Aerial Insectivores Research Articles

Modeling a cross-ecosystem subsidy: forest songbird response to emergent aquatic insects

Resource movements across ecosystem boundaries are important determinants of the diversity and abundance of organisms in the donor and recipient ecosystem. However the effects of cross-ecosystem movements of materials at broader spatial extents than a typical field study are not well understood. We tested the hypotheses that (1) variation in abundance of 57 forest songbird species within four foraging guilds is explained by modeled emergent aquatic insect biomass inputs from adjacent lakes and streams and (2) the degree of association varies across foraging guilds and species within guilds. We also sought to determine the importance of emergent aquatic insects while accounting for variation in local forest cover and edge. We spatially modeled the degree to which distribution and abundance of songbirds in different foraging guilds was explained by modeled emergent aquatic insect biomass. We used multilevel models to simultaneously estimate the responses of species in four different insectivorous guilds. Bird abundance was summarized from point counts conducted over 24 years at 317 points. Aerial insectivores were more abundant in areas with high estimated emergent insect biomass inputs to land (regression coefficient 0.30, P < 0.05) but the overall abundance of gleaners, bark-probers, and ground-foragers was not explained by estimated emergent insect abundance. The coursing aerial insectivores had the strongest association with emergent insects followed by willow flycatcher, olive-sided flycatcher, and alder flycatcher. Modeling cross-ecosystem movements of materials at broad spatial extents can effectively characterize the importance of this ecological process for aerial insectivorous songbirds.

Pteronotus gymnonotus(Chiroptera: Mormoopidae)

AbstractPteronotus gymnonotus (Wagner, 1843) is a medium-sized mormoopid commonly called the big naked-backed bat because of its wing membranes attached on the dorsal midline of the body, giving the appearance of a naked dorsum, and its large size in comparison to the other two species of naked-backed Pteronotus. The species is distributed across most of the Neotropical mainlands, ranging from southeastern Mexico through Central and South America south to northeastern Bolivia and central Brazil. P. gymnonotus has been considered as an uncommon species in most parts of its geographic distribution but there are some reports of more numerous local populations. It is an aerial insectivore that forages in open areas and gallery forests. Like other mormoopids P. gymnonotus usually forms large colonies roosting preferentially in hot and humid caves. It is currently listed as “Least Concern” according to the International Union for Conservation of Nature and Natural Resources Red List.

Mercury bioaccumulation in tropical bats from a region of active artisanal and small-scale gold mining.

Mercury negatively affects human and animal health. Artisanal and small-scale gold mining can be a major local source of mercury contamination, especially into aquatic systems in tropical areas. Animals associated with mercury-contaminated aquatic systems are at high risk of experiencing effects of this heavy metal, but it is not clear how far the effects may extend into nearby terrestrial systems. We report mercury contamination levels in bats in agricultural areas at increasing distances from gold mining (~3-89 km of distance). We hypothesized that bat mercury concentrations would differ between feeding guilds, land use types, and be higher at sites closer to gold mining areas. We collected 112 fur samples from 30 bat species and eight guilds, and provide the first reports of concentrations in 12 species. All mercury concentrations were below the level at which health is likely to be affected (10 ppm). We found guild-influenced differences among mercury concentration levels, with the highest concentrations in aerial insectivores and carnivores, and the lowest in canopy frugivores. Our results suggest insectivorous and carnivorous bats may still be at some risk even at sites distant from aquatic mercury contamination. We did not find an effect of agricultural land-use type on mercury concentrations within species or guilds, suggesting mercury contamination did not extend to agricultural sites from areas of gold mining activities, and that these agricultural activities themselves were not an important source of mercury. We conclude bats did not demonstrate a signature of mercury risk either as a result of proximity of gold mining, or as a result of agricultural activities.

