Varied Thrush Research Articles

Drivers of winter population cycles in the Varied Thrush (Ixoreus naevius)

The drivers of year-to-year difference in winter abundance patterns, particularly dramatic in the “eruptions” of many boreal seed-eating birds, are poorly understood. Varied Thrush ( Ixoreus naevius (Gmelin, 1789)), endemic to the Pacific Northwest of North America, is a boreal species that exhibits pronounced, often biennially cyclic, changes in winter abundance within most of its normal wintering range. Although the drivers of this variability have not previously been explored, it has been suggested that differences in acorn abundance, a key winter food resource, might be important. Here, we examine three hypotheses for the drivers of this pattern: the acorn crop within the bird's normal winter range, weather within the bird's winter range, and weather during the previous breeding season within the bird's breeding range. Analyses supported the importance of breeding season conditions, particularly breeding season rainfall, with more birds wintering following wetter years. No support was found for the hypotheses that winter conditions, neither the acorn crop nor winter weather, correlate with winter abundance patterns. For this forest species, year-to-year differences in winter abundance patterns are apparently not driven by the “pull” of winter food supply or winter conditions, but by environmental factors during the prior breeding season that presumably affect reproductive success and subsequent population size.

Year-round monitoring at a Pacific coastal campus reveals similar winter and spring collision mortality and high vulnerability of the Varied Thrush

Abstract Bird–window collisions are a leading cause of direct anthropogenic avian mortality, yet our state of knowledge regarding this threat relies heavily on eastern North American studies. Seasonal patterns of collision mortality may differ along the Pacific coast, and western North American species remain understudied. We therefore surveyed a stratified random sample of 8 buildings for collisions at the University of British Columbia, Vancouver, Canada over 45-day periods during 2 winters, 1 spring, 1 summer, and 1 fall season between January 22, 2015 and March 15, 2017. After accounting for the rate of scavenging and efficiency of observers in finding carcasses, we estimated that 360 collision fatalities (95% CI: 281–486) occurred over 225 days of collision monitoring. Collision mortality was highest in fall, but in contrast to most published research, collision mortality was intermediate in both winter and spring and was lowest in summer. In winter 2017, we performed point-count surveys to assess whether individual species are disproportionately vulnerable to collisions when accounting for population size and found that the Varied Thrush (Ixoreus naevius) was 76.9 times more likely to collide with buildings, relative to average species vulnerability in winter. To our knowledge, this is the first study to report the Varied Thrush as a species that is disproportionately vulnerable to collisions. Further studies are needed to assess the vulnerability of Western North American species and subspecies, and to determine whether similar patterns of seasonal collision mortality are found elsewhere.

Biotic factors influencing the unexpected distribution of a Humboldt marten (Martes caurina humboldtensis) population in a young coastal forest.

Pacific martens (Martes caurina) are often associated with mature forests with complex structure for denning, resting, and efficient hunting. Nonetheless, a small isolated population of the Humboldt subspecies of Pacific martens (Martes caurina humboldtensis) occupies a narrow strip of young, coastal forest (< 70 years old) but not inland mature forest in the central Oregon Coast Range. We examined factors contributing to this unexpected distribution of martens by 1) analyzing marten diets using DNA metabarcoding to assess 90 scats, 2) using camera traps to assess differences in the relative abundances of prey, competitors, and predators across a coastal to inland gradient of vegetation types, and 3) quantifying differences in extent of fruit-producing shrubs and vegetation structure within vegetation types. Diets of martens were diverse (12, 10, and 3 species of birds, mammals, and amphibians respectively), and most fall and winter scats contained fruit. Voles, mice, and varied thrushes (Ixoreus naevius) were dominant prey items. Voles, mice, and most birds, but not varied thrushes, were more commonly observed in the coastal shrub-dominated forest than in inland forest. The coastal shrub-dominated forest had the highest diversity of vertebrates and potential prey overall. Bobcats (Lynx rufus), a key potential predator, were more commonly detected in inland forest. Of potential competitors, western spotted skunks (Spilogale gracilis) were more commonly detected in inland forest, with gray foxes (Urocyon cinereoargenteus) and raccoons (Procyon lotor) detected almost exclusively in coastal forests. Vegetation in coastal forests appears to provide, at least seasonally, more prey and fruit, and more overhead shrub cover compared with inland forest. Remaining plausible hypotheses for the restricted distribution of marten to coastal forests include increased prey, fruit, and overhead cover, and reduced predation risk from bobcats.

Using bioacoustics to examine shifts in songbird phenology.

