Swainson's Thrush Research Articles

JUVENILE AND ADULT SURVIVAL OF SWAINSON'S THRUSH (CATHARUS USTULATUS) IN COASTAL CALIFORNIA: ANNUAL ESTIMATES USING CAPTURE-RECAPTURE ANALYSES

Abstract We estimated annual rates of survival for juvenile and adult Swainson's Thrushes (Catharus ustulatus) using capture-recapture analyses from 22 years of mist-neting data in coastal California (n= 2,651 individual captures). Our apparent survival estimate was 56% for adults and 25% for juveniles. We are the first to estimate an annual juvenile survival rate for a Neotropical migrant using capture-recapture probability estimates. Like most estimates of annual survival, we could not distinguish between dispersal away from our study area (which is likely high for juveniles) and mortality. Hence, survival is underestimated. However, our juvenile survival estimate did not include the period from fledging to independence, a time when mortality can be high. Many researchers have assumed juvenile survival to be half that of adult survival in population models (e.g. source-sink). Our juvenile to adult survival ratio was 45% (95% CI = 27 to 65%). We caution researchers from simply assuming that juvenile surv...

Abundance and Richness of Neotropical Migrants During Stopover at Farmstead Woodlots and Associated Habitats in Southeastern South Dakota

Woodland habitats are scarce in the northern Great Plains and were historically concentrated along river corridors. Over the past century, riparian habitats in this area have been much reduced, but new woodland habitats in the form of farmstead woodlots and shelterbelts have appeared. We used mist net sampling and point counts to document richness and abundance of Neotropical migrant birds in farmstead woodlot habitats during spring and fall migrations (1996–1997) in southeastern South Dakota. A total of 668 individuals of 30 Neotropical migrant species (excluding the taxa Coccyzus, Troglodytes, Mimidae, Icterus and Pheucticus, in which migratory and non-migratory individuals were difficult to distinguish) was captured in 4342 net hours (using 9-m, rather than the standard 12-m mist nets) in spring. The corresponding fall totals (again using 9-m nets) were 3250 net h, 231 individuals and 29 species. If fall captures in a ragweed (Ambrosia trifida) patch occurring within the woodlot are included, however, the fall totals were 5107 net h, 1211 individuals and 29 species. Overall densities of Neotropical migrants from point counts were 1302 birds · km−2 in spring and 898 birds · km−2 in fall. Capture and point count data followed similar phenologies, with peak abundance during mid-May in spring and late August-early September in fall. Both methods indicated seasonal abundance differences for some species, with Swainson's thrush (Catharus ustulatus) and blackpoll warbler (Dendroica striata) more abundant in spring. Orange-crowned (Vermivora celata), Nashville (V. ruficapilla) and Wilson's (Wilsonia pusilla) warblers were more abundant in fall. Captures within the woodlot were evenly distributed among different microhabitats during spring migration, but fall captures occurred disproportionately in scrubby edge-related microhabitats, especially in ragweed, suggesting that seasonal shifts in microhabitat selection may occur within woodlots. Density and capture rate data were similar to previously reported values for riparian habitats in this area. Thus, a diverse assemblage of Neotropical migrants occurs in woodlots during migration, suggesting that woodlots are regularly used as stopover sites and supplement available natural woodland habitats along river corridors.

