Wilsonia Citrina Research Articles

Individual‐level biotic interactions and species distribution models

AbstractAimAccounting for biotic interactions in species distribution models is complicated by the fact that interactions occur at the individual‐level at unknown spatial scales. Standard approaches that ignore individual‐level interactions and focus on aggregate scales are subject to the modifiable aerial unit problem (MAUP) in which incorrect inferences may arise about the sign and magnitude of interspecific effects.LocationGlobal (simulation) and North Carolina, United States (case study).TaxonNone (simulation) and Aves (case study).MethodsWe present a hierarchical species distribution model that includes a Markov point process in which the locations of individuals of one species are modelled as a function of both abiotic variables and the locations of individuals of another species. We applied the model to spatial capture‐recapture (SCR) data on two ecologically similar songbird species—hooded warbler (Setophaga citrina) and black‐throated blue warbler (Setophaga caerulescens)—that segregate over a climate gradient in the southern Appalachian Mountains, USA.ResultsA simulation study indicated that the model can identify the effects of environmental variation and biotic interactions on co‐occurring species distributions. In the case study, there were strong and opposing effects of climate on spatial variation in population densities, but spatial competition did not influence the two species' distributions.Main ConclusionsUnlike existing species distribution models, the framework proposed here overcomes the MAUP and can be used to investigate how population‐level patterns emerge from individual‐level processes, while also allowing for inference on the spatial scale of biotic interactions. Our finding of minimal spatial competition between black‐throated blue warbler and hooded warbler adds to the growing body of literature suggesting that abiotic factors may be more important than competition at low‐latitude range margins. The model can be extended to accommodate count data and binary data in addition to SCR data.

Stabilizing selection on a plumage-based foraging adaptation: hooded warblers with average-sized white tail spots live longer.

Avian flush-pursuit insectivores typically use contrasting white plumage patches in their tails or wings to startle potential prey. Although experimental evidence indicates that the extent of white has been fine-tuned by natural selection to optimize foraging performance, the hypothesis that within-population plumage variation directly influences survival or lifetime reproduction and is subject to stabilizing selection has not been tested. Here, I provide such a test using data collected as part of a 14-year study of a colour-ringed breeding population of the hooded warbler (Setophaga citrina), a migratory flush-pursuit insectivore that shows inter-individual variation in the extent of white in the tail that is highly repeatable across moults and probably heritable. As expected under stabilizing selection, warblers with average-sized white tail patches achieved significantly higher apparent long-term survival than individuals with either a lesser or greater extent of white in the tail. Evidence of stabilizing selection was especially strong in males, an observation that is probably related to pronounced sexual habitat segregation on the wintering range. My results provide infrequently observed evidence of stabilizing selection operating in a natural population, and also illustrate how stabilizing selection can act on avian plumage traits outside the context of sexual and social signalling.

Brood parasitism of Hooded Warblers by Brown-headed Cowbirds: Severe impact on individual nests but modest consequences for seasonal fecundity and conservation

Abstract Brood parasitism by Brown-headed Cowbirds (Molothrus ater) often has pronounced negative effects on host nests. However, the extent to which parasitism reduces annual reproduction and presents conservation challenges for host species is unclear. We address this issue with data from a color-banded population of Hooded Warblers (Setophaga citrina) in Pennsylvania, where Hooded Warblers have increased dramatically despite frequent nest parasitism. Our analysis is based on both an extensive dataset (8 years, 847 nests) on the per-nest impacts of cowbird parasitism and female-based stochastic simulations that accurately reflect the reproductive biology and parasitism rate (30%) of our study population. Cowbird parasitism has multiple negative consequences for Hooded Warbler nests, including (1) reduced host clutch size; (2) increased nest abandonment; (3) increased risk of complete failure due to predation; and (4) in surviving nests increased egg loss, hatching failure, and nestling mortality. We estimate that parasitism reduces success of Hooded Warbler nests 68%, from 1.29 to 0.41 fledglings per nest. For females and populations, however, the consequences of nest parasitism are considerably less extreme; female annual fecundity decreases 25% for each nesting attempt parasitized, and population-level fecundity drops 5.6% for each 10% increase in the frequency of parasitism. These more modest impacts are attributable to (1) steep declines in rates of cowbird parasitism as the nesting season progresses, (2) rapid renesting following abandonment or failure of parasitized nests, and (3) regular double brooding, with second broods initiated in late June and July when the incidence of cowbird parasitism is low. Our results help resolve the paradox of how cowbird parasitism can have both severe consequences for individual host nests but more modest and sustainable conservation impacts on the seasonal fecundity of females and populations. They further underscore the importance of determining population-level effects of brood parasites before investing in costly management efforts.