Avian relationships with bark beetle outbreaks and underlying mechanisms in lodgepole pine and spruce-fir forests of Colorado

Bark beetle (Dendroctonus spp.) outbreaks have historically shaped the structure and function of western North American conifer forests by contributing to heterogeneous conditions needed to support various wildlife species. Previous studies of beetle impacts have primarily focused on pine-dominated systems within 1–6 years of outbreak, limiting our knowledge for informing wildlife habitat management to a relatively short timeframe and narrow range of forest types. Increases in extent and severity of outbreaks since 1900, caused in part by anthropogenic climate warming and forest management, elevates the value of understanding how bark beetle outbreaks impact wildlife populations. Our objectives were (1) to evaluate species and community relationships with outbreak severity (percent conifer mortality) and years since outbreak, (2) to evaluate potential environmental mechanisms underlying these relationships, and (3) to compare patterns across the two forest types for improved general knowledge. We studied avian species occupancy and richness in relation to outbreak conditions using two 18-year chronosequence datasets collected in 2013 and 2014 representing lodgepole forests (predominantly Pinus contorta) and spruce-fir forests (co-dominated by Picea engelmannii and Abies lasiocarpa) throughout Colorado. We employed hierarchical models to account for imperfect detection and spatial dependencies when analyzing population and community patterns apparent in data representing 73 bird species. We found various relationships and potential underlying mechanisms largely but not entirely consistent with a priori hypotheses based on species life histories and previous study. As expected, understory-associated birds related positively with outbreak conditions apparently following understory vegetative release. Consistent with our hypotheses, aerial insectivores and snag-associated species also related positively with outbreak conditions, although our data highlighted understory vegetation more so than canopy structure or snags as potential mechanistic factors. Contrary to our overall hypothesis for canopy-associated species, we did not observe many negative outbreak relationships for this group. Overall, bird species richness increased with years since outbreak, with clear increases in lodgepole forest. In contrast, the data from spruce-fir forest provided statistical support for fewer patterns (i.e., fewer with 90% credible intervals excluding zero), and they supported primarily negative relationships with outbreak severity. Our results suggest potential differences in ecological significance of bark beetle outbreaks in different forest types. At least in lodgepole forest, however, observed patterns were apparently consistent with the purported historical value of bark beetle outbreaks for promoting biodiversity (represented here by birds) despite recent increases in extent and severity.

Landscape structure influences the use of social information in an insectivorous bat

In anthropogenic landscapes, aerial insectivores are often confronted with variable habitat complexity, which may influence the distribution of prey. Yet, high mobility may allow aerial insectivores to adjust their foraging strategy to different prey distributions. We investigated whether aerial‐hunting common noctules Nyctalus noctula adjust their foraging strategy to landscapes with different habitat complexity and assumingly different prey distribution. We hypothesized that the movement behaviour of hunting common noctules and changes of movement behaviour in reaction towards conspecifics would depend on whether they hunt in a structurally poor cropland dominated landscape or a structurally rich forest dominated landscape. We tracked flight paths of common noctules in northeastern Germany using GPS loggers equipped with an ultrasonic microphone that recorded foraging events and presence of conspecifics. Above cropland, common noctules hunted mainly during bouts of highly tortuous and area restricted movements (ARM). Bats switched from straight flight to ARM after encountering conspecifics. In the forested landscape, common noctules hunted both during ARM and during straight flights. The onset of ARM did not correlate with the presence of conspecifics. Common noctules showed a lower feeding rate and encountered more conspecifics above the forested than above the cropland dominated landscape. We conjecture that prey distribution above cropland was patchy and unpredictable, thus making eavesdropping on hunting conspecifics crucial for bats during search for prey patches. In contrast, small scale structural diversity of the forested landscape possibly led to a more homogeneous prey distribution at the landscape scale, thus enabling bats to find sufficient food independent of conspecific presence. This suggests that predators depending on ephemeral prey can increase their foraging success in structurally poor landscapes by using social information provided by conspecifics. Hence, a minimum population density might be obligatory to enable successful foraging in simplified landscapes.