A CROSS-CANADA COMPARISON OF MASS CHANGE IN BIRDS DURING MIGRATION STOPOVER

I estimated hourly mass change at stopover sites for 14 species of migrant passerines from 15 sites across southern Canada by analyzing size-corrected mass of birds at first capture as a function of time of day of handling. Mean mass gains were 0.40% of lean body mass/h during spring and 0.53% during fall. Mass gain estimates varied significantly with season, site, and species, and were negatively related to condition of birds in the early morning. However, standard errors were large, such that few individual estimates were sig- nificantly different. Several sites with consistently low rates of mass gain had characteristics that probably reduced local food supply. Swainson's Thrushes ( Catharus ustulatus) also had consistently low rates of mass gain. I estimated the time required to accumulate sufficient mass to fuel a 10-h migratory flight, and found that the majority of estimated mass gains were sufficient for birds to refuel during ,1 week of stopover in southern Canada. At mean rates of mass gain from this study, migrants in southern Canada could potentially refuel completely during 2-3 days in both seasons, but true periods are likely somewhat longer. Analysis of mass change along migration routes (instead of across them, as in this study) is needed to detect whether there are differences among species in timing and location of maximum fuel deposition, as has been found in Europe. Received 2 November 2001, accepted 30 June 2002. Between migratory flights, birds must re- plenish energy stores in order to successfully complete their journeys, and the rate at which birds change mass during stopover should be an index of site quality. This measure reflects food abundance as well as incorporating ef- fects of weather conditions, levels of compe- tition and predator harassment, and other ex- ternal factors that could affect mass change. However, endogenous factors also affect fat- tening rates, and must be taken into account when interpreting mass change with respect to site quality. For example, birds with optimal fat stores should maintain mass rather than gain more, and optimal fuel loads may vary according to proximity to the final destination or to large geographic barriers such as the Gulf of Mexico. Finally, there may be differ- ences in migratory strategy among species, such as speed of migration or length of mi- gratory flights, that also could affect mass change patterns. Study of mass change at many sites across a large geographic area may help to tease these factors apart. The only example of such a study to date on nocturnally migrating pas- serines involved six species captured at 34 sites distributed from northern Europe to North Africa (Schaub and Jenni 2000). Re-

Songbird abundance in clear-cut and burned stands: a comparison of natural disturbance and forest management

To evaluate the efficacy of forest management to emulate natural disturbance, we compared bird abundances among burned and clear-cut, former black spruce (Picea mariana (Mill.) BSP) sites, after 5, 14, and 27 years of succession. Total bird density was lower in clear-cut sites resulting from fewer hermit thrushes, yellow warblers, Swainson's thrushes, and fox sparrows. Hermit thrushes were positively correlated with snag density while yellow warblers and Swainson's thrushes were positively associated with deciduous tree cover and negatively correlated with conifer cover. Only yellow-rumped warblers had higher densities on clear-cut sites, likely due to greater conifer cover. Bird densities and species richness peaked in the 14-year-old burns and exceeded that of mature forests reported for Labrador. This demonstrates the importance of natural early successional forests for birds. Although logged areas support several species found in natural young burns, logging does not precisely mimic fire. This suggests that forest managers should allow some forests to burn naturally.

Not as the crow flies: a historical explanation for circuitous migration in Swainson's thrush (Catharus ustulatus).

Many migratory songbirds follow circuitous migratory routes instead of taking the shortest path between overwintering and breeding areas. Here, we study the migration patterns in Swainson's thrush (Catharus ustulatus), a neartic-neotropical migrant songbird, using molecular genetic approaches. This species is presently separated into genetically distinct coastal and continental populations that diverged during the Late Pleistocene (as indicated by molecular dating), yet appear to have retained ancestral patterns of migration. Low nucleotide diversity, a star-like haplotype phylogeny and unimodal mismatch distributions all support the hypothesis that both the coastal and the continental populations have undergone recent demographic expansions. Nearctic-neotropical banding and genetic data show nearly complete segregation of migratory routes and of overwintering locations: coastal populations migrate along the Pacific Coast to overwintering sites in Central America and Mexico, whereas continental populations migrate along an eastern route to overwintering sites in Panama and South America. Nearctic-neotropical banding data also show that continental birds north, northwest and east of this migratory divide fly thousands of miles east before turning south. We conclude that circuitous migration in the Swainson's thrush is an artefact of a Late Pleistocene range expansion.

