Investigating flock-associated mimicry: examining the evidence for, and drivers of, plumage mimicry in the greater and lesser necklaced laughingthrush.

Visual mimicry is less understood in birds than in other taxa. The interspecific social dominance mimicry (ISDM) hypothesis asserts that subordinate species resemble dominant ones to reduce aggression. Plumage mimicry has also been consistently noted in mixed-species flocks (MSFs), suggesting a connection to grouping behaviour, although it is unclear whether this is linked to ISDM. We studied greater necklaced laughingthrush (GNLT, Pterorhinus pectoralis) and lesser necklaced laughingthrush (LNLT, Garrulax monileger), which were recently placed in different genera. Measurements of 162 museum specimens showed LNLT converging in sympatry with GNLT in necklace colour, but diverging in necklace to body ratio, with proportionally smaller necklaces. The species were closely associated in six of seven MSF systems from Nepal to China. In a study of foraging behaviour in Nepal, aggression was rare between the species, LNLT followed GNLT and had lower foraging rates when further from GNLT. Our data suggest a link between this MSF-associated mimicry and ISDM, and that the subordinate LNLT may be the mimic and gain more from the resemblance. The species spend much time together in dense and poorly lit vegetation, where the LNLTs resemblance to GNLTs potentially allows them to forage closer to GNLTs than would be otherwise possible.

Conservation: Different seasonal hotspots for migrating birds.

Migrating species are particularly vulnerable to habitat loss. A new study shows that migrating birds use seasonally different stopover hotspots, which need to be protected better.

High association strengths are linked to phenotypic similarity, including plumage color and patterns, of participants in mixed-species bird flocks of southwestern China.

Participants in mixed-species bird flocks (MSFs) have been shown to associate with species that are similar in body size, diet, and evolutionary history, suggesting that facilitation structures these assemblages. In addition, several studies have suggested that species in MSFs resemble each other in their plumage, but this question has not been systematically investigated for any MSF system. During the nonbreeding season of 2020 and 2021, we sampled 585 MSFs on 14 transects in 2 habitats of Tongbiguang Nature Reserve in western Yunnan Province, China. We performed social network analysis and the Multiple Regression Quadratic Assignment Procedure to evaluate the effect of 4 species traits (body size, overall plumage color, distinctive plumage patterns, and diet) and evolutionary history on species association strength at the whole-MSF and within-MSF levels. All 41 significant relationships showed that species with stronger associations were more similar in their various traits. Body size had the strongest effect on association strength, followed by phylogeny, plumage patterns, and plumage color; diet had the weakest effect. Our results are consistent with the hypotheses that the benefits of associating with phenotypically similar species outweigh the potential costs of interspecific competition, and that trait matching can occur in plumage characteristics, albeit more weakly than in other traits. Several explanations exist as to why similarities in plumage may occur in MSFs, including that they could reduce predators' ability to target phenotypically "odd" individuals. Whether trait matching in plumage occurs through assortative processes in ecological time or is influenced by co-evolution requires further study.

Wild mockingbirds distinguish among familiar humans.

Although individuals of some species appear able to distinguish among individuals of a second species, an alternative explanation is that individuals of the first species may simply be distinguishing between familiar and unfamiliar individuals of the second species. In that case, they would not be learning unique characteristics of any given heterospecific, as commonly assumed. Here we show that female Northern Mockingbirds (Mimus polyglottos) can quickly learn to distinguish among different familiar humans, flushing sooner from their nest when approached by people who pose increasingly greater threats. These results demonstrate that a common small songbird has surprising cognitive abilities, which likely facilitated its widespread success in human-dominated habitats. More generally, urban wildlife may be more perceptive of differences among humans than previously imagined.

Sensitivity of tropical montane birds to anthropogenic disturbance and management strategies for their conservation in agricultural landscapes.