Evidence of nest material kleptoparasitism in Worm-eating Warblers (Helmitheros vermivorum) in east-central Arkansas, USA.

Nest material kleptoparasitism likely evolved in birds to reduce the cost of searching for and collecting material themselves. Although nest material kleptoparasitism has been reported commonly in colonially nesting species, reports for solitary breeding species are infrequent, especially for neotropical migratory species. Here, we report potential and actual nest material kleptoparasitism in the Worm‐eating Warbler (Helmitheros vermivorum). We deployed video camera systems at passerine nests (n = 81) in east‐central Arkansas during summers 2011–2012. In one video, we observed a Worm‐eating Warbler stealing nesting material from a Hooded Warbler (Setophaga citrina) nest. One day later, we later observed a Worm‐eating Warbler landing within 0.5 m of the same warbler nest when the female was incubating, which possibly deterred a second theft of nesting material. In a third video recording, we observed another Worm‐eating Warbler landing within 1 m of an Indigo Bunting (Passerina cyanea) nest. As far as we could determine, neither of these latter two nest visits resulted in nest material kleptoparasitism. Potential benefits of nest material kleptoparasitism include reduced competition for limited nest materials, easy access to suitable material, reduced travel distance, and reduction of nest predation risk; however, costs include risk of attack by host or introducing parasites to one's nest. Importantly, this behavior could ultimately affect the behavioral and life history evolution of a species. We suggest further work should be conducted to determine the prevalence of nest material kleptoparasitism in Worm‐eating Warblers and other solitary breeding passerines, including efforts to quantify the benefits and costs of this behavior.

Stable isotope (C, N, O, and H) study of a comprehensive set of feathers from two Setophaga citrina.

Oxygen, hydrogen, carbon and nitrogen stable isotopes were measured on a comprehensive sampling of feathers from two spring Hooded Warblers (Setophaga citrina) in Texas to evaluate isotopic variability between feathers and during molt. Isotopic homogeneity within each bird was found across all four isotopic systems, supporting the hypothesis that molt in these neotropical migrants is fully completed on the breeding grounds. This homogeneity suggests that the isotopic composition of a single feather is may be representative of the whole songbird. However, each bird was found to have one or two outlier feathers, which could signify regrowth of lost feathers after prebasic molt.

Experience counts: The role of female age in morning incubation and brooding behavior in relation to temperature

Reproductive experience can impact how individuals allocate time and energy to reproduction and generate differences in reproductive behavior that leads to experience dependent variation in reproductive success. In order to understand if individual variation in parental behavior is related to environmental temperature and breeding experience, we observed the timing and duration of the first morning off bout in a wild, open cup nesting passerine bird during the incubation and early nestling period. We compared incubation behaviors and nest temperature of inexperienced (second year = first breeding season) and experienced (after second year) female hooded warblers Setophaga citrina. Females left the nest earlier on colder mornings suggesting an energetic constraint due to the long overnight on bout during colder temperatures. During incubation, females increased the duration of the first morning off bout with increasing temperature. Similarly, during the early nestling period, experienced females had shorter off bout duration on colder mornings and increased duration with warmer temperatures. In contrast, inexperienced females increased off bout duration with colder morning temperatures. Experienced females maintained higher nest temperatures and higher minimum nest temperatures compared to inexperienced females. We also found evidence that experienced females nested in microhabitats with higher minimum morning temperature which may buffer older females from colder daily extremes and enable older females to maintain higher nest temperatures. While we found no relationship between incubation and brooding behaviors and fledging success, the proportion of the clutch that hatched was positively related to minimum nest temperature. The ability of experienced females to maintain higher minimum nest temperatures and to adjust brooding behaviors during colder mornings is a potential mechanism that has consequences for nestling condition in a wild population. Our results highlight the need to examine experience‐related parental care behaviors in responding to environmental variation.