Effects of Thinning and Herbicide Treatments on Nest-Site Selection by Songbirds in Young Managed Forests

AbstractRoutine silvicultural practices continue to alter the structure and composition of forests after logging by removing deciduous vegetation from regenerating coniferous forests. We identified nest trees and surveyed vegetation in a 5 m radius surrounding songbird nests (nest patch) and compared the nest patches to available habitat in nine 11–22 year old conifer plantations (22–47 ha) where 90–96% of deciduous stems were removed by two treatments: manual thinning, and manual thinning plus application of glyphosate (herbicide). The control and two treatments were replicated three times. We characterized the nest patches of five species: Warbling Vireo (Vireo gilvus), Dusky Flycatcher (Empidonax oberholseri), Swainson's Thrush (Catharus ustulatus), American Robin (Turdus migratorius), and Chipping Sparrow (Spizella passerina). During three post-treatment years, areas treated with thinning plus herbicide remained depauperate of deciduous vegetation whereas thinned sites experienced deciduous regrowth. Despite variation in the density of deciduous trees and in the type of tree used for nesting in the control and treatments, nest patches were positively correlated with the amount of remaining deciduous vegetation, representing habitat that either escaped or recovered from silvicultural treatments. That relationship was stronger in areas with the fewest deciduous trees. Nests were more likely to be successful in areas with more willow. Within a mosaic of managed forest stands, birds appear to use the same proximate habitat cues for selecting a nest patch despite varying fitness consequences across different silviculture regimes. Although birds appeared to compensate for changes in stand habitat by finding patches of untreated vegetation or altering the type of tree they nested in, there was a reproductive cost for some species.

Orientation of Nocturnally Migrating Swainson's Thrush at Dawn and Dusk: Importance of Energetic Condition and Geomagnetic Cues

AbstractWe studied the early morning cage orientation of nocturnally migrating Swainson's Thrushes (Catharus ustulatus) during three fall migration seasons. The results were compared with earlier free-flight release tests under starry skies and were found to be consistent with continuation of migratory flights in the expected seasonally appropriate direction. Energetic condition proved decisive: fat birds chose directions in accordance with migration across the Gulf of Mexico, whereas lean birds oriented away from the coast, possibly in search of habitats suitable for refuelling. Whereas the orientation of fat Swainson's Thrushes was affected by experimental shifts of the magnetic field, the response during morning tests was larger than expected. A parallel series of orientation cage experiments performed during evening twilight showed a response to deflected magnetic fields that was close to the expected shift, which suggests a difference in integration of directional information between early morning and evening twilight activity. However, within-individual response to deflected magnetic fields was of the same magnitude during both morning and evening tests.

ORIENTATION OF NOCTURNALLY MIGRATING SWAINSON'S THRUSH AT DAWN AND DUSK: IMPORTANCE OF ENERGETIC CONDITION AND GEOMAGNETIC CUES

We studied the early morning cage orientation of nocturnally migrating Swainson's Thrushes (Catharus ustulatus) during three fall migration seasons. The results were compared with earlier free-flight release tests under starry skies and were found to be consistent with continuation of migratory flights in the expected seasonally appropriate direction. Energetic condition proved decisive: fat birds chose directions in accordance with migration across the Gulf of Mexico, whereas lean birds oriented away from the coast, possibly in search of habitats suitable for refuelling. Whereas the orientation of fat Swainson's Thrushes was affected by experimental shifts of the magnetic field, the response during morning tests was larger than expected. A parallel series of orientation cage experiments performed during evening twilight showed a response to deflected magnetic fields that was close to the expected shift, which suggests a difference in integration of directional information between early morning and evening twilight activity. However, within-individual response to deflected magnetic fields was of the same magnitude during both morning and evening tests.

EFFECTS OF THINNING AND HERBICIDE TREATMENTS ON NEST-SITE SELECTION BY SONGBIRDS IN YOUNG MANAGED FORESTS

Abstract Routine silvicultural practices continue to alter the structure and composition of forests after logging by removing deciduous vegetation from regenerating coniferous forests. We identified nest trees and surveyed vegetation in a 5 m radius surrounding songbird nests (nest patch) and compared the nest patches to available habitat in nine 11–22 year old conifer plantations (22–47 ha) where 90–96% of deciduous stems were removed by two treatments: manual thinning, and manual thinning plus application of glyphosate (herbicide). The control and two treatments were replicated three times. We characterized the nest patches of five species: Warbling Vireo (Vireo gilvus), Dusky Flycatcher (Empidonax oberholseri), Swainson's Thrush (Catharus ustulatus), American Robin (Turdus migratorius), and Chipping Sparrow (Spizella passerina). During three post-treatment years, areas treated with thinning plus herbicide remained depauperate of deciduous vegetation whereas thinned sites experienced deciduous regrowth....

