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Anthropogenic disturbance contributes to global change by reshaping the ecological niche space available to biological communities. Quantifying the range of functional response traits required for species persistence is central towards understanding the mechanisms underlying community disassembly in disturbed landscapes. We used intensive field surveys of cloud forest bird communities across seven replicate landscapes undergoing agricultural conversion in the Peruvian Andes to examine how a suite of 16 functional response traits related to morphology, diet, foraging behaviour and environmental niche breadth predict (1) species-specific abundance changes in countryside habitats compared to forest and (2) differential changes to the ecological niche space occupied by communities. Our analyses relied on (1) hierarchical distance sampling models to examine the functional predictors of abundance change across the agricultural land use gradient while accounting for imperfect detection and (2) n-dimensional hypervolumes to quantify the expansion and contraction of ecological niche space in countryside habitats. Key traits related to increased abundance in early successional and mixed-intensity agricultural areas included (1) morphological adaptations to dense understorey habitats, (2) plant-based diets (flowers, fruit and seeds) and (3) broad elevational range limits and habitat breadth. Species occupying mixed and high-intensity agricultural land use regimes had mean elevational range limits 20%-60% wider than species found within forests. Collectively, ecological niche space expanded within agricultural habitats for traits related to diet and environmental niche breadth, while contracting for foraging and dispersal traits. Such changes were driven by species with unique functional trait combinations. Our results reveal the dynamic changes to ecological niche space that underly community structure in disturbed landscapes and highlight how increased niche breadth can ameliorate disturbance sensitivity for generalist species. We emphasize that functional traits can be used to predict changes in community structure across disturbance gradients, allowing insights into specific mechanisms underlying community disassembly beyond emergent patterns of functional diversity. By identifying key functional trait groups that align with different countryside habitats, we demonstrate how conservation practitioners can contribute to the retention of avian functional diversity in agricultural landscapes throughout the world.La perturbación antropogénica contribuye al cambio global al remodelar el espacio de nicho ecológico disponible para las comunidades biológicas. Cuantificar la gama de rasgos de respuesta funcional requeridos para la persistencia de las especies es fundamental para comprender los mecanismos que subyacen al desensamble de la comunidad en los paisajes perturbados. Utilizamos muestreos de campo intensivos de las comunidades de aves del bosque nublado en siete paisajes replicados convertidos a uso agrícola en los Andes peruanos para examinar cómo un conjunto de 16 rasgos de respuesta funcional relacionados con la morfología, dieta, comportamiento de forrajeo, y la amplitud del nicho ambiental predicen (1) cambios en las abundancias de especies específicas en paisajes agrícolas (“countrysides”) en comparación con el bosque y (2) cambios diferenciales en el espacio del nicho ecológico ocupado por las comunidades. Nuestros análisis se basaron en (1) modelos jerárquicos de muestreo por la distancia para examinar los predictores funcionales del cambio de abundancia a través del gradiente de uso de suelo para agricultura teniendo en cuenta la detección imperfecta, y (2) “n-dimensional hypervolumes” para cuantificar la expansión y contracción del espacio de nicho ecológico en los hábitats agrícolas. Los rasgos clave relacionados con el aumento de la abundancia en áreas agrícolas de sucesión temprana y de intensidad mixta incluyeron (1) adaptaciones morfológicas a hábitats de sotobosque denso, (2) dietas basadas en plantas (flores, frutas y semillas), y (3) amplios límites de rango de elevación y amplitud de hábitat. Las especies que ocupan regímenes de suelo agrícola mixto y de alta intensidad tenían límites de rango de elevación promedio 20%-60% más amplios que las especies que se encuentran en los bosques. En conjunto, el espacio del nicho ecológico se expandió dentro de los hábitats agrícolas para los rasgos relacionadas con la dieta y la amplitud del nicho ambiental, mientras que se contrajo para los rasgos de forrajeo y dispersión. Dichos cambios fueron impulsados por especies con combinaciones de rasgos funcionales únicos. Nuestros resultados revelan los cambios dinámicos en el espacio del nicho ecológico que subyacen a la estructura de la comunidad en los paisajes perturbados y destacan cómo una mayor amplitud del nicho puede mejorar la sensibilidad a las perturbaciones para las especies generalistas. Enfatizamos que los rasgos funcionales pueden utilizarse para predecir los cambios en la estructura de la comunidad a través de gradientes de perturbación, lo que permite comprender los mecanismos específicos que subyacen al desensamble de la comunidad más allá de los patrones emergentes de diversidad funcional. Al identificar grupos de rasgos funcionales claves que se alinean con diferentes hábitats agrícolas como en “countrysides” demostramos cómo los profesionales de la conservación pueden contribuir a la retención de la diversidad funcional de las aves en los paisajes agrícolas en todo el mundo.