Females compensate for moult‐associated male nest desertion in Hooded Warblers

Uniparental offspring desertion occurs in a wide variety of avian taxa and usually reflects sexual conflict over parental care. In many species, desertion yields immediate reproductive benefits for deserters if they can re‐mate and breed again during the same nesting season; in such cases desertion may be selectively advantageous even if it significantly reduces the fitness of the current brood. However, in many other species, parents desert late‐season offspring when opportunities to re‐nest are absent. In these cases, any reproductive benefits of desertion are delayed, and desertion is unlikely to be advantageous unless the deserted parent can compensate for the loss of its partner and minimize costs to the current brood. We tested this parental compensation hypothesis in Hooded Warblers Setophaga citrina, a species in which males regularly desert late‐season nestlings and fledglings during moult. Females from deserted nests effectively doubled their provisioning efforts, and nestlings from deserted nests received just as much food, gained mass at the same rate, and were no more likely to die from either complete nest predation or brood reduction as young from biparental nests. The female provisioning response, however, was significantly related to nestling age; females undercompensated for male desertion when the nestlings were young, but overcompensated as nestlings approached fledging age, probably because of time constraints that brooding imposed on females with young nestlings. Overall, our results indicate that female Hooded Warblers completely compensate for male moult‐associated nest desertion, and that deserting males pay no reproductive cost for desertion, at least up to the point of fledging. Along with other studies, our findings support the general conclusion that late‐season offspring desertion is likely to evolve only when parental compensation by the deserted partner can minimize costs to the current brood.

Development of the corticosterone stress response differs among passerine species

Glucocorticoids are steroid hormones which increase dramatically in response to a physical or perceived stressor. However, developing young of altricial species typically have a damped glucocorticoid stress response. The developmental hypothesis posits that the physiological stress response should develop concurrently with an individual’s ability to respond to a challenge. The dampened response may benefit an organism, as chronic exposure to glucocorticoids can have short- and long-term detrimental effects, and altricial young are unable to escape most stressors. However, we do not know if or why species with similar ontogeny vary in their development of the physiological stress response. We assessed levels of baseline and stress-induced corticosterone (the main avian glucocorticoid) in six passerine species with varying life-history strategies, including a brood parasite, the brown-headed cowbird (Molothrus ater). Circulating baseline corticosterone levels increased with nestling age for all species. Stress-induced corticosterone levels sampled at 15-min post-capture significantly increased with nestling age at a similar rate and magnitude in brown-headed cowbirds, eastern phoebes (Sayornis phoebe), hooded warblers (Setophaga citrina), red-winged blackbirds (Agelaius phoeniceus), and song sparrows (Melospiza melodia). However, gray catbird (Dumetella carolinensis) nestlings showed an extremely dampened elevation in corticosterone in response to stress, even near fledge. Gray catbirds are unusual among songbirds, as they are open-ended song learners. Stress during development can profoundly influence avian song learning and performance abilities. However, further study is necessary to determine if there is a relationship between nestling adrenocortical activity and open- versus closed-ended song learning.