Tropical montane bird communities are hypothesized to be highly sensitive to anthropogenic disturbance because species are adapted to a narrow range of environmental conditions and display high rates of endemism. We assessed avian sensitivity at regional and continental scales for a global epicenter of montane bird biodiversity, the tropical Andes. Using data from an intensive field study of cloud forest bird communities across 7 landscapes undergoing agricultural conversion in northern Peru (1800-3100 m, 2016-2017) and a pan-Andean synthesis of forest bird sensitivity, we developed management strategies for maintaining avian biodiversity in tropical countrysides and examined how environmental specialization predicts species-specific sensitivity to disturbance. In Peru, bird communities occupying countryside habitats contained 29-93% fewer species compared with those in forests and were compositionally distinct due to high levels of species turnover. Fragments of mature forest acted as reservoirs for forest bird diversity, especially when large or surrounded by mixed successional vegetation. In high-intensity agricultural plots, an addition of 10 silvopasture trees or 10% more fencerows per hectare increased species richness by 18-20%. Insectivores and frugivores were most sensitive to disturbance: abundance of 40-70% of species declined in early successional vegetation and silvopasture. These results were supported by our synthesis of 816 montane bird species studied across the Andes. At least 25% of the species declined due to all forms of disturbance, and the percentage rose to 60% in agricultural landscapes. The most sensitive species were those with narrow elevational ranges and small global range sizes, insectivores and carnivores, and species with specialized trophic niches. We recommend protecting forest fragments, especially large ones, and increasing connectivity through the maintenance of early successional vegetation and silvopastoral trees that increase avian diversity in pastures. We provide lists of species-specific sensitivities to anthropogenic disturbance to inform conservation status assessments of Andean birds.

Sensibilidad de aves montanas a perturbaciones antropogénicas y estrategias de manejo para su conservación en paisajes agrícolas Resumen Se ha hipotetizado que las comunidades de aves tropicales montanas son sumamente sensibles a la perturbación antropogénica porque las especies están adaptadas a una reducida gama de condiciones ambientales y tienen altas tasas de endemismo. Evaluamos la sensibilidad aviar a escalas regional y continental para un epicentro global de biodiversidad de aves montanas, los Andes tropicales. Utilizando datos de un estudio intensivo de campo de comunidades de aves de bosques nublados en 7 paisajes bajo conversión agrícola en el norte de Perú (1800 - 3100 m, 2016-2017) y una síntesis pan-Andina de sensibilidad de aves de bosque, desarrollamos estrategias de manejo para el mantenimiento de la biodiversidad de aves en campiñas tropicales y examinamos cómo la especialización ambiental predice la sensibilidad de cada especie a la perturbación. Las comunidades de aves ocupando hábitats campestres tropicales en Perú contenían 29 - 93% menos especies en comparación con las de bosques y tuvieron una composición distinta debido a los altos niveles de recambio de especies. Los fragmentos de bosque maduro fungieron como reservorios para la diversidad de aves de bosque, especialmente cuando eran extensos y estaban rodeados por vegetación secundaria mixta. En las parcelas con actividad agrícola intensiva, la adición de 10 árboles silvopastoriles o 10% más de cercos por hectárea incrementó la riqueza de especies en 18 - 20%. Las insectívoras y frugívoras fueron más sensibles a la perturbación: la abundancia de 40 - 70% de especies declinó en la vegetación secundaria temprana y en la silvopastura. Estos resultados fueron sustentados por nuestra síntesis de 816 especies de aves montanas estudiadas en los Andes. Por lo menos 25% de estas especies declinaron debido a todas las formas de perturbación, y el porcentaje incrementó a 60% en paisajes agrícolas. Las especies más sensibles fueron aquellas con rangos altitudinales estrechos y extensiones de distribución pequeñas, las insectívoras y carnívoras y las especies con nichos tróficos especializados. Recomendamos la protección de fragmentos de bosque, especialmente los extensos, y el incremento de la conectividad mediante el mantenimiento de vegetación secundaria temprana y árboles silvopastoriles que incrementan la diversidad de aves en los pastizales. Proporcionamos listas de la sensibilidad de cada especie a la perturbación antropogénica para contribuir a las evaluaciones del estatus de conservación de aves Andinas.