Skyrocketing Flights as a Previously Unrecognized Behaviour of Open-Space Foraging Bats

The airspace of the lower troposphere is an important habitat for many animals, including open-space foraging bats. Yet, we are missing a detailed understanding of how bats forage in the seemingly unstructured open space. As part of an exploratory study, we present data on flight activity for Taphozous theobaldi (Dobson 1872), an aerial insectivore known to hunt in the open space close to mountain ridges in central Thailand. Using logger data from five individuals, covering a total period of 26 observation days, we show that T. theobaldi remained airborne for 4.8 ± 3.1 h per night (mean ± SD). We confirmed that T. theobaldi performed altitudinal flights with quick ascents, short horizontal flights followed by descents. Each night, T. theobaldi engaged on average in 2.9 ± 1.4 altitudinal flights during which they reached an average peak altitude of 306 ± 53 m above cave elevation. On average, bats spent about 14% of the total foraging time in altitudinal flights. We conclude that T. theobaldi is a regular, but most likely not an obligatory exploiter of high altitudes. We speculate that T. theobaldi may ascend to higher altitudes in search for insects, for example migrating or dispersing insects at higher air layers or swarms of hill-topping insects. Short altitudinal flights with fast ascents and descents (a slender bell-shaped flight trajectory over several 100 m altitude) may present a so far unrecognized flight behaviour of bats that may be more widespread among other aerial insectivores.

Elucidating the Consequences of a Warming Climate for Common Bat Species in North-Western Europe

Climate change presents a serious threat to global biodiversity. In the extreme north of Europe, for example, some boreal bat species have been predicted to become extinct by the end of the century. We have used bat activity data derived from car-based surveys across Ireland to examine the effects of weather and climate using an approach that teases apart their spatial and temporal effects. This allows more accurate modelling of the impacts of weather than has heretofore been possible. Our results also support previous work that suggests the standard approach for combining the spatio-temporal variation in weather covariates is unsound. We used the results of this modelling exercise to forecast the effects of predicted climate change on three widespread Irish species for the years 2046–2059. Nyctalus leisleri, in its international stronghold, is predicted to undergo a decline in the north-west with increases likely in the south-east. Pipistrellus pipistrellus is predicted to undergo similar changes to N. leisleri, with declines in the north. Pipistrellus pygmaeus may increase in coastal locations. Predicted declines in the north of Ireland run somewhat contrary to expectations since increased ambient temperature there may have been anticipated to favour bat population growth. This work is a promising step towards disentangling the spatial and temporal impacts of weather and, from there, assessing the effects of climate change on bats in Europe. Pipistrelle species, in particular, are likely to be north-western Europe's most abundant consumers of aerial nocturnal insects and future conservation plans must consider potential losses of these species from agricultural areas and how any resulting reduction in ecosystem services can be mitigated.

Threats from the air: Damselfly predation on diverse prey taxa.

To understand the diversity and strength of predation in natural communities, researchers must quantify the total amount of prey species in the diet of predators. Metabarcoding approaches have allowed widespread characterization of predator diets with high taxonomic resolution. To determine the wider impacts of predators, researchers should combine DNA techniques with estimates of population size of predators using mark-release-recapture (MRR) methods, and with accurate metrics of food consumption by individuals. Herein, we estimate the scale of predation exerted by four damselfly species on diverse prey taxa within a well-defined 12-ha study area, resolving the prey species of individual damselflies, to what extent the diets of predatory species overlap, and which fraction of the main prey populations are consumed. We identify the taxonomic composition of diets using DNA metabarcoding and quantify damselfly population sizes by MRR. We also use predator-specific estimates of consumption rates, and independent data on prey emergence rates to estimate the collective predation pressure summed over all prey taxa and specific to their main prey (non-biting midges or chironomids) of the four damselfly species. The four damselfly species collectively consumed a prey mass equivalent to roughly 870 (95% CL 410-1,800) g, over 2months. Each individual consumed 29%-66% (95% CL 9.4-123) of its body weight during its relatively short life span (2.1-4.7days; 95% CL 0.74-7.9) in the focal population. This predation pressure was widely distributed across the local invertebrate prey community, including 4 classes, 19 orders and c. 140 genera. Different predator species showed extensive overlap in diets, with an average of 30% of prey shared by at least two predator species. Of the available prey individuals in the widely consumed family Chironomidae, only a relatively small proportion (0.76%; 95% CL 0.35%-1.61%) were consumed. Our synthesis of population sizes, per-capita consumption rates and taxonomic distribution of diets identifies damselflies as a comparatively minor predator group of aerial insects. As the next step, we should add estimates of predation by larger odonate species, and experimental removal of odonates, thereby establishing the full impact of odonate predation on prey communities.