The role of light in partitioning ecological niche space remains a frontier in understanding the assembly of terrestrial vertebrate communities and their response to global change. Leveraging recent advances in biologging technology and intensive field surveys of cloud forest bird communities across an agricultural land use gradient in the Peruvian Andes, we demonstrate that eye size predicts (1) the ambient light microenvironment used by free-ranging birds, (2) their foraging niche, and (3) species-specific sensitivity to agricultural land use change. For 15 species carrying light sensors (N = 71 individuals), light intensity levels were best explained by eye size and foraging behavior, with larger-eyed species using darker microenvironments. Across the cloud forest bird community (N = 240 species), hyperopic ("far-sighted") foragers, (e.g., flycatchers), had larger eyes compared to myopic ("near-sighted") species (e.g., gleaners and frugivores); eye size was also larger for myopic insectivores that foraged in the forest understory. Eye size strongly predicted sensitivity to brightly lit habitats across an agricultural land use gradient. Species that increased in abundance in mixed intensity agriculture, including fencerows, silvopasture, and pasture, had smaller eyes, suggesting that light acts as an environmental filter when communities disassemble in a human-disturbed landscape. We suggest that eye size represents a novel functional trait contributing to terrestrial vertebrate community assembly and sensitivity to habitat disturbance.

Birds require additional resources for raising young, and the breeding currency hypothesis predicts that insectivorous species exploit large soft-bodied prey during the breeding season, but shift to small, likely hard-bodied, prey during the non-breeding season. To test this hypothesis, we examined prey use by Cerulean Warblers (Setophaga cerulea), foliage-gleaning Nearctic-Neotropical migrants, during the breeding and non-breeding seasons. We collected data on foraging behavior during the breeding season (including observations of prey items fed to young) in upland mixed-oak forest in southeastern Ohio in 2009 and 2010 and, during the non-breeding season, in shade coffee in the Cordillera de Merida, Venezuela, in 2008–2009. Cerulean Warblers captured 7% more large prey (visible prey extending beyond the bill) during the breeding than the non-breeding season, but foraged at similar rates during both seasons. Large, soft-bodied prey appeared to be especially important for feeding young. We found that adults delivered large prey on >50% of provisioning visits to nests and 69% of identifiable large prey fed to nestlings were greenish larvae (likely Lepidoptera or caterpillars) that camouflage against leaves where they would tend to be captured by foliage-gleaning birds. Availability of specific taxa appeared to influence tree species foraging preferences. As reported by other researchers, we found that Cerulean Warblers selected trees in the genus Carya for foraging and our examination of caterpillar counts from the central Appalachian Mountains (Butler and Strazanac 2000) showed that caterpillars with greenish coloration, especially Baileya larvae, may be almost twice as abundant on Carya than Quercus. Our results provide evidence for the breeding currency hypothesis, and highlight the importance of caterpillars to a foliage-gleaning migrant warbler of conservation concern. RESUMEN Comportamiento forrajero del Chipe Ceruleo durante la temporada reproductiva y no reproductiva: evidencia de la hipotesis de divisa de crianza Las aves requieren recursos adicionales a fin de criar a los pichones, y la hipotesis de divisa de crianza predice que las especies insectivoras explotan presas grandes de cuerpo blando durante la temporada reproductiva, pero que cambian a presas pequenas de cuerpo duro en la temporada no reproductiva. A fin de poner a prueba esta hipotesis, examinamos el uso de presas por parte del Chipe Ceruleo (Setophaga cerulea), forrajeadores de follaje y migrantes Neartico-Neotropico, durante la temporada reproductiva y no reproductiva. Colectamos datos del comportamiento forrajero durante la temporada reproductiva (incluyendo observaciones de presas entregadas a los pichones) en un bosque de tierras altas compuesto de varias especies de roble en el sur este de Ohio en 2009 y 2010 y, durante la temporada no reproductiva, en una plantacion sombreada de cafe en la Cordillera de Merida, Venezuela, en 2008–2009. Los Chipes Ceruleos capturaron presas un 7% mas grandes (presa visible extendiendose mas alla del pico) durante la temporada reproductiva que durante la temporada no reproductiva, pero forrajearon a igual tasa durante ambas temporadas. Las presas grandes y de cuerpo blando parecian ser especialmente importantes para alimentar a los pichones. Encontramos que los adultos entregaban presas en mas del 50% de las visitas de entrega de presas al nido y el 69% de las presas identificables entregadas a los pichones era larvas verduzcas (probablemente Lepidoptera u orugas) que se camuflaban contra las hojas donde tendian a ser capturadas por aves forrajeadoras de follaje. La disponibilidad de taxas especificos parece influenciar las preferencias en el forrajeo arboreo. Como ha sido reportado por otros investigadores, encontramos que los Chipes Ceruleos seleccionaron arboles dentro de genero Carya para el forrajeo y nuestro conteo de orugas del centro de las Montanas Apalaches (Butler and Strazanac 2000) mostraron que las orugas con coloracion verduzcas, especialmente las larvas de Baileya, deben ser al menos el doble de abundantes en Carya que en Quercus. Nuestros resultados proveen evidencia a la hipotesis de divisa de crianza, y resaltan la importancia de las orugas en un chipe forrajeador de follaje y migrante con problemas de conservacion.