Brood parasitism of a Hooded Warbler (Setophaga citrina) nest by a cuckoo (Coccyzus sp.) in Pennsylvania

Brood parasitism of Hooded Warblers (Setophaga citrina) by Brown-headed Cowbirds (Molothrus ater) has been well documented, but brood parasitism of this host by other bird species has not been recorded. Hooded Warblers are known to abandon nests with parasitic eggs early in the egg-laying phase, but are not known to remove parasitic eggs from their nests or to replace these foreign eggs with their own. In June 2017, a Hooded Warbler nest in northeastern Pennsylvania was observed to contain a cuckoo (Coccyzus sp.) egg. Within a span of 6 days, the cuckoo egg was gone and a third Hooded Warbler egg was observed in its place.

The trade-off between molt and parental care in Hooded Warblers: Simultaneous rectrix molt and uniparental desertion of late-season young

ABSTRACT Temporal overlap between parental care and molt occurs frequently in birds, but few studies have examined how individuals manage conflicts between these 2 demanding phases of the annual cycle. The potential for trade-offs between molt and parental care is especially high in the Hooded Warbler (Setophaga citrina) because (1) all rectrices are replaced simultaneously during primary molt, leaving birds temporarily without a functional tail; and (2) the tail plays an important role in foraging, as birds use their white tail spots and tail-flicking behavior to startle insect prey. I examined how simultaneous rectrix molt affected late-season parental care in a color-banded population of Hooded Warblers in northwest Pennsylvania, USA. Of 62 adults initiating rectrix molt before the end of parental care, 43 (69%) deserted their late-season nestlings and fledglings, leaving the mate to provide all remaining parental care. Because females initiate rectrix molt significantly later than males, most instance...

Chipe encapuchado (Setophaga citrina) y otros registros notables de aves de Guerrero, México

Presentamos una lista de registros de distribución notable para 12 especies de aves en el estado de Guerrero, México, las cuáles observamos y colectamos durante el trabajo de campo llevado a cabo de 2006-2013 en diferentes ambientes del estado. Uno de los registros (Setophaga citrina) es nuevo para el estado, y otras cinco especies (e.g., Pelecanus erythrorynchos, Mycteria americana, Setophaga dominica) ampliaron su distribución geográfica a una provincia biótica donde su presencia no había sido previamente registrada. Confirmamos la presencia de otras seis especies (e.g., Elanus leucurus, Setophaga palmarum) con escasa información distribucional. Los registros presentados en esta nota científica muestran la importancia de continuar el trabajo de inventario en ambientes inexplorados e incluso en áreas que aparentemente están bien documentadas. Este tipo de información junto con estudios sobre la biogeografía, taxonomía y ecología, ayudarán a comprender mejor la historia natural de las aves y su conservación.

Binational climate change vulnerability assessment of migratory birds in the Great Lakes Basins: Tools and impediments.

Climate change is a global concern, requiring international strategies to reduce emissions, however, climate change vulnerability assessments are often local in scope with assessment areas restricted to jurisdictional boundaries. In our study we explored tools and impediments to understanding and responding to the effects of climate change on vulnerability of migratory birds from a binational perspective. We apply and assess the utility of a Climate Change Vulnerability Index on 3 focal species using distribution or niche modeling frameworks. We use the distributional forecasts to explore possible changes to jurisdictional conservation responsibilities resulting from shifting distributions for: eastern meadowlark (Sturnella magna), wood thrush (Hylocichla mustelina), and hooded warbler (Setophaga citrina). We found the Climate Change Vulnerability Index to be a well-organized approach to integrating numerous lines of evidence concerning effects of climate change, and provided transparency to the final assessment of vulnerability. Under this framework, we identified that eastern meadowlark and wood thrush are highly vulnerable to climate change, but hooded warbler is less vulnerable. Our study revealed impediments to assessing and modeling vulnerability to climate change from a binational perspective, including gaps in data or modeling for climate exposure parameters. We recommend increased cross-border collaboration to enhance the availability and resources needed to improve vulnerability assessments and development of conservation strategies. We did not find evidence to suggest major shifts in jurisdictional responsibility for the 3 focal species, but results do indicate increasing responsibility for these birds in the Canadian Provinces. These Provinces should consider conservation planning to help ensure a future supply of necessary habitat for these species.