Network structure of avian mixed-species flocks decays with elevation and latitude across the Andes.

Birds in mixed-species flocks benefit from greater foraging efficiency and reduced predation, but also face costs related to competition and activity matching. Because this cost-benefit trade-off is context-dependent (e.g. abiotic conditions and habitat quality), the structure of flocks is expected to vary along elevational, latitudinal and disturbance gradients. Specifically, we predicted that the connectivity and cohesion of flocking networks would (i) decline towards tropical latitudes and lower elevations, where competition and activity matching costs are higher, and (ii) increase with lower forest cover and greater human disturbance. We analysed the structure of 84 flock networks across the Andes and assessed the effect of elevation, latitude, forest cover and human disturbance on network characteristics. We found that Andean flocks are overall open-membership systems (unstructured), though the extent of network structure varied across gradients. Elevation was the main predictor of structure, with more connected and less modular flocks upslope. As expected, flocks in areas with higher forest cover were less cohesive, with better defined flock subtypes. Flocks also varied across latitude and disturbance gradients as predicted, but effect sizes were small. Our findings indicate that the unstructured nature of Andean flocks might arise as a strategy to cope with harsh environmental conditions. This article is part of the theme issue 'Mixed-species groups and aggregations: shaping ecological and behavioural patterns and processes'.

A classification scheme for mixed-species bird flocks.

The literature on mixed-species flocks references a wide variety of bird associations. These studies, however, have used an array of unstructured characteristics to describe flocks, ranging from the temporal occurrence of flocking to the identity and behavioural features of constituent members, with little consensus on which key traits define and characterize a mixed-species flock. Moreover, although most studies report species-specific roles, there is no clear consensus about what these roles signify nor how to define them. This lack of consistency limits our ability to compare flocks from different habitats, regions and species pools. To unify this sizable body of literature, we reviewed and synthesized 538 studies on mixed-species flocks. We propose 13 categories to classify mixed-species flocks using behavioural and physical traits at the flock and participant level, as well as the habitat where the flock occurs. Lastly, we discuss the historical terminology for different species roles and propose definitions to clarify and distinguish among nuclear, leader, sentinel, and flock-following species. We envision that these guidelines will provide a universal language for mixed-species flock research, paving the way for future comparisons and new insight between different regions and systems. This article is part of the theme issue 'Mixed-species groups and aggregations: shaping ecological and behavioural patterns and processes'.

Can convergence in mixed-species flocks lead to evolutionary divergence? Evidence for and methods to test this hypothesis.

One of the most fundamental goals of modern biology is to achieve a deep understanding of the origin and maintenance of biodiversity. It has been observed that in some mixed-species animal societies, there appears to be a drive towards some degree of phenotypic trait matching, such as similar coloration or patterning. Here we build on these observations and hypothesize that selection in mixed-species animal societies, such as mixed-species bird flocks, may drive diversification, potentially leading to speciation. We review evidence for possible convergent evolution and even outright mimicry in flocks from southwestern China, where we have observed several cases in which species and subspecies differ from their closest relatives in traits that match particular flock types. However, understanding whether this is phenotypic matching driven by convergence, and whether this divergence has promoted biodiversity, requires testing multiple facets of this hypothesis. We propose a series of steps that can be used to tease apart alternative hypotheses to build our understanding of the potential role of convergence in diversification in participants of mixed-species societies. Even if our social convergence/divergence hypothesis is not supported, the testing at each step should help highlight alternative processes that may affect mixed-species flocks, trait evolution and possible convergence. This article is part of the theme issue 'Mixed-species groups and aggregations: shaping ecological and behavioural patterns and processes'.

Changing patterns of nest predation and predator communities along a tropical elevation gradient.

