Bicknell's Thrush Research Articles

Coinfection rates of avian blood parasites increase with latitude in parapatric host species.

Animals are frequently coinfected with multiple parasites concurrently, and advances in our sampling of these complex intra-host parasite communities have revealed important ecological impacts on their hosts. However, the spatial distributions and environmental determinants of parasite coinfection remain infrequently studied. Here, we investigated the drivers of haemosporidian blood parasite coinfection in the Bicknell's thrush (Catharus bicknelli) and grey-cheeked thrush (Catharus minimus), parapatric sister species that occur across a broad latitudinal range in northern North America. Using 298 samples from across the distributions of these species, we found high overall infection (86%) and coinfection (41%) rates within host populations. Coinfection rates within populations were highly variable across sampling sites, ranging from 7 to 75%. Latitude was a more important predictor of coinfection frequency than host species identity, with coinfections becoming more abundant at higher latitudes. The 2 host species exhibited similar parasite faunas, and an analysis of the co-occurrence patterns among haemosporidians showed that host species identity was largely not a factor in structuring which parasites were found within coinfections. To our knowledge, this is the first study to illustrate a reverse latitudinal gradient in coinfection frequency in a eukaryotic parasite system. Further work is necessary to determine whether vector ecology or some other factor is the primary proximate driver of this pattern.

Mercury exposure in songbird communities along an elevational gradient on Whiteface Mountain, Adirondack Park (New York, USA).

Mercury (Hg) is a potent neurotoxin that biomagnifies within food webs. Adverse effects have been documented for avian species related to exposure of elevated Hg levels. High elevation, boreal forests generally receive higher atmospheric Hg deposition and regional studies have subsequently identified elevated blood Hg concentrations in songbird species inhabiting these montane habitats. The overall goal of this study was to investigate spatial and seasonal Hg exposure patterns in songbird species along an elevational gradient on Whiteface Mountain in the Adirondack Park of New York State. Songbird blood samples were collected from June-July in 2009 and 2010 along an elevational gradient at 13 study plots (450-1400 m) with a focus on Catharus thrushes, including the hermit thrush, Swainson's thrush, and Bicknell's thrush. The main results of this study documented: (1) an overall linear pattern of increasing blood Hg concentrations with increasing elevation, with additional analysis suggesting a nonlinear elevational pattern of increasing blood Hg concentrations to 1075 m, followed by decreasing concentrations thereafter, for all Catharus thrush species across the elevational gradient; and (2) an overall nonlinear seasonal pattern of increasing, followed by decreasing blood Hg concentrations across target species. Avian exposure patterns appear driven by elevated atmospheric Hg deposition and increased methylmercury bioavailability within high elevation habitats as compared with low elevation forests. Seasonal patterns are likely influenced by a combination of complex and dynamic variables related to dietary selection and annual molting cycles. Considering that few high elevation analyses have been conducted within the context of regional songbird research, this project complements the results from similar studies and highlights the need for further monitoring efforts to investigate environmental Hg contamination within avian communities.

Patterns of blood mercury variation in two long-distance migratory thrushes on Mount Mansfield, Vermont.

We investigated mercury (Hg) blood concentrations in Bicknell's thrush (Catharus bicknelli) and Swainson's thrush (C. ustulatus), congeneric long-distance migratory songbirds, from 2000-2017 at a montane forest site in north-central Vermont. We analyzed variation in blood Hg of both species using mixed-effects models, incorporating atmospheric wet Hg deposition data from a nearby sampling location. Although Hg deposition varied among years and seasonally, we detected no temporal trend in either atmospheric deposition or blood Hg, nor evidence of a relationship between the two. Sampling date had the strongest effect on blood Hg concentration, which declined seasonally, followed by age and sex of the individual. The data did not support an effect of species. We believe that the absence of a clear relationship between local atmospheric deposition and thrush blood Hg concentrations suggests that Hg cycling dynamics, mechanisms of transfer, and timing of uptake by montane forest biota are complex and poorly understood. The blood Hg concentrations of ~0.07-0.1 μg/g we documented in Bicknell's and Swainson's thrush are below those found to negatively impact physiological or reproductive endpoints in other invertivorous terrestrial passerines. To better evaluate the validity of Bicknell's thrush as a bioindicator of MeHg availability in montane forest ecosystems, we recommend (1) effects-based investigations, (2) a more robust understanding of Hg and MeHg cycling, (3) more clear geospatial and temporal links between Hg deposition and biotic uptake, and (4) more thorough documentation of Hg burdens across the species' annual cycle.

