parasitism and high rates of failure of parasitized (and all) nests (see above).

Few data. Leaves nest after 10–14 d; fledged at 10 d at a nest where young were removed and replaced daily and parents agitated, 12–14 d at undisturbed nests (10 Stanwood, C. J. (1913). The olive-backed thrush (Hylocichla ustulata swainsoni) at his summer home. Wilson Bulletin 25:118-137. Close , 42 Campbell, R. W., N. K. Dawe, I. McTaggart-Cowan, J. M. Cooper, G. W. Kaiser, M. C. E. McNall, and G. E. J. Smith (1997). The Birds of British Columbia. Volume 3. Passerines: Flycatchers Through Vireos. University of British Columbia Press, Vancouver, BC, Canada. Close , 96 Evans, D. M., J. C. Bednarz, C. R. Davis and J. C. Hovis. (1998). The effects of forest landscape modification and management on Neotropical migratory songbird populations in west-central Idaho. State University: Arkansas State Univ. Close ). Active in nest prior to fledging, with frequent stretching and preening, hopping from nest cup to rim (10 Stanwood, C. J. (1913). The olive-backed thrush (Hylocichla ustulata swainsoni) at his summer home. Wilson Bulletin 25:118-137. Close ). Fledging event not reported; role of parents in triggering departure from nest (vocal encouragement, food enticement, withholding food) unreported, although at 1 frequently disturbed nest, parents called repeatedly to young after they were replaced in nest, and nest was empty 1 h later (10 Stanwood, C. J. (1913). The olive-backed thrush (Hylocichla ustulata swainsoni) at his summer home. Wilson Bulletin 25:118-137. Close ). Departs nest in Juvenile plumage; capable of standing, perching, and short, fluttery flight. Fledglings remain close to nest tree for 1–2 d, begin to wander subsequently (DEM). One fledgling observed 300 m from nest 5 d postfledging at a site in Idaho, being fed by a banded parent (J. Hovis pers. comm.). Duration of dependence on and association with parents unknown; 21–31 d for Wood Thrush (35 Roth, R.R., M. S. Johnson, and T. J. Underwood. 1996. Wood Thrush (Hylocichla mustelina). In The Birds of North America, No. 246 (A. Poole and F. Gill, eds.). The Academy of Natural Sciences, Philadelphia, PA, and The American Ornithologists' Union, Washington D.C. Close ).

No information.