Demographic Costs Associated with Differences in Habitat Space Occupancy

Delimiting the habitat characteristics describing the environmental conditions required by a species has become a critical tool for predicting organismal responses to environmental change. Grinnell emphasized the effects of environmental factors on the ability of a population to maintain a positive growth rate, yet few studies have included demographic or reproductive data in analyses of the Grinnellian niche. Identifying differences in habitat exploitation patterns in response to structural variation in the environment presents an incomplete description of the ability of species to adapt to changing habitats if demographic traits are not included. We estimated the vegetation characteristics used by individuals within a population of hooded warblers (Setophaga citrina) across a spatial transect that includes three structurally different forest habitats. We predicted individuals should select similar structural characteristics within each habitat and have similar reproductive success across sample sites. In the two years post burn, adults were present but no young fledged indicating the habitat requirements necessary for reproduction were absent in this habitat. We found significant differences in habitat space occupied by individuals in unaltered and harvested habitats. Nesting habitats used by female warblers differed from available habitat. Fledging success was lower in the harvested habitat 10 to 12 years post-harvest. In the harvested habitat, fledging success was greater on mesic slopes but decreased along a habitat gradient to xeric ridgetops, suggesting compensation in habitat use does not ameliorate fitness costs. In contrast, there was no difference in the number of fledged young along this gradient in the unaltered habitat. Based solely on occupancy data, traditional ecological niche models would incorrectly conclude the environmental characteristics found across the three forested habitats are included in the Grinnellian niche of the hooded warbler. However, examination of demographic and environmental data simultaneously allows differentiation between occupied habitat space and niche space.

Avian occupancy response to oak woodland and savanna restoration

ABSTRACTOak (Quercusspp.) woodlands and savannas throughout the Mid‐South, USA have become closed‐canopy forests through succession in the absence of fire, contributing to declines in disturbance‐dependent avian species. Restoration could ameliorate these trends, but effects of such management practices on avian communities of the Mid‐South are poorly understood. We implemented an experiment on the Cumberland Plateau, Tennessee with treatments involving fall (Oct) or spring (Mar) fire with woodland (14 m2/ha) or savanna (7 m2/ha) residual basal area and unmanaged control stands (20 ha each). We conducted fixed‐radius point counts 3 times annually (2010–2012) and analyzed trends in naïve occupancy for 41 species. For 20 species with adequate data (10 early‐, 10 late‐successional), we performed multiple‐season, robust‐design occupancy modeling and accounted for detection bias. We then used mixed‐model polynomial regression to define relationships between occupancy and measures of live and dead basal area, midstory density, and herbaceous groundcover. Restoration substantially altered forest structural characteristics but did not affect the occupancy of most late‐successional species. In contrast, the presence of early‐successional species increased as live basal area decreased and herbaceous groundcover increased. Only 3 of 41 species (hooded warbler [Setophaga citrina], ovenbird [Seiurus aurocapilla], and worm‐eating warbler [Helmitheros vermivorum]) exhibited reduced occupancy as woodland and savanna conditions were approached. The presence of all other species, including 13 regionally and 2 continentally declining, remained constant or increased as restoration progressed. Woodland conditions of 10 m2/ha live basal area and 20% herbaceous groundcover maximized occupancy of early‐successional species with little consequences for late‐successional species. Proceeding further toward savanna conditions negatively affected species associated with lower strata of mature‐forests but further increased the presence of some early‐successional species. Midstory density and dead basal area were poor predictors of occupancy and thus may be necessary but not sufficient in promoting early‐successional species. Our results suggest implementing oak woodland and savanna management in the Mid‐South could benefit disturbance‐dependent birds of high conservation priority with minimal negative impacts on the presence of late‐successional species. © 2016 The Wildlife Society.