Population decline in tree swallows (Tachycineta bicolor) linked to climate change and inclement weather on the breeding ground.

Population decline and the threat of extinction are realities currently facing many species. Yet, in most cases, the detailed demographic data necessary to identify causes of population decline are unavailable. Using 43years (1975-2017) of data from a box-nesting population of tree swallows (Tachycineta bicolor), we identified reduced survival of offspring as a probable demographic cause of population decline. Poor fledging success was associated with increased predation and poor weather conditions during early nestling development. Low juvenile survival and subsequent recruitment was linked to poor weather conditions during the post-fledging period and may also be linked to conditions on the wintering grounds. Regional weather conditions during critical stages of breeding (early nestling and post-fledging) have become progressively worse over the 43-year study period. None of the other factors linked to offspring survival have similarly deteriorated. Overall, our results suggest tree swallows should be added to the growing list of species challenged by climate change, and that other species of aerial insect specialists may face similar impacts of climate change.

Barn Swallow (Hirundo rustica) fledglings use crop habitat more frequently in relation to its availability than pasture and other habitat types

Abstract Populations of birds that forage on aerial insects have been declining across North America for several decades, but the main causes of and reasons for geographical variation in these declines remains unclear. We examined the habitat use and survival of post-fledging Barn Swallows (Hirundo rustica) near Vancouver, British Columbia, Canada, using VHF radio telemetry. We predicted that fledgling Barn Swallows hatched in higher-quality natal habitat (pasture) would fledge at higher quality, stay closest to the nest, disproportionately use higher-quality habitat during the post-fledge stage, and have higher survival rates in the region. Contrary to our predictions, we found that natal habitat (crop, pasture, or non-agriculture) had no effect on fledgling quality or movement distance. Barn Swallow fledglings used crop habitat more frequently in relation to its availability than other habitat types, including pasture. Barn Swallows had low post-fledging survival rates (0.44; 95% CI: 0.35–0.57), which could negatively influence the population trend of the species in this region. While natal habitat had only minor effects, crop habitat appears to be important for fledgling Barn Swallows and, therefore, a decline in this habitat type could have further negative implications for an already declining species.

Heat dissipation capacity influences reproductive performance in an aerial insectivore.

Climatic warming is predicted to increase the frequency of extreme weather events, which may reduce an individual's capacity for sustained activity because of thermal limits. We tested whether the risk of overheating may limit parental provisioning of an aerial insectivorous bird in population decline. For many seasonally breeding birds, parents are thought to operate close to an energetic ceiling during the 2-3 week chick-rearing period. The factors determining the ceiling remain unknown, although it may be set by an individual's capacity to dissipate body heat (the heat dissipation limitation hypothesis). Over two breeding seasons we experimentally trimmed the ventral feathers of female tree swallows (Tachycineta bicolor) to provide a thermal window. We then monitored maternal and paternal provisioning rates, nestling growth rates and fledging success. We found the effect of our experimental treatment was context dependent. Females with an enhanced capacity to dissipate heat fed their nestlings at higher rates than controls when conditions were hot, but the reverse was true under cool conditions. Control females and their mates both reduced foraging under hot conditions. In contrast, male partners of trimmed females maintained a constant feeding rate across temperatures, suggesting attempts to match the feeding rate of their partners. On average, nestlings of trimmed females were heavier than controls, but did not have a higher probability of fledging. We suggest that removal of a thermal constraint allowed females to increase provisioning rates, but additionally provided nestlings with a thermal advantage via increased heat transfer during maternal brooding. Our data provide support for the heat dissipation limitation hypothesis and suggest that depending on temperature, heat dissipation capacity can influence reproductive success in aerial insectivores.