Timber harvesting has been proposed as a management tool to enhance breeding habitat for the Cerulean Warbler (Setophaga cerulea), a declining Neotropical–Nearctic migratory songbird that nests in the canopy of mature eastern deciduous forests. To evaluate how this single-species management focus might fit within an ecologically based management approach for multiple forest birds, we performed a manipulative experiment using four treatments (three intensities of timber harvests and an unharvested control) at each of seven study areas within the core Cerulean Warbler breeding range. We collected pre-harvest (one year) and post-harvest (four years) data on the territory density of Cerulean Warblers and six additional focal species, avian community relative abundance, and several key habitat variables. We evaluated the avian and habitat responses across the 3–32 m2 ha−1 residual basal area (RBA) range of the treatments. Cerulean Warbler territory density peaked with medium RBA (∼16 m2 ha−1). In contrast, territory densities of the other focal species were negatively related to RBA (e.g., Hooded Warbler [Setophaga citrina]), were positively related to RBA (e.g., Ovenbird [Seiurus aurocapilla]), or were not sensitive to this measure (Scarlet Tanager [Piranga olivacea]). Some species (e.g., Hooded Warbler) increased with time post-treatment and were likely tied to a developing understory, whereas declines (e.g., Ovenbird) were immediate. Relative abundance responses of additional species were consistent with the territory density responses of the focal species. Across the RBA gradient, greatest separation in the avian community was between early successional forest species (e.g., Yellow-breasted Chat [Icteria virens]) and closed-canopy mature forest species (e.g., Ovenbird), with the Cerulean Warbler and other species located intermediate to these two extremes. Overall, our results suggest that harvests within 10–20 m2 ha−1 RBA yield the largest increases in Cerulean Warblers, benefit additional disturbance-dependent species, and may retain closed-canopy species but at reduced levels. Harvests outside the optimum RBA range for Cerulean Warblers can support bird assemblages specifically associated with early or late (closed-canopy) successional stages.

