Kent P. McFarland

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 Bicknells 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 Swainsons 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 Bicknells 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.

Transoceanic migration by a 12 g songbird.

Many fundamental aspects of migration remain a mystery, largely due to our inability to follow small animals over vast spatial areas. For more than 50 years, it has been hypothesized that, during autumn migration, blackpoll warblers (Setophaga striata) depart northeastern North America and undertake a non-stop flight over the Atlantic Ocean to either the Greater Antilles or the northeastern coast of South America. Using miniaturized light-level geolocators, we provide the first irrefutable evidence that the blackpoll warbler, a 12 g boreal forest songbird, completes an autumn transoceanic migration ranging from 2270 to 2770 km (mean ± s.d.: 2540 ± 257) and requiring up to 3 days (62 h ± 10) of non-stop flight. This is one of the longest non-stop overwater flights recorded for a songbird and confirms what has long been believed to be one of the most extraordinary migratory feats on the planet.

WINTER BIRD COMMUNITIES IN FOUR HABITATS ALONG AN ELEVATIONAL GRADIENT ON HISPANIOLA

Abstract We used five years of mist-net-capture and point-count data to quantify avian diversity in four habitats along a 1750-m elevational gradient in the Dominican Republic. These habitats include desert thorn scrub, dry forest, pine forest, and montane broadleaf forest, which together comprise more than two-thirds of existing forest on Hispaniola. In midwinter samples we recorded 74 species of landbirds, including 22 species of latitudinal migrants and 19 endemics. The highest diversity and species richness were found in pine forest and dry forest, but the highest capture rate of individuals was in desert thorn scrub. Abundance of migrant individuals was highest in pine forest, whereas pine and montane broadleaf forest contained the highest proportion of endemic species and individuals, and more habitat specialists. Among mist-net captures, insectivorous species and individuals predominated in all habitats except in dry forest, where more omnivorous individuals were captured. A more complex pattern was found in point-count detections: insectivorous species and individuals predominated in most habitats; omnivorous species and individuals were most frequently counted in montane broadleaf and dry forest, respectively; and nectarivorous individuals were most common in desert thorn scrub. Data presented here represent the most complete quantitative record of avian abundance and distribution on Hispaniola. This study not only details the value of these four habitats to various suites of species, but also emphasizes the importance of montane broadleaf and pine forests to large numbers of Neotropical migrants and Hispaniolan endemics, some of which are narrowly restricted to these habitats. Comunidad de Aves de Invierno en Cuatro Hábitats a lo largo de una Gradiente Altitudinal en la Isla Española Resumen. Utilizamos cinco años de datos de captura con redes ornitológicas y de conteos en puntos para cuantificar la diversidad de aves en cuatro hábitats a lo largo de un gradiente altitudinal de 1750 m en la República Dominicana. Estos hábitats incluyen matorral espinoso seco, bosque seco, bosque de coníferas y bosque latifoliado montano, los que en conjunto ocupan más de dos tercios del área boscosa del país. En enero y febrero, registramos 74 especies de aves terrestres, incluyendo 22 especies migratorias latitudinales y 19 endémicas. La mayor diversidad y riqueza de especies fue encontrada en el bosque de coníferas y en el bosque seco, pero la mayor tasa de captura de individuos se presentó en el matorral espinoso seco. La abundancia de individuos migratorios fue mayor en el bosque de coníferas, mientras que los bosques de coníferas y latifoliado montano presentaron la mayor proporción de individuos y especies endémicas y el mayor número de especialistas de hábitat. Entre las capturadas, las especies e individuos insectívoros predominaron en todos los hábitats, excepto en el bosque seco dónde se capturaron más individuos omnívoros. Entre las aves registradas en los puntos de conteo, los insectívoros también predominaron en la mayoría de los hábitats, pero registramos más especies omnívoras e individuos omnívoros en el bosque latifoliado nublado y bosque seco respectivamente. Los individuos nectarívoros fueron más comunes en el matorral espinoso seco. Los datos presentados aquí representan la cuantificación más completa de la abundancia y distribución de aves en Española. Esta investigación no sólo describe en detalle el valor de estos hábitats para los distintos grupos de especies, sino que también enfatiza la gran importancia de los bosques latifoliado montano y de coníferas para un gran número de aves migratorias Neotropicales y endémicas de Española, algunas de las cuales se encuentran estrechamente restringidas a estos hábitats.