English

Abstract. Surface mining transforms landscapes and ecosystem functions through the removal of vegetation and soil. Losses of vegetation correlate with declines, displacement, and transformations of songbird communities. Mine reclamation is a legislative requirement that can influence wildlife communities. The purpose of this study was to examine the avian community responses to mine reclamation practices and, as a proxy, assess the potential benefits and limitations of current reclamation approaches. Avian point counts were carried out at 202 plots on mined and surrounding non-mined areas throughout the Shale Hills Region of Alabama. These mines were reclaimed across a 26 year time-frame and using a variety of reclamation techniques. Six of the thirty-six bird species observed in high enough densities for detailed analysis showed differences of interest between reclaimed and random non-mined sites. Two species showed negative density responses: Carolina Chickadees (Poecile carolinensis), Hooded Warblers (Setophaga citrina). Conversely, densities of Field Sparrows (Spizella pusilla), Gray Catbirds (Dumetella carolinensis), Pine Warblers (Setophaga pinus), and Prairie Warblers (Setophaga discolor) responded positively to mine reclamation. We found that most mine reclamation in the Shale Hills Region of Alabama tended to shift habitat towards open canopy, edge, and grassland habitats. Though our study area tended to have fairly open forest structures (average basal areas around 13 m 2 /ha and average canopy closures around 50%), species that responded negatively were often associated with older sites with more closed canopies. Reclamation techniques should incorporate diverse canopy vegetation and thick mid-story cover to promote more complex vertical forest structure.

Estimating the Effects of Habitat and Biological Interactions in an Avian Community.

We used repeated sightings of individual birds encountered in community-level surveys to investigate the relative roles of habitat and biological interactions in determining the distribution and abundance of each species. To analyze these data, we developed a multispecies N-mixture model that allowed estimation of both positive and negative correlations between abundances of different species while also estimating the effects of habitat and the effects of errors in detection of each species. Using a combination of single- and multispecies N-mixture modeling, we examined for each species whether our measures of habitat were sufficient to account for the variation in encounter histories of individual birds or whether other habitat variables or interactions with other species needed to be considered. In the community that we studied, habitat appeared to be more influential than biological interactions in determining the distribution and abundance of most avian species. Our results lend support to the hypothesis that abundances of forest specialists are negatively affected by forest fragmentation. Our results also suggest that many species were associated with particular types of vegetation as measured by structural attributes of the forests. The abundances of 6 of the 73 species observed in our study were strongly correlated. These species included large birds (American Crow (Corvus brachyrhynchos) and Red-winged Blackbird (Agelaius phoeniceus)) that forage on the ground in open habitats and small birds (Red-eyed Vireo (Vireo olivaceus), House Wren (Troglodytes aedon), Hooded Warbler (Setophaga citrina), and Prairie Warbler (Setophaga discolor)) that are associated with dense shrub cover. Species abundances were positively correlated within each size group and negatively correlated between groups. Except for the American Crow, which preys on eggs and nestlings of small song birds, none of the other 5 species is known to display direct interactions, so we suspect that the correlations may have been associated with species-specific responses to habitat components not adequately measured by our covariates.

Stand-level bird response to experimental forest management in the Missouri Ozarks