Orientation of Nocturnally Migrating Swainson's Thrush at Dawn and Dusk: Importance of Energetic Condition and Geomagnetic Cues

Orientation of Nocturnally Migrating Swainson's Thrush at Dawn and Dusk: Importance of Energetic Condition and Geomagnetic Cues

Response of Passerine Birds to Forest Edge in Coast Redwood Forest Fragments

Abstract The response of passerine birds to forest edge was examined in old-growth and mature second-growth coast redwood (Sequoia sempivirens) forest in northern California. The study objectives were to determine which common passerine species are sensitive to edges during the breeding season and to estimate edge width for forest interior species. Response to edge was measured along twelve 100 × 400 m plots extending from the edge into the forest interior to obtain relative density of birds. Plots were surveyed 4 to 5 times in 1996 and 8 to10 times in 1997. We found that 14 common passerines showed a gradient of edge sensitivity. Steller's Jay (Cyanocitta stelleri) and Swainson's Thrush (Catharus ustulatus) had higher relative densities near edges than in the forest interior (P < 0.05) and were categorized as edge species. Brown Creeper (Certhia americana), Winter Wren (Troglodytes troglodytes), Pacific-slope Flycatcher (Empidonax difficilis), and Varied Thrush (Ixoreus naevius) had lower relative densities near edges (P < 0.05) and were categorized as interior birds. Based on exponential regression models, estimated edge widths were 140 m for Varied Thrushes, 85 m for Brown Creepers, 120 m for Winter Wrens, and 125 m for Pacific-slope Flycatchers. Creation of edges would probably benefit Steller's Jays (which may be a nest predator), may not benefit Swainson's Thrushes, and may be detrimental to species that avoid edges. We recommend that edge effects be taken into consideration when planning for the conservation of bird species in the region.

Birds disperse ixodid (Acari: Ixodidae) and Borrelia burgdorferi-infected ticks in Canada.

A total of 152 ixodid ticks (Acari: Ixodidae) consisting of nine species was collected from 82 passerine birds (33 species) in 14 locations in Canada from 1996 to 2000. The Lyme disease spirochete Borrelia burgdorferi Johnson, Schmidt, Hyde, Steigerwaldt & Brenner was cultured from the nymph of a blacklegged tick, Ixodes scapularis Say, that had been removed from a common yellowthroat, Geothlypis trichas L., from Bon Portage Island, Nova Scotia. As a result of bird movement, a nymphal I. scapularis removed from a Swainson's thrush, Catharus ustulatus incanus (Godfrey), at Slave Lake, Alberta, during spring migration becomes the new, most western and northern record of this tick species in Canada. Amblyomma longirostre Koch, Amblyomma sabanerae Stoll, and Ixodes baergi Cooley & Kohls are reported for the first time in Canada. Similarly, Amblyomma americanum L., Arnblyomma maculatum Koch, and ixodes muris Bishopp & Smith are reported for the first time on birds in Canada. After removal of an I. muris gravid female from a song sparrow, Melospiza melodia Wilson, at St. Andrews, New Brunswick, eggs were laid, which developed into larvae, and this new tick-host record demonstrates that birds have the potential to start a new tick population. We conclude that passerine birds disperse several species of ixodid ticks in Canada, and during spring migration translocate ticks from the United States, and Central and South America, some of which are infected with B. burgdorferi.

A stable-isotope approach to delineate geographical catchment areas of avian migration monitoring stations in North America.