Shade coffee has been identified as an important habitat for Nearctic-Neotropical migrants during the non-breeding season, including species of conservation concern such as Cerulean Warblers (Setophaga cerulea). To better understand habitat features important for migrants in shade coffee, we studied the foraging behavior of migrants in mixed-species flocks at six shade-coffee farms in the Cordillera de Merida, Venezuela, in 2008–2009 and the El Nino drought year of 2009–2010. We examined interspecific differences in foraging behavior and tree species selection of three foliage-gleaning migrants, Blackburnian (Setophaga fusca), Cerulean, and Tennessee (Oreothlypis peregrina) warblers, and aerial-foraging American Redstarts (Setophaga ruticilla). For morphologically similar Blackburnian and Cerulean warblers, we also examined factors influencing foraging rates (attack and movement rates), capture of large prey, and maneuver/substrate type. We found that aerial-foraging American Redstarts foraged lower, used more aerial maneuvers, showed no tree species selection, and were less likely to forage in flocks than foliage-gleaners. Although foraging rates were similar for Blackburnian and Cerulean warblers, the three foliage-gleaners differed in foraging height and use of maneuvers. Cerulean Warblers foraged lower than the other two species, whereas Blackburnian Warblers used the greatest proportion of woody gleans. All three foliage-gleaners selected Inga spp. (a commonly planted shade tree in shade-coffee farms) for foraging, and Blackburnian and Cerulean warblers captured a greater proportion of large prey in Inga spp. than in other tree species. During the drought year, Blackburnian and Cerulean warblers captured half as many large prey and used a greater proportion of woody-gleans. We found that interactions among behavioral, floristic, and environmental drivers influenced the foraging behavior of migrants wintering in shade coffee. Our results support those of previous studies suggesting that migrants partition resources behaviorally during the non-breeding season, that foliage-gleaners may benefit from the presence of shade trees, especially Inga spp., in agroforestry systems, and that drought may influence the foraging behavior of foliage-gleaning migrants, presumably due to reduced prey availability. RESUMEN Comportamiento de forrajeo de reinitas migrantes en bandadas mixtas en cafe bajo sombra en Venezuela: diferencias inter-especificas, seleccion de especies de arboles y efectos de sequias El cafe bajo sombra ha sido identificado como un habitat importante para especies migratorias neartico-neotropicales durante la epoca no reproductiva, incluyendo especies con importancia para la conservacion como Setophaga cerulea. Con el fin de obtener un mejor entendimiento de las caracteristicas del habitat importantes para las especies migratorias en los cafes bajo sombra, estudiamos el comportamiento de forrajeo de migrantes en bandadas mixtas de especies, en seis fincas con cafe bajo sombra en la Cordillera de Merida, Venezuela, en 2008–2009 y durante los anos de sequia por El Nino en 2009–2010. Examinamos diferencias inter-especificas en los comportamientos de forrajeo y seleccion de especies de arboles en tres especies “buscadoras de follaje/foliage gleaners”, Setophaga fusca, Setophaga cerulea y Oreothlypis peregrina, ademas de una especie con comportamiento de “forrajeo aereo/aerial foraging”, Setophaga ruticilla. Para especies que son similares morfologicamente como Setophaga fusca y cerulea, examinamos tambien los factores que influencian las tasas de forrajeo (tasas de ataque y movimiento), captura de presas grandes y tipo de maniobra/substrato. Encontramos que Setophaga ruticilla forrajea mas bajo y utiliza mas maniobras aereas, no mostro ningun patron en la seleccion de especies de arboles y fue menos propensa a forrajear en bandadas mixtas que las buscadoras de follaje. A pesar que las tasas de forrajeo fueron similares en S. fusca y S. cerulea, las tres especies “buscadoras de follaje” difieren en la altura de forrajeo y uso de maniobras. S. cerulea forrajea mas bajo que las otras dos especies, mientras que S. fusca usa una mayor proporcion de estrategias de“busqueda en madera/woody-gleans”. Las tres especies buscadoras de follaje seleccionaron activamente Inga spp. (una especie de arbol utilizada comunmente como sobra en fincas de cafe bajo sombra) para forrajeo y S. fusca y S. cerulea capturaron una mayor proporcion de presas grandes en Inga spp. que las otras tres especies. Durante el ano de sequia, S. fusca y S. cerulea capturaron la mitad de presas grandes en comparacion con anos anteriores y utilizaron en una mayor proporcion estrategias de busqueda en madera. Nuestros resultados sugieren que interacciones entre caracteristicas de forrajeo, floristicas y del ambiente influencian el comportamiento de forrajeo de especies migratorias durante la epoca de invierno en cafe bajo sombra, y soportan los resultados de estudios previos que indican que las especies migratorias reparten los recursos mediante diferencias en el comportamiento durante las epocas no reproductivas. Nuestros resultados sugieren tambien que las especies buscadoras de follaje se pueden beneficiar de la presencia de arboles de sombrio, especialmente Inga spp., en sistemas agroforestales, y que la sequia puede, probablemente debido a la reduccion en la abundancia de presas, influenciar el comportamiento de forrajeo de especies migratorias buscadoras de follaje.