Tropical montane communities host the world's highest beta diversity of birds, a phenomenon usually attributed to community turnover caused by changes in biotic and abiotic factors along elevation gradients. Yet, empirical data on most biotic factors are lacking. Nest predation is thought to be especially important because it appears to be common and can change selective pressures underlying life history traits, which can alter competitive interactions. We monitored 2538 nests, 338 of which had known nest predators, to evaluate if nest predation changes along a tropical elevational gradient. We found that nest predation decreased with elevation, reflecting the loss of lowland predators that do not tolerate colder climates. We found different "super" nest predators at each elevation that accounted for a high percentage of events, suggesting that selection pressures exerted by nest predator communities may be less diffuse than has been hypothesized, at least for birds nesting in the understory.

The structure and organisation of an Amazonian bird community remains little changed after nearly four decades in Manu National Park.

Documenting patterns of spatiotemporal change in hyper-diverse communities remains a challenge for tropical ecology yet is increasingly urgent as some long-term studies have shown major declines in bird communities in undisturbed sites. In 1982, Terborgh et al. quantified the structure and organisation of the bird community in a 97-ha. plot in southeastern Peru. We revisited the same plot in 2018 using the same methodologies as the original study to evaluate community-wide changes. Contrary to longitudinal studies of other neotropical bird communities (Tiputini, Manaus, and Panama), we found little change in community structure and organisation, with increases in 5, decreases in 2 and no change in 7 foraging guilds. This apparent stability suggests that large forest reserves such as the Manu National Park, possibly due to regional topographical influences on precipitation, still provide the conditions for establishing refugia from at least some of the effects of global change on bird communities.

Wet and dry extremes reduce arthropod biomass independently of leaf phenology in the wet tropics.

Warming temperatures are increasing rainfall extremes, yet arthropod responses to climatic fluctuations remain poorly understood. Here, we used spatiotemporal variation in tropical montane climate as a natural experiment to compare the importance of biotic versus abiotic drivers in regulating arthropod biomass. We combined intensive field data on arthropods, leaf phenology and in situ weather across a 1700-3100 m elevation and rainfall gradient, along with desiccation-resistance experiments and multi-decadal modelling. We found limited support for biotic drivers with weak increases in some herbivorous taxa on shrubs with new leaves, but no landscape-scale effects of leaf phenology, which tracked light and cloud cover. Instead, rainfall explained extensive interannual variability with maximum biomass at intermediate rainfall (130 mm month-1 ) as both 3 months of high and low rainfall reduced arthropods by half. Based on 50 years of regional rainfall, our dynamic arthropod model predicted shifts in the timing of biomass maxima within cloud forests before plant communities transition to seasonally deciduous dry forests (mean annual rainfall 1000-2500 mm vs. <800 mm). Rainfall magnitude was the primary driver, but during high solar insolation, the 'drying power of air' (VPDmax ) reduced biomass within days contributing to drought related to the El Niño-Southern Oscillation (ENSO). Highlighting risks from drought, experiments demonstrated community-wide susceptibility to desiccation except for some caterpillars in which melanin-based coloration appeared to reduce the effects of evaporative drying. Overall, we provide multiple lines of evidence that several months of heavy rain or drought reduce arthropod biomass independently of deep-rooted plants with the potential to destabilize insectivore food webs.