Spatially explicit climate change projections for the recovery planning of threatened species: The Bicknell's Thrush (Catharus Bicknelli) as a case study

With climate change, natural resource managers are faced with the challenging task of planning the conservation of habitat for threatened species. Classified as "threatened" under the Species at Risk Act in Canada, the Bicknell's Thrush (Catharus Bicknelli - BITH) is a migratory bird whose range is highly restricted. Bioclimatic models project more than 50% loss of suitable habitat - high elevation dense balsam fir (Abies balsamea) forests - in the northeastern USA by 2050 due to climate change. We used BITH as a case study to demonstrate the value of forest landscape models (e.g., LANDIS-II) to support decision-making on conservation of habitat of threatened species. We modeled the impacts of forest management and natural disturbances, as well as climate-induced changes, on forest stand and landscape structure. Under RCP climate forcing scenarios 2.6, 4.5 and 8.5, simulations projected significant changes in dominant tree species biomass, from coniferous to broadleaved deciduous, implying important losses of mature balsam fir forest. Climate change was projected to have severe effects with major changes projected to occur after 2080 and losses of more than half of BITH suitable habitat by 2100 for “worst-case” climate change scenarios. This contrasts with results from bioclimatic models which do not capture the expected lags in vegetation responses to changing climate. Our results also suggest that innovative forest management strategies could help maintain BITH habitat abundance under RCP 2.6 and RCP 4.5 climate forcing. Such results can provide guidance for considering effective long-term conservation of habitat for threatened species in a changing world.

Modeling spatial variation in winter abundance to direct conservation actions for a vulnerable migratory songbird, the Bicknell's Thrush (Catharus bicknelli)

ABSTRACT The winter range of the Bicknell's Thrush (Catharus bicknelli), a globally vulnerable Nearctic–Neotropical migratory songbird, is restricted to the Greater Antilles. Most birds occur on Hispaniola, where the species exhibits sexual habitat segregation, and where the loss of its preferred wet broadleaf habitats has been severe. Conservation of female-dominated habitats has been identified as a top priority for the species' preservation. During 2010, we conducted standardized field surveys for the Bicknell's Thrush in forests of the Dominican Republic's Cordillera Septentrional, an area of high importance for females. We combined occurrence data with environmental variables to create a model of thrush distribution and habitat selection in the region. Elevation (densities peaked at ∼600 m), aspect (northeastern slopes), the amount of forest cover within 1 km, and forest density best described thrush presence and abundance. As an example of how predictive models of abundance can inform conservation, ...

Documentation of a hybrid Bicknell's Thrush (Catharus bicknelli) × Veery (C. fuscescens) using vocalization and genetic data

ABSTRACT We combined data on morphology, behavior, and gene sequences to document the first known case of hybridization within the thrush genus Catharus. During June 2008 at a montane forest site in south-central Vermont, we observed a male bird that repeatedly exhibited song characteristics of both Bicknell's Thrush (C. bicknelli) and Veery (C. fuscescens). Handheld examination determined the bird to be a yearling male with plumage characteristics predominantly of Veery and morphometric traits within the ranges of both Bicknell's Thrush and Veery. While the hybrid vocalized only with Bicknell's Thrush calls, its songs were reminiscent of both Veery and a mixture of Bicknell's Thrush and Veery. Mitochondrial DNA (NADH dehydrogenase 2 and cytochrome b genes) revealed the bird's mother to be a Veery, and nuclear data (β-fibrinogen 7 and myoglobin 2 introns) revealed DNA of both Veery and Bicknell's Thrush, supporting Bicknell's Thrush as the father of the hybrid. This hybridization is notable given that Bic...