Point counts are commonly used to assess changes in bird abundance, including analytical approaches such as distance sampling that estimate density. Point-count methods have come under increasing scrutiny because effects of detection probability and field error are difficult to quantify. For seven forest songbirds, we compared fixed-radii counts (50 m and 100 m) and density estimates obtained from distance sampling to known numbers of birds determined by territory mapping. We applied point-count analytic approaches to a typical forest management question and compared results to those obtained by territory mapping. We used a before–after control impact (BACI) analysis with a data set collected across seven study areas in the central Appalachians from 2006 to 2010. Using a 50-m fixed radius, variance in error was at least 1.5 times that of the other methods, whereas a 100-m fixed radius underestimated actual density by >3 territories per 10 ha for the most abundant species. Distance sampling improved accuracy and precision compared to fixed-radius counts, although estimates were affected by birds counted outside 10-ha units. In the BACI analysis, territory mapping detected an overall treatment effect for five of the seven species, and effects were generally consistent each year. In contrast, all point-count methods failed to detect two treatment effects due to variance and error in annual estimates. Overall, our results highlight the need for adequate sample sizes to reduce variance, and skilled observers to reduce the level of error in point-count data. Ultimately, the advantages and disadvantages of different survey methods should be considered in the context of overall study design and objectives, allowing for trade-offs among effort, accuracy, and power to detect treatment effects.

We document the first confirmed cases of polygyny and double brooding in the Eastern Wood-Pewee (Contopus virens). During an intensive study of the effects of shelterwood harvesting on canopy-nesting songbirds in southeastern Ohio, 2007–2010, we color-banded 79 Eastern Wood-Pewees and monitored 237 pewee nests. In 2007, we confirmed a color-banded male provisioning at two concurrently active nests; the male was polygynous in at least two consecutive years. In 2009, we observed an unbanded female feeding fledglings and subsequently shaping a nest from which young had recently fledged; the female successfully fledged two broods from the same nest. In addition to confirmed observations, we identified several other probable cases of polygyny and double brooding. In our upland oak system, we estimated rates of polygyny from 6–22% with the greatest occurrence of polygyny during a dry spring in 2007; rates of double brooding may have been as high as 6–12%. Males appeared to benefit from polygyny, becaus...

Studies of habitat selection are often of limited utility because they focus on small geographic areas, fail to examine behavior at multiple scales, or lack an assessment of the fitness consequences of habitat decisions. These limitations can hamper the identification of successful site-specific management strategies, which are urgently needed for severely declining species like Cerulean Warblers (Setophaga cerulea). We assessed how breeding habitat decisions made by Cerulean Warblers at multiple scales, and the subsequent effects of these decisions on nest survival, varied across the Appalachian Mountains. Selection for structural habitat features varied substantially among areas, particularly at the territory scale. Males within the least-forested landscapes selected microhabitat features that reflected more closed-canopy forest conditions, whereas males in highly forested landscapes favored features associated with canopy disturbance. Selection of nest-patch and nest-site attributes by female...