Long-term landscape-scale experiments allow for the detection of effects of silviculture on bird abundance. Manipulative studies allow for strong inference on effects and confirmation of patterns from observational studies. We estimated bird-territory density within forest stands (2.89–62 ha) for 19 years of the Missouri Ozark Forest Ecosystem Project (MOFEP), a 100-year experiment designed to study the effects of even-age and uneven-age management on wildlife. We spot-mapped territories of 15 species in 228 stands for 5 years before treatment and 14 years after treatment to assess the effects of stand-level silvicultural treatments (clearcut, select cut, thin, and no-harvest) applied within even-age, uneven-age, or no-harvest (control) management sites and year on avian territory density. We used 2 a priori contrasts to compare pre-treatment bird densities with densities during early (3–5 yr) and late (12–14 yr) post-treatment periods. The interaction of silvicultural treatment and year had significant effects on the densities of all 15 species. Densities of hooded warbler (Setophaga citrina), indigo bunting (Passerina cyanea), prairie warbler (Setophaga discolor), and yellow-breasted chat (Icteria virens) increased significantly 3–5 years post-treatment with the greatest changes in clearcuts, but densities 12–14 years post-treatment did not differ from pre-treatment densities. Densities of Acadian flycatcher (Empidonax virescens), wood thrush (Hylocichla mustelina), and especially ovenbird (Seiurus aurocapilla) had significant decreases in clearcut stands after treatment and lesser decreases in select cut or thin stands post-treatment. Densities of black-and-white warbler (Mniotilta varia), eastern wood-pewee (Contopus virens), and Kentucky warbler (Geothlypis formosa) increased in clearcut, thin, and select cut stands, but these increases were short-lived and sporadic by year after treatment. Densities of Acadian flycatcher and ovenbird remained lower in clearcut stands than no-harvest stands 13 years post-treatment. The results of this manipulative experiment were mostly consistent with our predictions of bird response to common silvicultural treatments in these forests. Managers can use these species-specific responses to silvicultural treatment to guide management decisions for target species or to balance management practices in a landscape to meet the needs of multiple species. © 2014 The Wildlife Society.

White tail spots and tail-flicking behavior enhance foraging performance in the Hooded Warbler

Many species of insectivorous birds have contrasting plumage patches that are often displayed during foraging. Although such displays are widely hypothesized to flush potential prey and enhance foraging performance, experimental evidence that they function in this way was previously available only from redstarts (Myioborus spp.). I provide additional evidence from an experimental study of the Hooded Warbler (Setophaga citrina) in northwestern Pennsylvania, USA. Hooded Warblers regularly flick their tails while foraging, revealing large white spots on the outer tail feathers. I tested the function of the white tail spots with plumage manipulation experiments. At nests containing 5-day-old nestlings, the attending adults were captured and assigned to one of two treatment groups. For experimental birds, I temporarily darkened the white tail spots with a brown marking pen; for sham-darkened controls, I applied a comparable amount of ink to the dark inner rectrices. Experimental birds with darkened tails showed significantly reduced foraging performance compared with controls, and the decline in foraging performance was driven almost entirely by a decreased frequency of aerial prey attack. High-definition video recording at nests showed that plumage manipulation also altered the types of prey females delivered to nestlings; females with darkened tails delivered significantly fewer winged insects, and proportionally more insect larvae, than controls. However, the types of prey delivered by males were unaffected by plumage manipulation. Tail-flicking behavior codevelops with independent foraging in juveniles and is a significant positive predictor of juvenile foraging performance, even when the effects of juvenile age and activity are controlled statistically. Collectively, these results provide strong support for the hypothesis that white tail spots and tail-flicking behavior of Hooded Warblers function to flush visually oriented winged prey and enhance foraging performance. They also raise questions about sexual dimorphism in the tail pattern and its relationship to possible sex differences in foraging strategies.

A comparison of similarity-based approaches in the classification of flight calls of four species of North American wood-warblers (Parulidae)