Monitoring patterns in biodiversity and phenology have become increasingly important given accelerating levels of anthropogenic change. Long‐term monitoring programs have reported earlier occurrence of spring activity, reflecting species response to climate change. Although tracking shifts in spring migration represents a valuable approach to monitoring community‐level consequences of climate change, robust long‐term observations are challenging and costly. Audio recordings and metrics of bioacoustic activity could provide an effective method for monitoring changes in songbird activity and broader biotic interactions. We used 3 years of spring and fall recordings at six sites in Glacier Bay National Park, Alaska, an area experiencing rapid warming and glacial retreat, to examine the utility of bioacoustics to detect changes in songbird phenology. We calculated the Acoustic Complexity Index (ACI), an algorithm representing an index of bird community complexity. Abrupt changes in ACI values from winter to spring corresponded to spring transition, suggesting that ACI may be an effective, albeit coarse metric to detect the arrival of migrating songbirds. The first peak in ACI shifted from April 16 to April 11 from 2012 to 2014. Changes in ACI were less abrupt in the fall due to weather events, suggesting spring recordings are better suited to indicate phenology. To ensure changes in ACI values were detecting real changes in songbird activity, we explored the relationship between ACI and song of three species: varied thrush (Ixoreus naevius), Pacific wren (Troglodytes pacificus), and ruby‐crowned kinglet (Regulus calendula). ACI was positively related to counts of all species, but most markedly with song of the varied thrush, the most common species in our recordings and a known indicator of forest ecosystem health. We conclude that acoustic recordings paired with bioacoustic indices may be a useful method of monitoring shifts in songbird communities due to climate change and other sources of anthropogenic disturbance.

Detection of &amp;lt;i&amp;gt;Borrelia americana&amp;lt;/i&amp;gt; in the Avian Coastal Tick, &amp;lt;i&amp;gt;Ixodes auritulus&amp;lt;/i&amp;gt; (Acari: Ixodidae), Collected from a Bird Captured in Canada

We document the first record of Borrelia americana in Canada. This Borrelia was detected in an avian coast tick, Ixodes auritulus (Acari: Ixodidae), collected from a Varied Thrush, Ixoreus naevius, along coastal British Columbia. Using real-time PCR and DNA sequencing of the flagellin gene, we determined that the borrelial amplicon from the I. auritulus nymph was 99% homologous with B. americana type strain SCW-41. Because patients infected with B. americana can be seronegative for Lyme disease, medical professionals should be willing to pursue molecular analyses and consider treatment for patients with Lyme disease-like symptoms.

Becoming Earth

Becoming Earth David Gessner and Eva Saulitis (bio) Introduction by David Gessner This is the best thing about Face-book,” I wrote on Eva Saulitis’s page last fall, a few months before she died. And it was. I only met Eva twice and only had one substantial conversation with her, at a hotel bar in Homer, Alaska, where, outside the window, whales breached and otters swam by on their backs while eating lunch on the tables of their bellies. I only knew the world outside that window superficially, but Eva knew it deeply. Her studies on the effects of oil on killer whales were groundbreaking, and by recording hundreds of hours of their calls she learned that the whales she studied, in Prince William Sound, essentially spoke their own language. She got to know whales as individuals. Her approach to research married the tough and tender-minded, and so did her approach to cancer. When she found out she was going to die she faced it bravely, but she also faced it publicly. Eva let us in and I am grateful for it. I knew her only briefly, but I began to follow her. I follow her still. In Eva’s writing, and, yes, on Facebook too, she wove a story that included traveling, losing her hair, exalting in nature, being scared shitless, playing with her nieces, getting sick after chemo, picking flowers. A story of admitting her illness, but taking her pleasures where she could. This was not a happy Facebook of awards and success. It was a darker one, and an honest one, a radiant one, and a deeply sad one. And a better one, it goes without saying. That was something she taught us in both her essays and her posts: how to tell better and more complicated stories. We at Ecotone are proud to publish this essay, one of her last. Click for larger view View full resolution © Cecil Sanders CC BY 2.0. via Flickr [End Page 208] Why do I need these landscapes? The image of the sea draws me out of myself, forces all my attention to the surface so that I can cast my thought into the depths once again … The roots of my astonishment at the world cling tight to my inner life, in a tangle of memories, experiences, atavisms from both my own childhood and that of our species. —Anna Kamienska It’s April again, and I’m still here, and I still have cancer, and “the roots of my astonishment at the world” sink deeper than ever past last fall’s leaf litter into the muck of breakup and resurrection. This year, spring is early in south coastal Alaska. Every morning, I step outside, and the woods around my house are already awake, speaking through the throat clearings of varied thrushes, earliest of songbirds to arrive. On Facebook, people are reporting things like One greater yellowlegs in Beluga Slough. And Today I saw my first sandhill crane. Just one crane is big news, gets a hundred likes. Last night, my husband Craig and I hiked around the nature trail near our home for the first time since fall, and a north wind blew across the Beluga wetlands, pressed down on the dead wheat-colored grasses, wrinkled the standing water. We were bundled up in scarves and hats. On the viewing platform, we could see ducks on Beluga Lake through binocs. We heard Canada geese fly over. In spring, during migration, over a hundred species of birds visit this place. Trumpeter swans and red-necked grebes nest along the lake. In winter, you can see a hawk owl, or even a gray. Though I know these basic things, I’m not a serious birder. More [End Page 209] than anything these days, it’s the sounds I’m after, the voices; it’s mere presence or absence that matters. This town is filled with serious birders, people who can identify the peeps, the migrating shorebirds of various species that for most of us are indistinguishable little mottled scurriers lifting from the mudflats into sheets-drying-in-wind flights. People from all over flock to the town for an annual...