Interest in regenerating oaks (Quercus spp.) has promoted use of partial harvesting techniques that create an open forest structure. From 2007 to 2009, we studied songbirds in mixed-oak forests in southeastern Ohio, comparing shelterwoods recently harvested to 50% stocking and closed-canopy mature second-growth. We surveyed birds using distance-based methods (56 line transects in 18 stands at 4 forests). We intensively investigated suitability of shelterwoods for canopy-nesting species by examining habitat preferences, as measured by settlement patterns, age distributions, and site fidelity; we also examined nesting success. Several midstory and ground-nesting species were 26–73% less abundant in shelterwood than unharvested stands, whereas shrub-nesting species increased >100% several years post-harvesting. Canopy-nesting species were 31–98% more abundant in shelterwoods, but cerulean warbler (Setophaga cerulea) responses varied by forest. Patterns of settlement and site fidelity were generally similar among stands. Proportions of young males were actually greater for several species in shelterwood than unharvested stands, which may have been a consequence of young birds colonizing newly created (or improved) habitat. Even in our predominantly forested study system, nesting success (>700 nests) was low, ranging from 15% to 19% for yellow-throated vireos (Vireo flavifrons) and cerulean warblers, to 27–36% for scarlet tanagers, blue-gray gnatcatchers (Polioptila caerulea) and eastern wood-pewees (Contopus virens). However, nest survival did not differ between shelterwood and unharvested stands, possibly because numbers of avian predators did not change with harvesting. Despite increased numbers of brown-headed cowbirds (Molothrus ater) in shelterwoods, only 2% of canopy nests in which young could be identified were parasitized. Although these results suggest shelterwood harvests containing abundant overstory trees can provide short-term breeding habitat for canopy songbirds, long-term responses of birds to partial harvesting may differ from those documented here depending on different management options employed. Management for oak regeneration will typically remove all overstory trees later in the cutting cycle, initially resulting in loss of nesting substrates and hence breeding habitat for canopy songbirds. © 2011 The Wildlife Society.

Although oaks (Quercus spp.) have historically dominated many forests in eastern North America, forest composition is changing due to anthropogenic impacts on disturbance regimes. Silvicultural practices, such as partial harvesting, are one component of management to promote oak regeneration. From 2007 to 2009 our research examined nest-site selection and nesting success for a guild of five canopy songbirds in upland mixed-oak forests in southeastern Ohio, USA. We monitored >700 nests across three state forests in both open canopy shelterwood stands harvested to approximately 50% stocking, and closed-canopy mature second-growth. Habitat attributes, including topography, canopy structure, and floristics, were measured at nest sites and random plots ⩽100 m from nests representing microhabitat available within the territory. Canopy songbirds selected specific topographic microclimates: Eastern Wood-pewees (Contopus virens) nested on xeric ridgetops, Blue-gray Gnatcachers (Polioptila caerulea) favored valleys, and Cerulean Warblers (Dendroica cerulea) preferred productive northeast-facing slopes. Nest sites differed among species in terms of concealment, nest support, topographic position on the slope, and basal area of trees >38 cm dbh. Four of the five focal species selected Quercus alba as the nest substrate more than twice as much as available, and three species avoided Quercus rubra. Daily survival rates of nests were negatively associated with basal area of red oak species (both Quercus velutina and Q. rubra) for several species individually and across the canopy-nesting guild. Additional factors related to success included time of season for Eastern Wood-pewees, nest age for Cerulean Warblers, and concealment and size of the support branch for Scarlet Tanagers (Piranga olivacea). In the long-term management for oak regeneration could benefit canopy songbirds, but our results indicate that white oaks, especially Q. alba, may be preferable to red oak species.

We studied the nesting of the Louisiana Waterthrush (Seiurus motacilla) in western Pennsylvania from 1996 to 2008. Eight of 143 pairs (5.6%) that successfully fledged young from their first nest attempted a second brood, the first cases of double brooding reported for this species. Given the potential for double brooding to increase annual productivity, its infrequency suggests strong costs offsetting this life-history strategy. Molting is a late-season constraint that may influence the occurrence of double brooding in this migratory species. The waterthrushes' nesting season extended from 14 April to 30 July. A single nesting cycle, from clutch initiation through post-fledging care, encompassed 48–55 days. The mean date of clutch initiation for first attempts of single-brooded pairs was 5 May, with at least two thirds of pairs renesting if their first nest attempt failed; the mean date of initiation of second clutches was 25 May. The mean dates of first and second clutches of eight double-brood...