MULTIPLE PATERNITY AND MULTIPLE MALE FEEDERS IN BICKNELL'S THRUSH (CATHARUS BICKNELLI)

Abstract Although multiple male feeders and multiple paternity for a single brood are taxonomically widespread among nonmigratory birds, they appear to be exceedingly rare among migrants. During a demographic study of Bicknells Thrush (Catharus bicknelli) in Vermont, we observed groups of 2–4 males and one female feed each of 14 broods (78% of 18 broods observed). One male and one female fed each of the remaining four broods. Three males fed broods of two females concurrently or consecutively. Genetic analysis using microsatellite markers revealed multiple paternity in 9 of 13 broods (69%). Four males had offspring in more than one brood in the same year, but no females did. Ten of 17 males (59%) sired offspring in the broods they fed, and 10 of 13 sires (77%) fed the broods in which they sired young. Females fed nestlings more than twice as often as males. Both males and females tended to increase feeding rates as brood size increased. Possible factors influencing the reproductive behavior of Bicknells Thrush include harsh montane weather, food limitation, a male-biased sex ratio, and reproductive synchrony. Additional research is needed to examine the interaction of ecological and demographic variables that shape the complex breeding ecology of Bicknells Thrush.

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 Bicknells 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 site, which were more often observed in the mid-canopy structure above 2 m. Birds at both sites displayed agonistic behaviors toward conspecifics and toward playback of conspecific vocalizations. Exclusive territoriality was the predominant winter social system, and we suggest that both arthropods and fruit are defensible resources for wintering Bicknells Thrushes.

MIGRANT STOPOVER AND POSTFLEDGING DISPERSAL AT A MONTANE FOREST SITE IN VERMONT

Abstract To investigate the use of high elevation fir forests by fall passage migrants, we conducted standardized mist-netting and banding at a 1150–1175 m elevation site on Mt. Mansfield in north-central Vermont during the autumns of 1995–1997. Overall, we captured 3024 individuals of 62 species in 10,048 cumulative net hours (30.1 birds/100 net hr). We divided species into 3 classes: (1) those breeding regularly on Mt. Mansfield above 916 m elevation (16 species), (2) those breeding only sporadically or at very low densities above 916 m (6 species), and (3) those occurring only as transients (40 species). Breeding species accounted for 68% of new captures, followed by transients (22%), and sporadic/low density breeders (10%). Of the ten most abundant species captured, only two, Black-throated Blue Warbler (Dendroica caerulescens) and Ovenbird (Seiurus aurocapillus), were transients, accounting for 50% and 9%, respectively, of all transient captures. Hatching-year birds accounted for 94% of known-age transients and 81% of known-age individuals among locally breeding species. Recapture rates of transients were extremely low (0.3%), while recapture rates among species known to breed locally (including presumed transient individuals) were higher (2.2%). Nearly 75% of all birds captured were very lean at first capture, and only 26% of recaptured individuals increased their fat scores between first and final captures. Weight changes of recaptured birds varied: 48% lost weight, 44% gained weight, and 8% maintained the same weight between first and final captures. Our data suggest that conditions on the Mt. Mansfield ridgeline are not conducive to prolonged migratory stopovers and that most migrants may be unable to meet their energetic requirements for continued migration. However, we believe that montane forest habitats may be preferentially selected by those migrants that use them for breeding. We further believe that montane fir forests may be an important postfledging dispersal habitat for Black-throated Blue Warblers and other low or mid-elevation breeding species, and that conservation planning for montane forest sites should carefully consider the needs of migrants outside the breeding season.