El aumento de las temperaturas está incrementando los extremos de precipitación, pero las respuestas de los artrópodos a las fluctuaciones climáticas siguen siendo poco conocidas. Aquí, utilizamos la variación espaciotemporal en el clima montano tropical como un experimento natural para comparar la importancia de los factores bióticos versus abióticos en la regulación de la biomasa de artrópodos. Combinamos datos de campo intensivos de artrópodos, fenología de las hojas y clima in situ a lo largo de un gradiente altitudinal de 1700 a 3100 m y un gradiente de precipitación, junto con experimentos de resistencia a la desecación y modelos multi-decenales. Encontramos evidencia limitada para los factores bióticos con aumentos débiles en algunos taxones de herbívoros en arbustos con hojas nuevas, pero no hubo efectos a escala de paisaje en la fenología de la hoja, que rastreaba la luz y la cubierta de nubes. En cambio, las precipitaciones explicaron la amplia variabilidad interanual con una biomasa máxima en precipitaciones intermedias (130 mm mes−1 ), ya que los tres meses de precipitaciones altas y bajas redujeron los artrópodos a la mitad. Basándose en 50 años de precipitación regional, nuestro modelo dinámico de artrópodos predijo cambios en el momento de los máximos de biomasa dentro del bosque nuboso antes de que las comunidades de plantas hicieran la transición al bosque seco estacional caducifolio (precipitación media anual 1000-2500 mm vs. <800 mm). La magnitud de las lluvias fue el principal factor, pero durante la alta insolación solar, el "poder de secado del aire" (VPDmax ) redujo la biomasa en cuestión de días, lo que contribuyó a la sequía relacionada con El Niño-Southern Oscillation (ENSO). Destacando los riesgos de la sequía, los experimentos demostraron la susceptibilidad de toda la comunidad a la desecación, excepto en el caso de algunas orugas en las que la coloración a base de melanina parece reducir los efectos de la desecación por evaporación. En resumen, proporcionamos múltiples líneas de evidencia de que varios meses de fuertes lluvias o sequías reducen la biomasa de artrópodos independientemente de las plantas de raíces profundas con el potencial de desestabilizar las redes alimentarias de los insectívoros.

Functional response traits and altered ecological niches drive the disassembly of cloud forest bird communities in tropical montane countrysides.

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.

Nesting success of red-winged blackbirds (Agelaius phoeniceus) in marshes in an anthropogenic landscape.

Recent analyses show significant population declines in many abundant avian species, especially marsh-nesting species including the red-winged blackbird (RWBL). Hypothesized causes include reduced nesting success resulting from changing land-use patterns and exposure to contaminants. Our goal was to test the hypothesis that landscape and nest characteristics as well as exposure to polychlorinated biphenyls (PCBs) correlate with nesting success. From 2008 to 2014, we measured clutch size, egg and nestling mass, hatching and fledging success and daily survival of 1293 RWBL nests from 32 marshes in the Hudson River valley of New York. Using generalized linear effect and survival models, we found that: (i) Julian date was negatively related to hatching success and clutch size but positively related to egg mass; (ii) nest height was negatively related to hatching success; (iii) nestling mass decreased with increased nest density and distance to edges; (iv) fledging success was significantly lower in nests closer to the ground that were far from water; and (v) clutch size and daily survival were higher in nests farther from water. Results showed that nesting success was correlated with variables associated with flooding, population density and predation and provided no support for the predicted negative effects of PCB exposure.

Adaptations to light predict the foraging niche and disassembly of avian communities in tropical countrysides.

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.

An efficient extension of N-mixture models for multi-species abundance estimation.

In this study we propose an extension of the N-mixture family of models that targets an improvement of the statistical properties of rare species abundance estimators when sample sizes are low, yet typical for tropical studies. The proposed method harnesses information from other species in an ecological community to correct each species' estimator. We provide guidance to determine the sample size required to estimate accurately the abundance of rare tropical species when attempting to estimate the abundance of single species.We evaluate the proposed methods using an assumption of 50 m radius plots and perform simulations comprising a broad range of sample sizes, true abundances and detectability values and a complex data generating process. The extension of the N-mixture model is achieved by assuming that the detection probabilities are drawn at random from a beta distribution in a multi-species fashion. This hierarchical model avoids having to specify a single detection probability parameter per species in the targeted community. Parameter estimation is done via Maximum Likelihood.We compared our multi-species approach with previously proposed multi-species N-mixture models, which we show are biased when the true densities of species in the community are less than seven individuals per 100 hectares. The beta N-mixture model proposed here outperforms the traditional Multi-species N-mixture model by allowing the estimation of organisms at lower densities and controlling the bias in the estimation.We illustrate how our methodology can be used to suggest sample sizes required to estimate the abundance of organisms, when these are either rare, common or abundant. When the interest is full communities, we show how the multi-species approaches, and in particular our beta model and estimation methodology, can be used as a practical solution to estimate organism densities from rapid inventory datasets. The statistical inferences done with our model via Maximum Likelihood can also be used to group species in a community according to their detectabilities.