Falling through the policy cracks: implementing a roadmap to conserve aerial insectivores in North America

Nebel et al discusses the implementation of a roadmap to conserve aerial insectivores in North America Aerial insectivores are subject to myriad anthropogenic stressors across their annual cycles but there is little evidence that a single factor explains the fluctuations observed among populations and species The expansion of intensive agricultural and urban land use especially over the last 30 years has introduced a range of threats impacting both habitat quantity and quality Direct impacts include habitat loss and degradation, particularly of wetlands, grasslands, and forests, and indirect impacts of stressors that reduce the quality of breeding, foraging, and roosting sites The response by Canadians to the COVID-19 pandemic amply demonstrates that rapid changes are possible when we work together to surmount challenges

The effects of landscape composition and configuration on Eastern Whip-poor-will (Caprimulgus vociferous) and Common Nighthawk (Chordeiles minor) occupancy in an agroecosystem

Vala, M. A., G. W. Mitchell, K. C. Hannah, J. Put, and S. Wilson. 2020. The effects of landscape composition and configuration on Eastern Whip-poor-will (Caprimulgus vociferous) and Common Nighthawk (Chordeiles minor) occupancy in an agroecosystem. Avian Conservation and Ecology 15(1):24. https://doi.org/10.5751/ACE-01613-150124

Southward Breeding Expansion of Tree Swallows in Alabama

Since first confirmation of nesting in 1987, there has been a steady southward expansion of breeding Tachycineta bicolor (Tree Swallow) in Alabama. Using 220 breeding season occurrence records, we performed a breakpoint analysis to quantify the breeding expansion rate of Tree Swallows in the state. Between 1988 and 2013, our models indicated Tree Swallows expanded ∼130 km south at a mean rate of 5.2 km/year. Possible drivers of this southward expansion include increased artificial nest box availability, aerial insect declines in more northern historic ranges, changing climate, and altered land-management practices. Our findings indicate Tree Swallows are following a similar southward breeding expansion pattern to that exhibited by Hirundo rustica (Barn Swallow) and Petrochelidon pyrrhonota (Cliff Swallow) in recent decades.

Antagonistic, synergistic and direct effects of land use and climate on Prairie wetland ecosystems: Ghosts of the past or present?

AbstractAimWetland loss and degradation threaten biodiversity to an extent greater than most ecosystems. Science‐supported responses require understanding of interacting effects of land use and climate change on wetland biodiversity.LocationAlberta, Canada.MethodsWe evaluated how current climate, climate change (as a ghost of the past), land use and wetland water quality relate to aquatic macroinvertebrates and birds.ResultsClimatic relationships and climate–land use interactions were observed on chironomid abundance, but not macroinvertebrate taxa richness (MTR) or odonate abundance, which responded to land use and water chemistry. Chironomid abundance was positively associated with cropland and negatively associated with total precipitation. Higher cropland cover and dissolved organic carbon synergistically interacted with total precipitation to affect chironomids. MTR was negatively related to salinity, yet greater area of non‐woody riparian vegetation attenuated salinity effects on MTR. Odonate abundance was negatively related to total phosphorus. Higher grassland cover also increased the negative relationship of total phosphorous to odonate abundance. Climatic relationships and climate–land use interactions were observed on bird species richness (BSR) and abundance of several bird functional groups. Higher BSR and abundances of several bird groups were positively related to average rainfall and greater warming temperatures over time. Area of non‐crop cover and wetlands was positively associated with most bird groups and BSR. Warming temperatures over time ameliorated the negative relationship of higher cropland or less shrubland on aerial insectivores and other bird groups.Main conclusionsClimate patterns and climate change are as important as land use pressures with stronger impacts on birds. Climate change was more influential than current climate and provided novel empirical evidence that progressively warmer, wetter conditions is benefiting some bird groups, including aerial insectivores, a group of conservation concern. Riparian vegetation ameliorated the negative impacts of climate and water quality gradients on MTR and could mitigate global change impacts in agricultural systems.