The Varied Thrush

The Varied Thrush

Temporal trends in bird abundance in response to thinning of lodgepole pine (Pinus contorta)

Forest managers are continually seeking methods of extracting more timber from the land base while maintaining appropriate wildlife habitat. A 7-year before–after control–impact (BACI) experiment was conducted in 60-year-old lodgepole pine (Pinus contorta Douglas ex Loudon) to determine the effects of thinning on forest birds. One year of pretreatment data was compared with 6 years of posttreatment data. Using linear trend analysis corrected for detection error, we found that Chipping Sparrow (Spizella passerina (Bechstein, 1798)), Dark-eyed Junco (Junco hyemalis (Linnaeus, 1758)), and Red-eyed Vireo (Vireo olivaceus (Linnaeus, 1766)) increased over time in thinned stands and declined in control stands. Alder Flycatcher (Empidonax alnorum Brewster, 1895), Hermit Thrush (Catharus guttatus (Pallas, 1811)), Rose-breasted Grosbeak (Pheucticus ludovicianus (Linnaeus, 1766)), and Blue-headed Vireo (Vireo solitarius (Wilson, 1810)) decreased in control and thinned stands but declined more in controls. Varied Thrush (Ixoreus naevius (J. F. Gemlin, 1789)) and White-winged Crossbill (Loxia leucoptera Gemlin, 1789) increased in control stands relative to thinned areas, where they were stable or declined. A significant interaction between treatment and year was observed for 24 species when year was categorical. Of these, species more likely found in control stands were Boreal Chickadee (Poecile hudsonicus (J.R. Forster, 1772)), Golden-crowned Kinglet (Regulus satrapa Lichtenstein, 1823), and Least Flycatcher (Empidonax minimus (W.M. Baird &amp; S.F. Baird, 1843)), whereas Ruffed Grouse (Bonasa umbellus (Linnaeus, 1766)), Tennessee Warbler (Vermivora peregrina (A. Wilson, 1811)), and Warbling Vireo (Vireo gilvus (Vieillot, 1808)) were more likely in thinned stands. Total bird abundance was higher in thinned stands, which is consistent with 83% of thinning studies in western coniferous forests. Thinning can be a useful tool for achieving wildlife management goals and increasing timber yields.

Reflections on Thrush Songs, Newt Tracks and Old-Growth Stands of Trees

The songs of varied thrush pull me from bed and out into a still-young morning. It took a full week, but I can resist their beckoning voices no longer. When the decision is made, I jerk off sheet and quilt, jump out of bed, and pile on several layers of shirts plus fleece jacket to ensure I'll stay warm in the cool, damp October air of an Oregon old-growth forest. To say it's early is a bit misleading; the clock reads a few minutes past seven when I step out the door, cross the lawn, and enter the old-growth stand only a stone's throw from where I've been sleeping, eating, and writing these last several days. The forest rings with the wake-up calls of varied thrushes, the predominant sound during this calm, quiet hour in the H. J. Andrews Experimental Forest. The songs aren't the gorgeously fluid, whistled melodies of the varied's Turdidae relatives, for instance the Swainson's and hermit's thrushes and American robin. But the distinctive voice is pleasing nonetheless, at least to these ears.