Avian, salamander, and forest floor mercury concentrations increase with elevation in a terrestrial ecosystem

High-elevation ecosystems of the northeastern United States are vulnerable to deposition and environmental accumulation of atmospheric pollutants, yet little work has been done to assess mercury (Hg) concentrations in organisms occupying montane ecosystems. The authors present data on Hg concentrations in ground-foraging insectivorous songbirds, a terrestrial salamander, and forest floor horizons sampled along a forested elevational gradient from 185 m to 1273 m in the Catskill Mountains, New York, USA. Mean Hg concentrations in Catharus thrushes and the salamander Plethodon cinereus increased with elevation, as did Hg concentrations in all forest floor horizons. Mean Hg concentrations in organic soils at approximately 1200 m elevation (503.5 ± 17.7 ng/g, dry wt) were 4.4-fold greater than those at approximately 200 m. Montane ecosystems of the northeastern United States, and probably elsewhere, are exposed to higher levels of atmospheric Hg deposition as reflected in accumulation patterns in the forest floor and associated high-elevation fauna. This information can be used to parameterize and test Hg transport and bioaccumulation models of landscape-specific patterns and may serve as a monitoring tool for decision makers considering future controls on Hg emissions. Further investigation is needed into the potential effects of increased Hg concentrations on high-elevation fauna.

A winter distribution model for Bicknell's Thrush (Catharus bicknelli), a conservation tool for a threatened migratory songbird.

Conservation planning and implementation require identifying pertinent habitats and locations where protection and management may improve viability of targeted species. The winter range of Bicknell’s Thrush (Catharus bicknelli), a threatened Nearctic-Neotropical migratory songbird, is restricted to the Greater Antilles. We analyzed winter records from the mid-1970s to 2009 to quantitatively evaluate winter distribution and habitat selection. Additionally, we conducted targeted surveys in Jamaica (n = 433), Cuba (n = 363), Dominican Republic (n = 1,000), Haiti (n = 131) and Puerto Rico (n = 242) yielding 179 sites with thrush presence. We modeled Bicknell’s Thrush winter habitat selection and distribution in the Greater Antilles in Maxent version 3.3.1. using environmental predictors represented in 30 arc second study area rasters. These included nine landform, land cover and climatic variables that were thought a priori to have potentially high predictive power. We used the average training gain from ten model runs to select the best subset of predictors. Total winter precipitation, aspect and land cover, particularly broadleaf forests, emerged as important variables. A five-variable model that contained land cover, winter precipitation, aspect, slope, and elevation was the most parsimonious and not significantly different than the models with more variables. We used the best fitting model to depict potential winter habitat. Using the 10 percentile threshold (>0.25), we estimated winter habitat to cover 33,170 km2, nearly 10% of the study area. The Dominican Republic contained half of all potential habitat (51%), followed by Cuba (15.1%), Jamaica (13.5%), Haiti (10.6%), and Puerto Rico (9.9%). Nearly one-third of the range was found to be in protected areas. By providing the first detailed predictive map of Bicknell’s Thrush winter distribution, our study provides a useful tool to prioritize and direct conservation planning for this and other wet, broadleaf forest specialists in the Greater Antilles.

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 Bicknells 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 Bicknells Thrush in the Dominican Republic and that nocturnal arboreal rat predation could influence the thrushs space-use strategies.

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 Bicknells 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 females were in poorer body condition than rainforest females. Females at the cloud forest site may face a greater likelihood of agonistic interactions with larger-bodied males and the thermoregulatory demands of roosting in colder night temperatures (0–12°C). We suggest that there are sex-specific advantages of wintering in these two habitats and that both are critical to supporting the full demographic structure of Bicknells Thrush populations. Rainforest habitats, which are highly vulnerable to agricultural development in the Dominican Republic, may be particularly important to female survival during the winter period.