A fine‐scale U.S. population estimate of a montane spruce–fir bird species of conservation concern

AbstractBicknell's Thrush (Catharus bicknelli) is one of the most range‐restricted bird species in North America. In the United States (U.S.), it occurs only in the disturbed montane forests of fir (Abies balsamea) and spruce (Picea spp.) in the northeastern states of New York, Vermont, New Hampshire, and Maine. Climate change is expected to substantially diminish this vegetation community (>50% extinction from current range) by 2300. Despite the increasingly tenuous conservation status of the species, fundamental demographic parameters remain unknown and overall population size is uncertain. We used N‐mixture models in a hierarchical Bayesian framework to predict population size and to elucidate spatial and temporal patterns of Bicknell's Thrush abundance in the northeastern United States. From 2011 to 2016, as part of Mountain Birdwatch, citizen‐scientists conducted 14,552 five‐minute point counts at 747 sampling locations along hiking trails. Observers detected Bicknell's Thrush at 49.4% of points and tallied 1079 observations of the species. Bicknell's Thrush abundance was parsimoniously modeled as a complex function of elevation, forest canopy cover, and latitude. Using our model results, and the estimated U.S. range of the Bicknell's Thrush, we produced the first fine‐scale (<1.0 ha resolution) abundance estimate and map of Bicknell's Thrush density across their U.S. range. We predicted the U.S. Bicknell's Thrush population in 2016 as 71,318 (95% credible interval: 56,080–89,748), and 95% of that population occurred above 805 m. Combining our results with existing estimates of population size in Canada suggests a global population size of <120,000; Bicknell's Thrush likely have one of the smallest population sizes of regularly occurring bird species within the contiguous United States and Canada. By cross‐referencing the U.S. Geological Survey Protected Areas Database, we estimated that 76.6% of Bicknell's Thrush habitat occurred on conserved lands across the United States and that this habitat supported 84.6% of the predicted population. The White Mountain National Forest (New Hampshire and Maine) is the largest conservator of Bicknell's Thrush habitat in the United States and supports ~31% of the predicted U.S. population. Our model provides a testable framework for assessing the success of future conservation and management actions on Bicknell's Thrush populations throughout their U.S. range.

Division within the North American boreal forest: Ecological niche divergence between the Bicknell's Thrush (Catharus bicknelli) and Gray‐cheeked Thrush (C. minimus)

Sister species that diverged in allopatry in similar environments are expected to exhibit niche conservatism. Using ecological niche modeling and a multivariate analysis of climate and habitat data, I test the hypothesis that the Bicknell's Thrush (Catharus bicknelli) and Gray‐cheeked Thrush (C. mimimus), sister species that breed in the North American boreal forest, show niche conservatism. Three tree species that are important components of breeding territories of both thrush species were combined with climatic variables to create niche models consisting of abiotic and biotic components. Abiotic‐only, abiotic+biotic, and biotic‐only models were evaluated using the area under the curve (AUC) criterion. Abiotic+biotic models had higher AUC scores and did not over‐project thrush distributions compared to abiotic‐only or biotic‐only models. From the abiotic+biotic models, I tested for niche conservatism or divergence by accounting for the differences in the availability of niche components by calculating (1) niche overlap from ecological niche models and (2) mean niche differences of environmental values at occurrence points. Niche background similarity tests revealed significant niche divergence in 10 of 12 comparisons, and multivariate tests revealed niche divergence along 2 of 3 niche axes. The Bicknell's Thrush breeds in warmer and wetter regions with a high abundance of balsam fir (Abies balsamea), whereas Gray‐cheeked Thrush often co‐occurs with black spruce (Picea mariana). Niche divergence, rather than conservatism, was the predominant pattern for these species, suggesting that ecological divergence has played a role in the speciation of the Bicknell's Thrush and Gray‐cheeked Thrush. Furthermore, because niche models were improved by the incorporation of biotic variables, this study validates the inclusion of relevant biotic factors in ecological niche modeling to increase model accuracy.