AVIAN NEST SURVIVAL IN COASTAL FORESTED BUFFER STRIPS ON PRINCE OF WALES ISLAND, ALASKA

Abstract In an effort to provide wildlife habitat and link blocks of forested habitat, coastal forested buffer strips in the Pacific Northwest are managed to mitigate effects of fragmentation that result from timber harvesting adjacent to a coastline. We examined the effect of coastal forest buffer strip width on avian nest survival on Prince of Wales Island, Alaska, in 2003 and 2004. We established nest monitoring plots in two buffer width treatments, narrow ( 350 m, n = 3), and monitored a total of 142 nests of six species: the Pacific-slope Flycatcher (Empidonax difficilis), Chestnut-backed Chickadee (Poecile rufescens), Winter Wren (Troglodytes troglodytes), Swainson's Thrush (Catharus ustulatus), Hermit Thrush (Catharus guttatus), and Varied Thrush (Ixoreus naevius). We modeled and compared the daily survival rate (DSR) of each species in both buffer width treatments. Point estimates for DSRs were slightly higher within wide buffers, but confidence intervals overlapped for al...

British Ornithologists’ Union Records Committee: 36th Report (November 2007)

British Ornithologists’ Union Records Committee: 36th Report (November 2007)

PRESENCE OF SALMON INCREASES PASSERINE DENSITY ON PACIFIC NORTHWEST STREAMS

Abstract The annual migration of Pacific salmon (Oncorhynchus spp.) to freshwater streams and lakes provides an important nutrient subsidy to terrestrial systems in North America. We investigated the effects of salmon and other habitat variables on abundance of Winter Wren (Troglodytes troglodytes), Swainson's Thrush (Catharus ustulatus), Varied Thrush (Ixoreus naevius), Pacific-slope Flycatcher (Empidonax difficilis), Golden-crowned Kinglet (Regulus satrapa), and Chestnut-backed Chickadee (Poecile rufescens) on the central coast of British Columbia. In our comparisons of salmon-bearing and non-salmon-bearing reaches of two rivers that had waterfall barriers to salmon partway upstream, we found that position above or below the falls and proximity to the stream were the major predictors of songbird abundance. Each species, except for Chestnut-backed Chickadee, had higher densities below the falls at both rivers. Our results suggest that salmon-derived nutrients influence songbird density, and thus benefit ...

Taxonomic recommendations for British birds: Fourth report

Taxonomic recommendations for British birds: Fourth report

BREEDING BIRDS OF RESEARCH NATURAL AREAS IN SOUTHEASTERN ALASKA

Abstract The Tongass National Forest encompasses a large expanse of temperate rainforest in southeastern Alaska and contains 12 designated Research Natural Areas (RNAs). Existing in as near a natural condition as possible, RNAs receive minimal commercial and recreational use. Because few bird studies have been conducted on RNAs, we used point counts and area searches to determine the occurrence and abundance of breeding landbirds present in vegetation communities of RNAs. Of 49 species of small landbirds detected during area searches, the most widely distributed birds among RNAs were rufous hummingbird (Selasphorus rufa), chestnut-backed chickadee (Poecile rufescens), winter wren (Troglodytes troglodytes), golden-crowned kinglet (Regulus satrapa), Swainson's thrush (Catharus ustulatus), hermit thrush (C. guttatus), varied thrush (Ixoreus naevius), and Townsend's warbler (Dendroica townsendi). The 8 most abundant species recorded on 187 point counts (>0.5 birds/point) were Pacific-slope flycatcher (Empidon...

RESPONSE OF BIRDS TO THINNING YOUNG DOUGLAS‐FIR FORESTS

Silvicultural practices, such as thinning, are increasingly performed both for commodity production and to help achieve biodiversity goals and promote ecological restoration. However, relatively little research has examined effects of thinning conifer forests on vertebrates. We experimentally manipulated stands using a randomized block design to evaluate influences of two thinning intensities on populations of diurnal breeding birds in western Oregon. We conducted point counts of birds seven times each year in 1994 (prior to treatment) and from 1995 through 2000 (subsequent to treatment). We analyzed data using multiple linear regression and information‐theoretic approaches to model selection. Of the 22 species for which we had sufficient data for analysis, detections of nine species decreased and eight species increased in thinned stands relative to controls, and there was no strong evidence that thinning influenced numbers of five species. Of the 17 species that responded to thinning, the magnitude of response of eight species varied with thinning intensity; for each of these species, response was greatest in the more heavily thinned stands. Although no species was extirpated from stands following thinning, detections of Hutton's Vireos (Vireo huttoni), Golden‐crowned Kinglets (Regulus satrapa), Brown Creepers (Certhia americana), Black‐throated Gray Warblers (Dendroica nigrescens), and Varied Thrushes (Ixoreus naevius) decreased to less than half of the detections in controls in one or more treatment types, suggesting thinning significantly reduces their numbers. In contrast, American Robins (Turdus migratorius), Townsend's Solitaires (Myadestes townsendi), and Hammond's Flycatchers (Empidonax hammondii) were rare or absent in controls but regularly present in thinned stands, and detections of Western Tanagers (Piranga ludoviciana), Evening Grosbeaks (Coccothraustes vespertinus), and Hairy Woodpeckers (Picoides villosus) increased by threefold or more in thinned stands relative to controls. Only Pacific‐slope Flycatchers (Empidonax difficilis), Warbling Vireos (Vireo gilvus), and Western Tanagers showed strong evidence of temporal trends in response. For these species, differences between numbers in controls and treated stands became more extreme through time. Our findings suggest that thinning densely stocked conifer stands in landscapes dominated by younger stands enhances habitat suitability for several species of birds, but that some unthinned patches and stands should be retained to provide refugia for species that are impacted by thinning.