Migration monitoring stations (MMSs) were established to provide information on population trends of North American passerines. However, apart from inferring general origins of birds, there has been no way to delineate geographical catchment areas sampled by MMSs. The ability to resolve MMS catchment areas would greatly enhance our ability to link and constrain population declines to specific geographical areas and thereby focus conservation efforts. Here, we utilize stable-hydrogen (delta D) and carbon (delta 13C) isotope values of rectrices of fall hatch-year (HY) and spring adult after-second year (ASY) Swainson's Thrush (Catharus ustulatus) moving through two MMSs in Canada to determine natal and molting catchment areas for those stations. Stable-carbon isotope signatures provided no information on geographical origins of birds. Conversely, delta D signatures provided good latitudinal geographical control, and this was also supported by analysis of feathers from two other species, Gray-cheeked Thrush (Catharus minimus) and Harris' Sparrow (Zonotrichia querula), also known to breed and molt in northern regions. The delta D values of fall HY birds moving through Delta Marsh MMSs in Manitoba were more negative than those moving through Long Point MMSs in southern Ontario, clearly indicating more northwesterly origins. However, based on delta D values alone, both MMSs sampled birds from broad catchment regions of the Canadian Boreal forest. The interpolated catchment area of the Delta MMS encompassed northwest Manitoba to northwestern Alberta. The interpolated catchment area of the Long Point MMS extended from north central Ontario and Quebec and into western Canada. The delta D values of birds moving northward in spring suggested that most ASY Swainson's Thrush molted at more southerly locations than their breeding sites. We show the stable-hydrogen isotope approach may be employed to significantly improve conventional observational techniques for avian population monitoring, and that MMSs provide a reliable means of associating population productivity with regional conservation issues.

BICKNELL'S THRUSHES (CATHARUS BICKNELLI) IN NEW BRUNSWICK CLEAR CUTS: THEIR HABITAT ASSOCIATIONS AND CO-OCCURRENCE WITH SWAINSON'S THRUSHES (CATHARUS USTULATUS)

We studied the use of regenerating clear cuts in the central highlands of New Brunswick by Bicknell's (Catharus bicknelli) and Swainson's (Catharus ustulatus) thrushes during the 1997 breeding season. Bicknell's Thrushes were found at 57 of the 90 points surveyed on six mountains. Swainson's Thrushes occupied 89 of 90 survey points. Bicknell's Thrushes had a lower probability of detection (P = 0.40) than Swainson's Thrushes (P = 0.76). Fixed-radius point counts yielded breeding density estimates for birds on individual mountains ranging from about 3–22 pairs/100 ha (average 16 pairs/100 ha) for Bicknell's Thrushes, and from about 39–69 pairs/100 ha (average 51 pairs/100 ha) for Swainson's Thrushes. White birch (Betula papyrifera), balsam fir (Abies balsamea), and pin cherry (Prunus pensylvanica) were the dominant tree species on regenerating sites. Points occupied by Bicknell's Thrushes tended to be at higher elevations than unoccupied points and to have a greater proportion of white birch stems, a greater number of stems in the 5–10 cm diameter class and a lower diversity of trees and shrubs. No attempt was made to characterize Swainson's Thrush habitat since this species was present at virtually all survey points. We believe that relatively young (average height about 4 m) mixed stands of regeneration with moderate to high stem densities provide important habitat for Bicknell's Thrushes in New Brunswick. Suggestions are made about the direction of future research needed to formulate habitat management and conservation plans for this species.

Ticks (Acari: Ixodidae) and spirochetes (spirochaetaceae: spirochaetales) recovered from birds on a Georgia Barrier Island.

From September 1997 through July 1999, 300 individuals and 46 species of birds were mist-netted and screened for ticks and spirochetes on St. Catherine's Island, Liberty County, GA. Seventy-six (25%) of the birds were parasitized by a meal intensity of 4.6 ticks. Seasonally, more birds were infested with ticks during the summer (50% in 1998, 34% in 1999) than in spring (15% in 1998, 11% in 1999) or fall (21% in 1997, 20% in 1998), mainly because of severe infestations on some birds by immature stages of the lone star tick, Amblyomma americanum (L.), during this season. Eight species ofticks were recovered from 14 species of birds during this study: A. americanum (74 nymphs, 168 larvae); the blacklegged tick, Ixodes scapularis Say (11 nymphs, 28 larvae), the Gulf Coast tick, Amblyomma maculatum Koch (two nymphs, 29 larvae); Ixodes minor Neumann (16 larvae); the rabbit tick. Haemaphysalis leporispalustris (Packard) (one nymph, 14 larvae); the bird tick Ixodes brunneus Koch (two larvae); the American dog tick, Dermacentor variabilis (Say) (one nymph); and Ixodes affinis Neumann (one larva). The Carolina wren was parasitized by more species of ticks (seven) than any other bird species, followed by the northern cardinal (five), white-throated sparrow (four) and painted bunting (three). Spirochetes were isolated in BSK II medium from one tick (a nymphal A. americanum) and from skin biopsies of 12 (4%) of the individual birds (three downy woodpeckers, three northern waterthrushes, two Carolina wrens, one American redstart, one pine warbler, one Swainson's thrush, and one white-eyed vireo) all in fall 1997. This concentrated phenology of spirochete isolations might reflect periodic amplification or recrudescence of spirochetes in reservoir avian hosts.