Interspecific aggression by the Swainson's Thrush (Catharus ustulatus) may limit the distribution of the threatened Bicknell's Thrush (Catharus bicknelli) in the Adirondack Mountains

ABSTRACT Bicknell's Thrush (Catharus bicknelli) is a threatened species that inhabits high-elevation spruce–fir forests in the northeastern United States. Bicknell's Thrush populations are predicted to shift upslope in response to global warming, leading to local extinctions on mountains that lack the elevational extent to accommodate such shifts. However, biotic interactions may influence how montane species respond to changing abiotic conditions. In territorial songbirds, for example, aggression from dominant low-elevation species has been hypothesized to combine with warming temperatures to “push” populations of subordinate montane species upslope at a faster rate than in the absence of interspecific aggression. This “push” hypothesis could apply to Bicknell's Thrushes: the related Swainson's Thrush (Catharus ustulatus) inhabits lower elevations, Bicknell's Thrushes are largely confined to mountaintops, and interspecific aggression by Swainson's Thrushes toward Bicknell's Thrushes has been anecdotally ...

Nest-Patch Characteristics of Bicknell's Thrush in Regenerating Clearcuts, and Implications for Precommercial Thinning

Abstract Catharus bicknelli (Bicknell's Thrush) is a rare and globally vulnerable songbird often found in regenerating clearcuts in the Canadian maritime provinces and Quebec. Previous studies have shown correlations between vegetation characteristics and occurrence and abundance of this species, but no study has described vegetation associated with Bicknell's Thrush nests in managed forests. From 2007–2010, we investigated nest-habitat selection of Bicknell's Thrush in the industrial forestry landscape of north-central New Brunswick. We compared vegetation composition and structure in 5-m-radius patches around nests to vegetation in a random control-patch within the home range of each Bicknell's Thrush. Precommercial thinning (PCT) is a forest-management treatment that may reduce the suitability of habitat for Bicknell's Thrush, thus we also examined the percent of the landscape treated by this practice around Bicknell's Thrush nests. We found that Bicknell's Thrush preferentially selected nest sites wit...

Site-specific variation in food resources, sex ratios, and body condition of an overwintering migrant songbird

ABSTRACT. Territorial social behavior of wintering Nearctic—Neotropic migrant songbirds places males and females in direct conflict over access to winter space and resources. Outcomes of this intersexual competition can vary by species and habitat, but information has been collected for only a small subset of migrant species. We investigated the available food resources, sex ratios, and body condition of territorial Bicknell's Thrushes (Catharus bicknelli) wintering in the Dominican Republic between 1999 and 2008 at two ecologically distinct wet-forest sites, one in high-elevation cloud forest and the other in mid-elevation rainforest. Arthropod abundance was greater in cloud forest habitat, which was occupied by proportionally more males, the larger-bodied sex (74% male). By contrast, both sexes occurred at parity in rainforest habitat (53% male), where soft-bodied fruit was the predominant dietary resource. Body condition of cloud forest males was comparable to that of rainforest males, but cloud forest...

Stable-hydrogen isotope turnover in red blood cells of two migratory thrushes: application to studies of connectivity and carry-over effects

ABSTRACT Understanding turnover rates of stable isotopes in metabolically active tissues is critical for making spatial connections for migratory birds because samples provide information about pre-migratory location only until the tissue turns over to reflect local values. We calculated stable-hydrogen isotope (δ2H) turnover rate in the red blood cells of two long-distance migratory songbirds, Bicknell's Thrushes (Catharus bicknelli) and Swainson's Thrushes (Catharus ustulatus), using samples collected at a breeding site in New Brunswick, Canada. Blood from both species captured early in the breeding site was more positive in δ2H than blood sampled later in the summer, but did not match blood values for wintering Bicknell's Thrushes. An asymptotic exponential model was used to estimate turnover of red blood cell δ2H and yielded a half-life estimate of 21 days and 14 days for Bicknell's and Swainson's thrushes, respectively. Red blood cells of both species approached the local breeding site value one month after the first individuals were detected at the site. For Bicknell's Thrushes, estimated δ2H in blood at arrival (−72‰) was closer to blood collected at wintering sites (mean −61‰) than to expected breeding site δ2H (−120‰). Discrimination values calculated for red blood cells collected at the breeding site for both species were greater than expected based on studies using keratin. Turnover during migration currently limits the use of blood sampled early in the breeding season for connectivity/carry-over effect studies. However, direct tracking technology such as geolocators can provide information about migration duration, timing, and stopovers that can be used to improve isotopic turnover equations for metabolically active tissues.