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 &lt; 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 &lt; 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.

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...

Song Post and Foraging Site Characteristics of Breeding Varied Thrushes in Northwestern California

Abstract We used a hierarchical approach to describe habitat characteristics of song posts and foraging sites used by Varied Thrushes (Ixoreus naevius) in coastal redwood (Sequoia sempervirens) forests of northwestern California. We measured mesohabitat (0.04-ha circular plots) and microhabitat (0.5-m radius) scale attributes centered on occupied and random song posts and foraging locations at four study sites during March–August of 1994 and 1995. Ninety-five percent of song posts were in trees or snags. Male thrushes used song posts with low foliage density near the top of large conifers (microhabitat scale), located on steeper slopes, surrounded by a high density of trees, and centered in drainages closer to water (mesohabitat scale) as compared to random locations. Varied Thrushes foraged predominantly on the ground early in the breeding season, then subsequently included fruit in their diet after the young had fledged. Although many variables were correlated with ground foraging locations, microhabitat foliage density had the greatest explanatory power, indicating thrushes selected foraging locations primarily at the microhabitat scale, and emphasizing the importance of measuring habitat characteristics at the appropriate spatial scale. Abrupt forest edges, such as those produced by clearcuts, may reduce habitat suitability for Varied Thrushes possibly explaining their absence from small forest fragments during the breeding season.

SONG POST AND FORAGING SITE CHARACTERISTICS OF BREEDING VARIED THRUSHES IN NORTHWESTERN CALIFORNIA

Abstract We used a hierarchical approach to describe habitat characteristics of song posts and foraging sites used by Varied Thrushes (Ixoreus naevius) in coastal redwood (Sequoia sempervirens) forests of northwestern California. We measured mesohabitat (0.04-ha circular plots) and microhabitat (0.5-m radius) scale attributes centered on occupied and random song posts and foraging locations at four study sites during March–August of 1994 and 1995. Ninety-five percent of song posts were in trees or snags. Male thrushes used song posts with low foliage density near the top of large conifers (microhabitat scale), located on steeper slopes, surrounded by a high density of trees, and centered in drainages closer to water (mesohabitat scale) as compared to random locations. Varied Thrushes foraged predominantly on the ground early in the breeding season, then subsequently included fruit in their diet after the young had fledged. Although many variables were correlated with ground foraging locations, microhabita...

Effect of Birds and Bears on Seed Germination of Fleshy-Fruited Plants in Temperate Rainforests of Southeast Alaska

The effects of avian and mammal seed dispersers on seed germination were investigated for six common plant species (five shrubs and one herb) in the temperate rainforests of southeast Alaska. Patterns of germination behaviour were compared between defecated seeds and control (unpassed) seeds for a period of two years. Seed passage through vertebrate frugivores enhanced germination in two species, Rubus spectabilis and Sambucus racemosa, and a similar trend was observed in Ribes bracteosum. In contrast, it did not affect seed germination in Oplopanax horridus, Streptopus amplexifolius and Vaccinium ovalifolium/alaskaense. No differences in germination behaviour were found between bear- and bird-treated seeds, suggesting that seed retention time (much greater in bears than in birds) in the guts does not relevantly affect germination. Differences among frugivore effects were detected only for Sambucus, with varied thrushes enhancing germination more than American robins and black bears. The two colour morphs of Rubus spectabilis showed similar germination patterns, suggesting that seed dispersers do not exert an important selective pressure contributing to the maintenance of the polymorphism, at least through the mechanism of a differential effect on germination. The results of this study support the idea that the advantages of animal seed dispersal lie more in seed movement away from the parent plant than in seed treatment within the disperser's guts.