Linking Breeding and Wintering Grounds of Bicknell's Thrushes Using Stable Isotope Analyses of Feathers

AbstractPrevious studies have shown that natural abundance of deuterium (D), and to a lesser extent 13C, in feathers of migrant songbirds in North America can be used to infer geographic origins of molt. We used that approach to investigate whether Bicknell's Thrush (Catharus bicknelli) populations could be segregated on the breeding grounds at six sites in eastern North America to evaluate contributions from the breeding areas to wintering populations. Further, we tested our isotopic model using feathers from hatching-year migrant Swainson's Thrush (Catharus ustulatus) and Gray-cheeked Thrush (Catharus minimus) moving through southern Manitoba and Ontario and found that δD values were consistent with northerly boreal forest natal sites in northwestern and central Canada. Despite a strong latitudinal gradient in δD of average growing season precipitation over the breeding range, we found considerable overlap in δD values of Bicknell's Thrush among all breeding sites. No strong pattern in δ13C values of feathers with latitude was apparent. Nevertheless, the more negative δD values of feathers from wintering birds in the Dominican Republic compared to breeding area samples suggested that birds from more northerly locations in North America contributed to this population of wintering birds. High variance in feather δD values within breeding populations also suggested high natal dispersal, considerable movement of birds among island habitats on the breeding grounds, or both. Even higher variances in feather δD values from small study areas on the wintering grounds, compared to similar-sized areas in the breeding range, suggest that many different breeding populations mix in winter habitat.

Interspecific variability of prevalence in blood parasites of adult passerine birds during the breeding season in Alaska.

Blood parasite prevalence based on microscopic examination of stained blood smears was determined in adults of 11 passerine bird species sampled during their breeding season (May and June 1997-98) in interior Alaska (USA). These species included primarily Nearctic migratory species such as the dark-eyed junco (Junco hyemalis) and neotropical migratory species such as the blackpoll warbler (Dendroica striata), alder flycatcher (Empidonax alnorum), Swainson's thrush (Catharus ustulatus), northern waterthrush (Seiurus noveboracensis), and bank swallow (Riparia riparia) as well as one long-distance palearctic migrant, the arctic warbler (Phylloscopus borealis). The more prevalent parasites were Leucocytozoon dubreuili (73% of the sampled turdinids), L. fringillinarum (42% of the sampled fringillids and parulids), and Trypanosoma avium (39% of the sampled hosts). Other parasites (H. fallisi: 18% of the sampled turdinids; Haemoproteus paruli: 14% of the sampled parulids; H. fringillae: 5% of the sampled fringillids; microfilariae: 4% of the sampled hosts) were observed less frequently. Plasmodium vaughani was found only in two yellow warblers (Dendroica petechia). Overall parasite prevalence varied between 0% in the alder flycatcher to >80% in Swainson's thrush, arctic warbler, and Townsend's warbler (Dendroica townsendi). Prevalence of various hematozoa also was bird species-dependent. No relationship was observed between prevalence and either foraging (aerial versus trees/shrubs) or nesting habits (ground versus arboreal) or general location of the wintering area of the different species examined. Prevalence also was unrelated to average dates of arrival on breeding grounds and, therefore, to potential duration of exposure to local insect vectors before capture. Differences in blood parasite prevalence among species breeding in a same region and in the same type of habitat may result from differences in host specificity such as immunological resistance to infection or blood meal preference by potential vectors and/or in behavioral adjustments/physiological traits that alter exposure to vectors.

