Christopher C. Rimmer

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.

Ancient differentiation in the single-island avian radiation of endemic Hispaniolan chat-tanagers (Aves: Calyptophilus)

The simple geographic structure of island systems often makes them tractable for studies of the patterns and processes of biological diversification. The Calyptophilus chat‐tanagers of Hispaniola are of general evolutionary interest because their multiple lineages might have arisen on a single island, of conservation concern because several isolated populations are nearly extinct, and taxonomically ambiguous because they have been variously lumped or split into one to four species. To explore the context of diversification of the seven extant Calyptophilus populations, we conducted a multilocus coalescent analysis based on sequences of mitochondrial ND2 and three nuclear intron loci. We then compared patterns of phylogeographic genetic variation with the morphological differences that distinguish these populations. Mitochondrial haplotypes formed two reciprocally monophyletic groups separated by a large magnitude of nucleotide divergence. Intron structure largely paralleled the geographic grouping pattern of the mitochondrial DNA (mtDNA), but these groups were only reciprocally monophyletic at one of the three introns. Also, the magnitude of between‐group divergence was much lower in the introns than mtDNA genealogies. Multilocus coalescent analyses inferred a nonzero divergence time between these two major geographic groups, but suggested that they have experienced a low level of gene flow. All four markers showed substantial allele sharing within each of the two groups, demonstrating that many now separated montane populations do not have long histories of isolation. Considered in concert, our multilocus phylogeographic reconstructions support the recognition of two species within the Calyptophilus complex, and raise the possibility that these taxa differentiated prior to the fusion of the two palaeo‐islands that form present‐day Hispaniola.

Timing of the Definitive Prebasic Molt in Yellow Warblers at James Bay, Ontario

The relationship between breeding, molt, and migration of Yellow Warblers (Dendroica petechia) was studied on the southwest coast of James Bay, Ontario from 1982 to 1984. Breeding chronology was similar in each year, with a mean fledging date of 11 to 12 July. Color-banded individuals regularly overlapped molt and care of fledglings. Knownbreeding males began remigial molt 8.1 days after young fledged (range = 1-19 days), females 12.3 days (range = 9-15 days). The population mean date of molt onset was 17 July for males, and 22 July for females. Females molted significantly more rapidly than males, and both sexes completed molt on the mean date of 28 August. There were marked behavioral differences between the sexes during molt. Males typically remained on territories, advertising their presence with song. Females showed little site attachment, wandering over the study area or dispersing. Both sexes gained weight during the early and middle periods of molt, but lost weight prior to migration. Most birds began migrating during the final stages of remigial molt. Depletion of insect food and deteriorating weather in late summer appear to have selected for a rapid, intense molt of 35 to 45 days, during which some birds may become virtually flightless. The importance of maintaining thermoregulatory abilities and high rates of feeding may constrain further reduction in molt duration. The need for maintaining peak flight efficiency during fledgling care and migration may have partially determined the scheduling of molt in this population.

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.

Timing and extent of the molts of adult red-eyed vireos on their breeding and wintering grounds

The molt of adult Red-eyed Vireos (Vireo olivaceus) on their breeding grounds was studied at three widely separated sites in northeastern North America: North Point, James Bay (NPJB), Ontario; Vermont Institute of Natural Science (VINS), Woodstock, Vermont; and Powdermill Nature Reserve (PNR), Rector, Pennsylvania. Unlike other North American vireos and most other North American passerines, adult Red-eyed Vireos undergo an incomplete molt on the breeding grounds. This molt typically replaces most or all of the body plumage, about half of the wing coverts, the tertials, and the central rectrices. Extent of molt did not differ among localities; however, timing of the molt was 1-2 weeks earlier and onset of molt occurred over a longer period at PNR than at VINS or NPJB. Based on examination of museum specimens, flight feathers not molted on the breeding grounds are replaced on the wintering grounds beginning in early January ; average duration of primary molt on the wintering grounds is 78 days. It is unclear whether or not feathers molted on the breeding grounds are molted again at this time. This issue needs to be resolved before Humphrey-Parkes terminology can be applied to breeding and wintering ground molt episodes in this species. Selective advantages associated with a protracted nesting season and with early onset of migration both may have had a role in shaping the molt strategy of adult Red-eyed Vireos. Rapid onset of fall migration following nesting in this seasonally frugivorous species may have been influenced by the phenology of fruit availability and abundance en route to and especially on its Amazonian wintering grounds.

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.