Numerous methods are available for analysis of avian vocalizations, but few research efforts have compared recent methods for calculating and evaluating similarity among calls, particularly those collected in the field. This manuscript compares a suite of methodologies for analyzing flight calls of New World warblers, investigating the effectiveness of four techniques for calculating call similarity: (1) spectrographic cross-correlation, (2) dynamic time warping, (3) Euclidean distance between spectrogram-based feature measurements, and (4) random forest distance between spectrogram-based feature measurements. We tested these methods on flight calls, which are short, structurally simple vocalizations typically used during nocturnal migration, as these signals may contain important ecological or demographic information. Using the four techniques listed above, we classified flight calls from three datasets, one collected from captive birds and two collected from wild birds in the field. Each dataset contained an equal number of calls from four warbler species commonly recorded during acoustic monitoring: American Redstart, Chestnut-sided Warbler, Hooded Warbler, and Ovenbird. Using captive recordings to train the classification models, we created four similarity-based classifiers which were then tested on the captive and field datasets. We show that these classification methods are limited in their ability to successfully classify the calls of these warbler species, and that classification accuracy was lower on field recordings than captive recordings for each of the tested methods. Of the four methods we compared, the random forest technique had the highest classification accuracy, enabling correct classification of 67.6% of field recordings. To compare the performance of the automated techniques to manual classification, the most common method used in flight call research, human experts were also asked to classify calls from each dataset. The experts correctly classified approximately 90% of field recordings, indicating that although the automated techniques are faster, they remain less accurate than manual classification. However, because of the challenges inherent to these data, such as the structural similarity among the flight calls of focal species and the presence of environmental noise in the field recordings, some of the tested automated classification techniques may be acceptable for real-world applications. We believe that this comparison of broadly applicable methodologies provides information that will prove to be useful for analysis, detection and classification of short duration signals. Based on our results, we recommend that a combination of feature measurements and random forest classification can be used to assign flight calls to species, while human experts oversee the process.

Results of 20 years of experimental forest management on breeding birds in Ozark forests of Missouri, USA

Understanding the relationship between forest management and bird populations requires understanding the effects of silvicultural practices on avian demography at large spatio-temporal scales. The Missouri Ozark Forest Ecosystem Project (MOFEP) is a long-term, large-scale manipulative experiment testing the effects of even-aged (3–13ha cuts over 10–15% of the site; n=3), uneven-aged (0.03–3.14ha cuts over 57% of the site; n=3), and no harvest forest management on ecosystem level responses. We report on the effects of these management systems on the density and reproductive success of 11 songbird species from 5years of pre-harvest (1991–1995) to 14years of post-harvest (1997–2010). Density of four of the five mature forest species were lower after harvest in all management types and did not return to pre-harvest density, even in no harvest sites. Ovenbird (Seiurus aurocapilla) and Wood Thrush (Hylocichla mustelina) responded most negatively to even-aged management in the early post-harvest period (1997–2003) where density was significantly lower than in no harvest sites. Kentucky Warbler (Geothlypis formosa) density increased on uneven-aged and even-aged sites during early post-harvest, but returned to pre-harvest density on both management types by 14years post-harvest. Acadian Flycatcher (Empidonax virescens) and Worm-eating Warbler (Helmitheros vermivorus) had lower density in all treatments post-harvest. Among the six early-successional species, density of Blue-winged Warbler (Vermivora cyanoptera), Hooded Warbler (Setophaga citrina), Indigo Bunting (Passerina cyanea) White-eyed Vireo (Vireo griseus) and Yellow-breasted Chat (Icteria virens) were significantly higher in even-aged and uneven-aged sites than in no harvest sites after harvest. Density of Prairie Warbler (Setophaga discolor) were significantly higher only in even-aged management after harvest. Prairie Warbler, Hooded Warbler, and White-eyed vireo appeared on the study sites following harvest while Indigo Bunting, Prairie Warbler, and Yellow-breasted Chat were absent from the study area 14years post-harvest. Nest survival of mature forest and early-successional species did not change significantly from the pre- to the late post-harvest period or with forest management. Brood parasitism rates remained low from pre-harvest to late post-harvest, but parasitism rates were higher for early-successional species (4%) than mature forest species (1%). Although forest management had variable effects on species, we suggest a modified version of even-aged management could maximize benefits to early-successional species while minimizing decreases in mature forest bird species in central hardwood forests. Rather than the current prescription to harvest 10–15% of the mature stands every 15years, we recommend harvesting approximately half the number of mature stands with a shorter re-entry period of 8–10years.