We used Mayfield logistic regression and an information-theoretic approach to examine habitat characteristics associated with nesting success of Wood Thrushes (Hylocichla mustelina) across an urban to forested gradient in southwestern Pennsylvania in 2003 and 2004. Both nest placement and number of understory stems provided equally plausible models. Mayfield success was 20% higher for nests >3 m above ground level while dense understory was associated with low nest height. Wood Thrush nests in the forest interior averaged 2 m higher with a third less understory than edge nests. Urbanization and distance to the forest edge were not useful predictors of Wood Thrush nest success. The analysis was confounded by low breeding density at the most urbanized sites, but we found moderate success (42%, n = 63) across a fragmented landscape with minimal core forest area. Interior nests in a large contiguous forest were twice as successful (60%, n = 31) compared to edge nests (25%, n = 33) adjacent to a small...

We documented the juvenal plumage of the Green-breasted Mountain-gem (Lampornis sybillae) during mist-netting operations in the cloud forest at La Tigra National Park, Honduras from February to April 2006. A recently-fledged juvenile of this species was caught on 17 March and, contrary to previous suggestions, we found the throat and breast were mottled green. Ninety-eight immature mountain-gems intermediate between this juvenal plumage and that of adults were also caught during our study. Both males and females of the Green-breasted Mountain-gem apparently begin replacing juvenal throat feathers soon after fledging and prior to molting flight feathers. A high capture rate of young hummingbirds at the end of the dry season, including recent fledglings and individuals showing only traces of juvenal plumage, suggests a protracted breeding season that we estimate to last at least from November through March. We also caught adults in a variety of stages of flight-feather molt, perhaps part of a trans...

Summary 1. We compared breeding ecology of the Louisiana waterthrush (Seiurus motacilla) on acidified and circumneutral streams in the Appalachian Highlands of Southwestern Pennsylvania from 1996 to 2005. 2. Headwater streams impacted by acid mine drainage and/or acidic precipitation showed reduced pH (range 4.5–5.5) compared to four circumneutral streams (pH c. 7). Acid-sensitive taxa, including most mayflies (Ephemeroptera), were almost completely absent from acidified streams, whereas several acid-tolerant taxa, especially stonefly (Plecoptera) genera Leuctra and Amphinemura, were abundant. 3. Louisiana waterthrush breeding density (c. 1 territory km−1) was significantly reduced on acidified streams compared to circumneutral streams (>2 territories km−1). Territories on acidified streams were almost twice as long as on circumneutral streams. Territories usually were contiguous on circumneutral streams, but they were often disjunct on acidified streams. Breeding density declined on one acidified stream that we studied over a 10-year period. 4. Clutch initiation was significantly delayed on acidified streams, on average by 9 days in comparison to circumneutral streams, and first-egg dates were inversely related to breeding density. Birds nesting along acidified streams laid smaller clutches, and nestlings had shorter age-adjusted wing lengths. Stream acidity had no effect on nest success or annual fecundity (fledglings/female). However, the number of young fledged km−1 was nearly twice as high on circumneutral streams as on acidified streams. 5. Acidified streams were characterized by a younger, less site-faithful breeding population. Individuals were less likely to return multiple years to breed, allowing inexperienced breeders to settle on acidified streams. Pairing success was lower on acidified streams, and we observed four cases of waterthrushes emigrating from territories on acidified streams to nearby circumneutral streams in the following year. 6. We conclude that acidified headwaters constitute lower quality habitat for breeding Louisiana waterthrush. However, breeding birds can apparently compensate for reduced prey resources to fledge young on acidified streams by increasing territory size, foraging in peripheral non-acidified areas, and by provisioning young with novel prey.