Radio-transmitters do not affect seasonal mass change or annual survival of wintering Bicknell's Thrushes

Studies of the effects of transmitters on passerines have provided mixed results, but many have revealed no negative impacts. Most such studies have been conducted during the breeding season and, as a result, little is known about the possible effects of transmitters on wintering birds. We examined the effects of transmitters on Bicknell's Thrushes ( Catharus bicknelli ) wintering in the Dominican Republic. We used long-term mark-recapture data to compare birds with radio-transmitters ( N = 5 years, 64 individuals) to birds banded, but not radio-tagged (N = 10 years, 164 individuals). For a subset of birds in each category, we measured the change in mass between early-winter and late-winter and, for all birds, we calculated return rates and modeled annual survival by sex and age classes. Return rates of radio-tagged (16%) and banded-only individuals (17%) did not differ, and survival models including the effect of transmitter attachment were poorly supported. We also found no sex or age-specific effects of transmitter attachment. Radio-tagged and banded-only birds did not differ either in the proportion of birds gaining mass during the winter or amount of mass gained. Our results indicate that attaching light-weight transmitters ( <5% of body mass) to wintering Bicknell's Thrushes did not adversely affect either body condition or annual survival. RESUMEN. Radiotransmisores no afectan los cambios estacionales en masa o sobrevivencia anual en individuos invernales de Catharus bicknell Estudios sobre el efecto de radiotransmisores en paserinos, han provisto resultados mixtos, aunque muchos no han revelado impacto negativo. La mayor ´ oa de estos estudios han sido conducidos durante lade reproducci ´ on, y como resultado, se conoce poco sobre el posible efecto de los radiotransmisores en aves invernales. Examinamos el efecto de transmisores en Catharus bicknell, que pasaron el invierno en Rep ´ ublica Dominicana. Utilizamos datos de recaptura de aves marcadas previamente, para comparar con aves con radiotransmisores (N = 5 a ˜ nos, 64 individuos), y aves anilladas, pero sin radiotransmisores (N = 10 a ˜ 164 individuos). Para un subconjunto de aves, en cada categormedimos el cambio en masa entre temprano y luego tarde en el invierno, y para todas las aves calculamos la tasa de retorno y modelamos la sobrevivencia anual por sexo y grupo de edad. No encontramos diferencias en la tasa de retorno entre aves con radiotransmisores (16%) e individuos solamente anillados (17%) y los modelos de sobrevivencia, que incluyeron el efecto de los radiotransmisores, tuvieron poco apoyo. Tampoco encontramos efectos especde los radiotransmisores relacionados con la edad o sexo de las aves. Con respecto a la proporcien ganancia de peso durante el invierno o la ganancia de peso en general, no hubo diferencia, entre aves con radiotransmisores o anilladas. Nuestros resultados indican que el colocarles un radiotransmisor liviano ( < 5% de la masa corporal) a individuos invernales del Zorzal de Bicknell no tiene efecto adverso en su condici ´ corporal o en la sobrevivencia anual.