Breeding origins and migratory connectivity at a northern roost of Vaux’s Swift, a declining aerial insectivore

ABSTRACT Populations of Vaux’s Swift (Chaetura vauxi), like those of many aerial insectivores, are rapidly declining. Determining when and where populations are limited across the annual cycle is important for their conservation. Establishing the linkages between wintering and breeding sites and the strength of the connections between them is a necessary first step. In this study, we analyzed 3 stable isotopes (δ13C, δ15N, δ2H) from feathers collected during spring migration from Vaux’s Swifts that perished during a stopover on Vancouver Island, British Columbia, Canada. We previously analyzed claw tissue (grown during winter) from the same individuals, revealing that the swifts likely wintered in 2 or 3 locations/habitats. Here, we used stable isotope analysis of flight feathers presumed to have been grown on, or near, the breeding grounds to determine the likely previous breeding locations and presumed destinations for the swifts. Stable isotope values (δ13C, δ15N, δ2H) showed no meaningful variation between age classes, sexes, or with body size. Surprisingly, ~26% of the birds sampled had feather isotope values that were not consistent with growth on their breeding grounds. For the remaining birds, assigned breeding origins appeared most consistent with molt origins on Vancouver Island. Overall, migratory connectivity of this population was relatively weak (rM = 0.07). However, the degree of connectivity depended on how many winter clusters were analyzed; the 2-cluster solution suggested no significant connectivity, but the 3-cluster solution suggested weak connectivity. It is still unclear whether low migratory connectivity observed for Vaux’s Swift and other aerial insectivores may make their populations more or less vulnerable to habitat loss; therefore, further efforts should be directed to assessing whether aerial insectivores may be habitat limited throughout the annual cycle.

Urbanization‐driven homogenization is more pronounced and happens at wider spatial scales in nocturnal and mobile flying insects

AbstractAimWe test whether urbanization drives biotic homogenization. We hypothesize that declines in abundance and species diversity of aerial insects are exacerbated by the urbanization‐driven loss of species with low habitat generalism, mobility and warm‐adaptedness. We predict this homogenization to be more pronounced for nocturnal taxa, and at wider scales for mobile taxa.LocationBelgium.Time periodSummers 2014–2015.Major taxa studiedLepidoptera.MethodsWe compare communities along urbanization gradients using a shared, replicated and nested sampling design, in which butterflies were counted within 81 grassland and macro‐moths light‐trapped in 12 woodland sites. We quantify taxonomic and functional community composition, the latter via community‐weighted means and variation of species‐specific traits related to specialization, mobility and thermophily. Using linear regression models, variables are analysed in relation to site‐specific urbanization values quantified at seven scales (50–3,200 m radii). At best‐fitting scales, we test for taxonomic homogenization.ResultsWith increasing urbanization, abundance, species richness and Shannon diversity severely declined, with butterfly and macro‐moth declines due to local‐ versus landscape‐scale urbanization (200 vs. 800–3,200 m radii, respectively). While taxonomic homogenization was absent for butterflies, urban macro‐moth communities displayed higher nestedness than non‐urban communities. Overall, communities showed mean shifts towards generalist, mobile and thermophilous species, displaying trait convergence too. These functional trait models consistently fit best with urbanization quantified at local scales (100–200 m radii) for butterfly communities, and at local to wider landscape scales (200–800 m radii) for macro‐moth communities.Main conclusionsUrban communities display functional homogenization that follows urbanization at scales linked to taxon‐specific mobility. Light pollution may explain why homogenization was more pronounced for the nocturnal taxon. We discuss that urbanization is likely to impact flying insect communities across the globe, but also that impacts on their ecosystem functions and services could be mitigated via multi‐scale implementation of urban green infrastructure.

