Hatching asynchrony in birds, which occurs when incubation begins before egg laying is complete, has been a topic of study for many decades. The “nest failure hypothesis” posits that the distribution of the risk of nest predation across the nesting cycle (from egg laying to the fledging of young) determines the optimal degree of hatching asynchrony. If such risk is higher earlier in the nesting cycle, then much asynchrony (e.g., incubation on the first egg laid) is generally favored. Alternatively, if the risk of nest predation is concentrated later in the cycle, then greater synchrony (a late start to incubation) is favored. Adult mortality during incubation favors greater hatching synchrony. Existing models suggest that optimal hatching asynchrony should depend on the ratio of daily mortality risks prehatching and posthatching, or early and late in the nesting cycle. We show that these classic theoretical expectations depend critically on the assumption of a single nesting attempt per breeding season. Allowing for multiple nesting attempts per season leads to a large range of possible outcomes, often with large deviations from the expectations of single-attempt models. We show further that daily nest survival probabilities across the entire nesting cycle combine to influence optimal hatching asynchrony, not just a subset of values, or a ratio of values, as suggested by earlier theory. In addition, assumptions about the transitions in daily survival values between nesting stages (abrupt vs. gradual changes) are also important determinants of hatching asynchrony. Overall, however, a consideration of multiple nesting attempts does not alter the general expectation that high risk early in the nesting cycle favors hatching asynchrony, and that greater synchrony is favored by high risk later in the cycle or adult mortality during incubation.

Avian eggshells exhibit a variety of colors and patterns whose functions include camouflage, detection of brood parasites, and facilitation of embryonic development. In addition, eggshell coloration, specifically blue-green coloration, may signal a female's quality to her mate. This signaling hypothesis relies on the assumption that biliverdin, the pigment responsible for blue-green eggshell coloration, is an indicator of female quality. Biliverdin is the product of heme degradation and has antioxidant and immunomodulating properties, supporting the possibility that if only a limited amount of biliverdin can be produced, deposition of this pigment into eggshells may be costly to females. However, these putative physiological functions of biliverdin were established by mammal-based studies, even though mammals produce an enzyme—biliverdin reductase—that readily converts biliverdin into bilirubin. Thus, the assumed physiological properties of biliverdin stem from data based on the end-product bilirubin, rather than biliverdin itself. Therefore, to assess the physiological role of biliverdin in birds, it is essential to quantify biliverdin in nonmammalian tissues in a manner that is repeatable, uses relatively accessible laboratory equipment, and is accurate at very low concentrations. Here, we develop a protocol that quantifies biliverdin in avian liver and spleen, the 2 organs that are predominately responsible for biliverdin production. Biliverdin concentration in both tissues was highly repeatable (R = 0.99) and could be accurately quantified at concentrations as low as 3.1 nM, which was less than the concentration of biliverdin in every sample that we tested. Recovery was calculated based on additions of biliverdin standard to each sample, and recoveries typically ranged from 60% to 95% for all 16 species (13 families, 4 orders) of birds in this study. Use of this assay will allow for the quantification of biliverdin concentration in a tissue-specific manner, which will be critical in determining the type and extent of biliverdin's physiological role.