Stable‐hydrogen isotope measures of natal dispersal reflect observed population declines in a threatened migratory songbird

AbstractAimMeasuring dispersal is crucial for estimating demographic rates that inform conservation plans for rare and threatened species. We evaluated natal dispersal patterns in Bicknell's thrush (Catharus bicknelli) across most of the breeding range using a 10‐year data set of stable‐hydrogen isotope ratios in feathers (δ2HF) grown on the natal area and sampled 1 year later at the first breeding site.LocationNorth‐eastern United States and south‐eastern Canada.MethodsWe used δ2HF values of adult thrushes sampled at 25 breeding sites as prior information for assigning first‐time breeders to their natal site. We calculated the minimum distance birds moved from their natal to first breeding site and fit these data to three statistical distributions for characterizing the importance of long‐distance dispersal: the exponential, Weibull and half‐Cauchy. Finally, we assessed differences in the probability of dispersal across the breeding range and through time to understand spatio‐temporal variation in demographic connectivity.ResultsThe δ2HF values of first‐time breeders were lower compared with those of adults, a difference that was greater at the southern compared with northern breeding range extreme. Assignment tests accounting for age differences in δ2HF suggested that most birds dispersed &lt; 200 km from their natal area and within the centre of the breeding range, whereas comparatively few individuals dispersed up to 700 km. A Weibull distribution provided the best fit to these data. Two of three corrections for age differences in δ2HF indicated that natal dispersal probability declined by 30–38% from 1996 to 2005.Main conclusionsOur findings suggest that estimating natal dispersal with δ2HF measurements may contribute to understanding the resilience of geographically isolated Bicknell's thrush populations. Declining natal dispersal may be symptomatic of observed population declines and could compound this trend by limiting demographic exchange between habitat patches predicted to be increasingly isolated by natural and anthropogenic habitat changes.

The relative contribution of local habitat and landscape context to metapopulation processes: a dynamic occupancy modeling approach

Changes in site occupancy across habitat patches have often been attributed to landscape features in fragmented systems, particularly when considering metapopulations. However, failure to include habitat quality of individual patches can mask the relative importance of local scale features in determining distributional changes. We employed dynamic occupancy modeling to compare the strength of local habitat variables and metrics of landscape patterns as drivers of metapopulation dynamics for a vulnerable, high‐elevation species in a naturally fragmented landscape. Repeat surveys of Bicknell's thrush Catharus bicknelli presence/non‐detection were conducted at 88 sites across Vermont, USA in 2006 and 2007. We used an organism‐based approach, such that at each site we measured important local‐scale habitat characteristics and quantified landscape‐scale features using a predictive habitat model for this species. We performed a principal component analysis on both the local and landscape features to reduce dimensionality. We estimated site occupancy, colonization, and extinction probabilities while accounting for imperfect detection. Univariate, additive, and interaction models of local habitat and landscape context were ranked using AICc scores. Both local and landscape scales were important in determining changes in occupancy patterns. An interaction between scales was detected for occupancy dynamics indicating that the relationship of the parameters to local‐scale habitat conditions can change depending on the landscape context and vice versa. An increase in both landscape‐ and local‐scale habitat quality increased occupancy and colonization probability while decreasing extinction risk. Colonization and extinction were both more strongly influenced by local habitat quality relative to landscape patterns. We also identified clear, qualitative thresholds for landscape‐scale features. Conservation of large habitat patches in high‐cover landscapes will help ensure persistence of Bicknell's thrushes, but only if local scale habitat quality is maintained. Our results highlight the importance of incorporating information beyond landscape characteristics when investigating patch occupancy patterns in metapopulations.

Sex and Age Ratios of Bicknell's Thrush Wintering in Hispaniola

Abstract We investigated sex and age ratios of wintering Bicknell's Thrush (Catharus bicknelli) across a geographic gradient of sites on Hispaniola. The island-wide proportion male was 0.64 (n = 127), which is comparable to the known male bias in breeding areas. The proportion male varied geographically on Hispaniola, suggesting some level of habitat segregation. Male-biased ratios occurred at two sites whereas sex ratios at five sites did not differ from parity. The island-wide proportion adult was 0.72 and age ratios were significantly adult-biased at two sites. We assessed vegetative structure at all sites and the proportion of male thrushes increased significantly with density of understory vegetation. Age ratios were not associated with vegetation characteristics. Neither sex nor age ratios varied significantly with elevation. Our data suggest the possibility of sexual habitat segregation with males preferentially occupying cloud forest sites characterized by a thick understory of vines and sapling...