Analysis of Body Size, Web Size, and Diet in Two Congeneric Orb-Weaving Spiders (Araneae: Araneidae) Syntopic in Florida Scrub

The yellow garden spider, Argiope aurantia Lucas (Araneae: Araneidae), is widespread in North America, whereas the Florida garden spider, Argiope florida Chamberlin & Ivie (Araneae: Araneidae), is restricted to scrub habitats in the southeastern USA. Both orb-weavers occur syntopically in native oak scrub at the Archbold Biological Station in Highlands County, Florida. We hypothesized that female spiders of both species might have a high degree of dietary niche overlap because their orb-webs are located near each other, despite the fact that they may differ in body size and web characteristics. We located 180 side-by-side pairs of mature A. aurantia and A. florida, and assigned them at random into 3 treatment groups of 60 pairs each. Argiope aurantia in group 1 were more heavy-bodied and longer legged than A. florida. In group 2, the orb-webs of both species were hung at the same height aboveground, but A. aurantia webs were larger with fewer sticky spirals than those of A. florida. Over the course of 5 d, the 2 spider species in group 3 captured almost the same number of aerial arthropods (80 for A. aurantia; 76 for A. florida). We identified most prey (94%) to genus or species. Dietary niche overlap at the ordinal level was extensive (70%), but it shrank dramatically when calculated at the generic/ species level (20%). We concluded that not only were A. aurantia consuming substantially larger prey than A. florida, but also that they were feeding on different kinds of aerial insects moving through gaps in oak scrub.

Local habitat association does not inform landscape management of threatened birds

Species that use open patches in forested landscapes often select clearcuts. However, it is unknown whether local associations with clearcuts translate to an effect of clearcut amount in the surrounding landscape on occupancy or abundance at local sites. This question is important because forest management decisions are made at landscape scales. We examined whether the amount of clearcut in the surrounding landscape influenced site occupancy of two threatened aerial insectivores, Common Nighthawk and Eastern Whip-poor-will. Both species nest in/near clearcuts at a local-scale. We used acoustic recorders placed on edges of recent clearcuts (≤ 15 years old, n = 49 sites) to measure presence-absence. We estimated occupancy in relation to the proportion of clearcut and open wetland within the surrounding landscapes at spatial extents between 0.5 and 5.0 km. Occupancy of Eastern Whip-poor-will was not related to clearcut amount in the surrounding landscape at any scale. Common Nighthawk occupancy was lower in sites surrounded by landscapes with higher proportion of older (11–15 years old) clearcuts. Both species’ occupancy was higher in sites where the surrounding landscapes had higher proportions of open wetland. Two possible mechanisms for our results include multi-scale selection of breeding sites or demographic responses to higher productivity in wetlands than clearcuts; both need further study. Our results show how the association of species with clearcut habitats at a local scale does not necessarily translate to a higher occurrence of those species at the landscape scale at which management decisions are made.

Nonbreeding season movements of a migratory songbird are related to declines in resource availability

AbstractThere have been an increasing number of observations of itinerancy in migratory songbirds, where individuals move among 2 or more widely separated areas during the “stationary” nonbreeding season. Knowledge of such movements and an understanding of what drives them are important for predicting how migratory populations will respond to environmental change. In this study, we investigated nonbreeding movements of the Tree Swallow (Tachycineta bicolor), an aerial insectivore that breeds across North America and spends the nonbreeding season around the Gulf of Mexico, Florida, Mexico, Central America, and the Caribbean. With year-round tracking data obtained from 133 light-level geolocators deployed at 12 breeding sites ranging from Alaska to Nova Scotia to North Carolina, we show that 44% of individuals made at least one large-scale movement (range: 301–1,744 km) within the nonbreeding range. The frequency of itinerancy decreased with longitude, such that 75% of individuals made a movement in the western portion of the nonbreeding range compared to only 31% in the east. Using the Normalized Difference Vegetation Index (NDVI) as a proxy for resource availability, we found that when individuals did move, they were more likely to move from sites where resources were deteriorating faster (a more negative change in NDVI prior to departure) than their destination sites. There was also evidence that individuals moved to destination sites with higher NDVI and temperature in the autumn, but not in the winter. Our results suggest movements of Tree Swallows during the nonbreeding season are influenced by resource availability, but because not all individuals used multiple nonbreeding sites, the density of individuals at a site and the level of competition may have also been a factor influencing nonbreeding season movements.