Functional Traits, Flocking Propensity, and Perceived Predation Risk in an Amazonian Understory Bird Community.

Within a community, different species might share similar predation risks, and, thus, the ability of species to signal and interpret heterospecific threat information may determine species' associations. We combined observational, experimental, and phylogenetic approaches to determine the extent to which evolutionary history and functional traits determined flocking propensity and perceived predation risk (response to heterospecific alarm calls) in a lowland Amazonian bird community. We predicted that small birds that feed myopically and out in the open would have higher flocking propensities and account for a higher proportion of positive responses to alarms. Using generalized linear models and the incorporation of phylogeny on data from 56 species, our results suggest that phylogenetic relationships alongside body size, foraging height, vegetation density, and response to alarm calls influence flocking propensity. Conversely, phylogenetic relationships did not influence response to heterospecific alarm calls. Among functional traits, however, foraging strategy, foraging density, and flocking propensity partially explained responses to alarm calls. Our results suggest that flocking propensity and perceived predation risk are positively related and that functional ecological traits and evolutionary history may explain certain species' associations.

Squeezed at the top: Interspecific aggression may constrain elevational ranges in tropical birds.

Tropical montane species are characterized by narrow elevational distributions. Recent perspectives on mechanisms maintaining these restricted distributions have emphasized abiotic processes, but biotic processes may also play a role in their establishment or maintenance. One historically popular hypothesis, especially for birds, is that interspecific competition constrains ranges of closely related species that "replace" each other along elevational gradients. Supporting evidence, however, is based on patterns of occurrence and does not reveal potential mechanisms. We experimentally tested a prediction of this hypothesis in two genera of tropical songbirds, Catharus (Turdidae) and Henicorhina (Troglodytidae), in which species have nonoverlapping elevational distributions. Using heterospecific playback trials, we found that individuals at replacement zones showed aggressive territorial behavior in response to songs of congeners. As distance from replacement zones increased, aggression toward congener song decreased, suggesting a learned component to interspecific aggression. Additionally, aggressive responses in Catharus were asymmetric, indicating interspecific dominance. These results provide experimental evidence consistent with the hypothesis that interspecific competitive interactions restrict ranges of Neotropical birds. Our results also underscore the need to consider biotic processes, such as competition, when predicting how species' ranges will shift with climate change. Asymmetric aggression could be particularly important. For example, if warming in montane landscapes allows upslope range expansion by dominant competitors, then high-elevation subordinate species could be forced into progressively smaller mountaintop habitats, jeopardizing viability of their populations.

Linking snake behavior to nest predation in a Midwestern bird community.

Nest predators can adversely affect the viability of songbird populations, and their impact is exacerbated in fragmented habitats. Despite substantial research on this predator-prey interaction, however, almost all of the focus has been on the birds rather than their nest predators, thereby limiting our understanding of the factors that bring predators and nests into contact. We used radiotelemetry to document the activity of two snake species (rat snakes, Elaphe obsoleta; racers, Coluber constrictor) known to prey on nests in Midwestern bird communities and simultaneously monitored 300 songbird nests and tested the hypothesis that predation risk should increase for nests when snakes were more active and in edge habitat preferred by both snake species. Predation risk increased when rat snakes were more active, for all nests combined and for two of the six bird species for which we had sufficient nests to allow separate analyses. This result is consistent with rat snakes being more important nest predators than racers. We found no evidence, however, that nests closer to forest edges were at greater risk. These results are generally consistent with the one previous study that investigated rat snakes and nest predation simultaneously. The seemingly paradoxical failure to find higher predation risk in the snakes' preferred habitat (i.e., edge) might be explained by the snakes using edges at least in part for non-foraging activities. We propose that higher nest predation in fragmented habitats (at least that attributable to snakes) results indirectly from edges promoting larger snake populations, rather than from edges directly increasing the risk of nest predation by snakes. If so, the notion of edges per se functioning as ecological "traps" merits further study.