RESPONSE OF PASSERINE BIRDS TO FOREST EDGE IN COAST REDWOOD FOREST FRAGMENTS

Abstract The response of passerine birds to forest edge was examined in old-growth and mature second-growth coast redwood (Sequoia sempivirens) forest in northern California. The study objectives were to determine which common passerine species are sensitive to edges during the breeding season and to estimate edge width for forest interior species. Response to edge was measured along twelve 100 × 400 m plots extending from the edge into the forest interior to obtain relative density of birds. Plots were surveyed 4 to 5 times in 1996 and 8 to10 times in 1997. We found that 14 common passerines showed a gradient of edge sensitivity. Steller's Jay (Cyanocitta stelleri) and Swainson's Thrush (Catharus ustulatus) had higher relative densities near edges than in the forest interior (P < 0.05) and were categorized as edge species. Brown Creeper (Certhia americana), Winter Wren (Troglodytes troglodytes), Pacific-slope Flycatcher (Empidonax difficilis), and Varied Thrush (Ixoreus naevius) had lower relative densit...

LINKING BREEDING AND WINTERING GROUNDS OF BICKNELL'S THRUSHES USING STABLE ISOTOPE ANALYSES OF FEATHERS

Abstract Previous studies have shown that natural abundance of deuterium (D), and to a lesser extent 13C, in feathers of migrant songbirds in North America can be used to infer geographic origins of molt. We used that approach to investigate whether Bicknell's Thrush (Catharus bicknelli) populations could be segregated on the breeding grounds at six sites in eastern North America to evaluate contributions from the breeding areas to wintering populations. Further, we tested our isotopic model using feathers from hatching-year migrant Swainson's Thrush (Catharus ustulatus) and Gray-cheeked Thrush (Catharus minimus) moving through southern Manitoba and Ontario and found that δD values were consistent with northerly boreal forest natal sites in northwestern and central Canada. Despite a strong latitudinal gradient in δD of average growing season precipitation over the breeding range, we found considerable overlap in δD values of Bicknell's Thrush among all breeding sites. No strong pattern in δ13C values of f...

Breeding Bird Communities in Boreal Forest of Western Canada: Consequences of “Unmixing” the Mixedwoods

Abstract Silvicultural practices following clearcutting in boreal forest may encourage the creation of monospecific, single-aged stands having less vegetation heterogeneity and diversity than original stands. We conducted point counts in central Saskatchewan, Canada, 1993–1995, in pure and mixedwood stands dominated by black spruce (Picea mariana), jackpine (Pinus banksiana), trembling aspen (Populus tremuloides), or white spruce (Picea glauca). Mixedwood stands supported more individuals and more species than pure stands. Higher abundance in mixedwood stands relative to pure stands was consistent among nesting guilds and migration strategies. Rarefaction revealed similar patterns, although pure trembling aspen stands were predicted to support more species than aspen-dominated mixedwood stands. Increased avian diversity in mixedwood stands was not solely the result of the mixing of bird species associated with coniferous or deciduous forest types. Chipping Sparrow (Spizella passerina), Pine Siskin (Carduelis pinus), White-winged Crossbill (Loxia leucoptera), Red-breasted Nuthatch (Sitta canadensis), Swainson's Thrush (Catharus ustulatus), and Tennessee Warbler (Vermivora peregrina) were more abundant in mixedwood stands than pure stands. Black-throated Green Warbler (Dendroica virens), Magnolia Warbler (D. magnolia), and Blackburnian Warbler (D. fusca) were abundant in stands dominated by white spruce but were absent from jackpine or black spruce. Other species such as American Redstart (Setophaga ruticilla) and Chestnut-sided Warbler (D. pensylvanica) relied exclusively on pure trembling aspen, particularly stands with dense shrub cover. Several bird species in the boreal forest will be adversely affected by forestry practices that target mature to old aspen and white spruce mixedwoods and promote reduction in mixedwood compositions of regenerating stands.