Winter Territoriality and Spatial Behavior of Bicknell's Thrush (Catharus bicknelli) at Two Ecologically Distinct Sites in the Dominican Republic

ABSTRACT. We used radiotelemetry to investigate the spatial behavior of wintering Bicknell's Thrushes (Catharus bicknelli) at a mid-elevation rainforest site and a high-elevation cloud-forest site in the Dominican Republic. We also analyzed blood stable carbon isotopes and fecal samples to compare thrush diets at these two floristically and climatically distinct sites. Birds consumed a primarily fruit-based diet at the mid-elevation site and a primarily arthropod-based diet at the high-elevation site. Despite these dietary differences, individuals at both sites defended and maintained exclusive, minimally overlapping core use areas and home ranges. The mean size of both core use areas and home ranges was similar between males and females and between adults and first-winter birds. Presence of nonterritorial or “floater” individuals was low at both sites (2.7% and 5.6%). Birds at the arthropod-dominated site were observed significantly more often on or within 1 m of the ground than birds at the fruit-heavy ...

Mercury bioaccumulation and trophic transfer in the terrestrial food web of a montane forest

We investigated mercury (Hg) concentrations in a terrestrial food web in high elevation forests in Vermont. Hg concentrations increased from autotrophic organisms to herbivores < detritivores < omnivores < carnivores. Within the carnivores studied, raptors had higher blood Hg concentrations than their songbird prey. The Hg concentration in the blood of the focal study species, Bicknell's thrush (Catharus bicknelli), varied over the course of the summer in response to a diet shift related to changing availability of arthropod prey. The Bicknell's thrush food web is more detrital-based (with higher Hg concentrations) in early summer and more foliage-based (with lower Hg concentrations) during late summer. There were significant year effects in different ecosystem compartments indicating a possible connection between atmospheric Hg deposition, detrital-layer Hg concentrations, arthropod Hg concentrations, and passerine blood Hg concentrations.

Designing spatially explicit incentive programs for habitat conservation: A case study of the Bicknell's thrush wintering grounds

The Bicknell's thrush ( Catharus bicknelli) is among North America's migratory bird species of greatest conservation concern. Protecting habitat at both ends of its range has been identified as a high priority action. An estimated 90% of the species' global population overwinters on the island of Hispaniola, where ongoing habitat loss is a severe problem. Preventing further loss of Bicknell's thrush habitat on Hispaniola will require, among other efforts, establishing conservation areas on private lands, where success will depend on cooperation with landowners. This paper considers a voluntary incentive program to landowners in the Dominican Republic to yield a cost-effective network of protected habitat to sustain overwintering Bicknell's thrush populations. TAMARIN, a Geographic Information System (GIS) based model, evaluates the economic and ecological considerations for a voluntary bid scenario where landowners sell their property rights to a government agency or non-governmental organization. Results indicate that two scientific reserves in northeastern Dominican Republic could be connected under one viable forest fragment valued at US $5.5 × 10 6, increasing the total protected land by 87% to 19,357 ha. Incorporating the spatial variability of economic costs and biological benefits into a conservation program, such as the Bicknell's Thrush Habitat Protection Fund will help planners achieve habitat conservation at least cost.

Predation of a Wintering Migratory Songbird by Introduced Rats: Can Nocturnal Roosting Behavior Serve as Predator Avoidance?

Abstract. Many studies have demonstrated the deleterious effects of introduced predators on resident populations of island birds, but few have quantified their effect on the survival and space-use behavior of migratory species. We used radio telemetry to investigate the winter survival and roosting patterns of Bicknell's Thrush (Catharus bicknelli) at two sites in the Dominican Republic. Depredation by introduced rats was the only cause of mortality among 53 radio-tagged individuals monitored between January and March over multiple years; five (9%) marked individuals were depredated. Predator trapping revealed the presence of both the black rat (Rattus rattus) and Norway rat (R. norvegicus) and that the density of rats was higher in broadleaf cloud forest than in nearby pine forest. Some thrushes that used cloud forest exclusively during the day roosted at night in adjacent pine habitat. We suggest that introduced rats exert predation pressure on wintering Bicknell's Thrush in the Dominican Republic and t...