Yellow-shafted Flickers (Colaptes auratus auratus subspecies group of the Northern Flicker) occasionally have orange to red flight feathers in eastern North America far from the hybrid zone with the Red-shafted Flicker (C. a. cafer subspecies group). Blocks of feathers of anomalous color tend to show bilateral symmetry and may differ from one year to the next in the same bird. It has been suggested that hybridization with cafer must be the source of the red color and that selection for that color could explain the high incidence of red or orange shafts in eastern flickers in some localities. Complex, though largely unproven, physiological mechanisms have been hypothesized to explain the variegated look. We evaluated a simpler, dietary explanation for the variation: that the pigment rhodoxanthin acquired exogenously at the time of feather molt comes to rest alongside the carotenoids normally present in these feathers. An exogenous source of rhodoxanthin exists in the berries of nonnative bush honeysuckles (Lonicera spp.) now naturalized in eastern North America and the American Midwest. We confirm the presence of rhodoxanthin and a probable metabolite, rather than the 4-keto-carotenoids found in the Red-shafted Flicker, in the red flight feathers of 2 Yellow-shafted Flickers from eastern North America. In these individuals, dietary rhodoxanthin appeared to interfere with the production of lutein, the main carotenoid in auratus. A fairly abrupt appearance of red color in earlier-molted primaries (usually p4 or p5) followed by its gradual fading in subsequent primaries in a large series of aberrantly colored flickers from eastern North America and the American Midwest supports a dietary explanation. We use data on the timing of replacement of primaries in the Northern Flicker at Manomet in Plymouth, eastern Massachusetts, to infer that these aberrantly colored Yellow-shafted Flickers on average acquired the unusual red pigment in early August.

Vaquero-Alba and colleagues published a study in The Auk: Ornithological Advances comparing objective color measurements of plumage taken in the field directly on a bird's body to those taken in the lab on collected feathers arranged to emulate the appearance of a bird's natural plumage. Although the field measures of plumage color were less repeatable than lab measures, the authors concluded that measurements taken in the field were more representative of a bird's “true color.” Accordingly, they recommend that researchers should bring spectrophotometers into the field to measure color on live birds. We question the assumption that their field measurements represent true color and highlight concerns regarding their experimental design and methodology. Because they did not measure color of live birds in the lab or the color of plucked feathers in the field, they cannot directly test whether the assessment of color in the field on a live bird is superior. Also, rather than assume field measures are the most accurate or precise way to assess plumage color, we suggest cross-validation with other methodologies, such as digital photography, pigment biochemistry, or measures of a known color standard in both environments. Importantly, researchers should be aware of the limitations and advantages of various methods for measuring plumage color so they can use the method most appropriate for their study.

Modeling metapopulation dynamics is potentially a useful and powerful tool for ecologists and conservation biologists. However, the key processes driving metapopulation dynamics are colonization and extinction events, which are notoriously difficult to study. A recent global assessment of Adelie Penguin (Pygoscelis adeliae) breeding populations reported multiple potential colonization and extinction events from occupancy observations using recent high-resolution satellite imagery and examination of historical occupancy literature, but emphasized that the events should be confirmed with direct observations. We evaluated these conclusions by developing a comprehensive database of direct observations of Adelie Penguin breeding site occupancy across East Antarctica. The database allowed 16 of 19 proposed colonization and extinction events in this region to be evaluated using direct observations that were concurrent in space and time (same breeding season) with observations from satellite imagery. We concluded that none of the 16 proposed colonization and extinction events had occurred. One true extinction event may have been correctly identified from satellite imagery, but a conclusive evaluation was not possible because the direct and satellite observations were not concurrent in time. The evaluation identified several sources of error in observations from satellite imagery, including errors of omission through failure to detect small colonies and errors of commission through misidentification of other biological and physical features as Adelie Penguin guano. The occupancy database corrected issues of poor precision and accuracy in locations of historical occupancy data. Our results improve understanding of key parameters for Adelie Penguin metapopulation dynamics and facilitate improved interpretation of satellite imagery in the future.

Breeding strategies can shape the timing of other events and processes, including arrival on the breeding grounds, prebasic molt, and departure for fall migration. We studied these relationships in sympatric Saltmarsh Sparrows (Ammodramus caudacutus) and Seaside Sparrows (A. maritimus), 2 closely related species with notably different breeding strategies. On average, females of both species arrived on the breeding grounds later, initiated molt later, and departed from the breeding grounds later than did conspecific males. Furthermore, we found that female Saltmarsh Sparrows—which mate with multiple males and care for nests, eggs, and chicks alone—were last to arrive on the breeding grounds and last to initiate molt, had the shortest molt duration, and were last to depart for the nonbreeding grounds. Both species exhibited protandry, but Seaside Sparrows averaged earlier arrival on the breeding grounds than Saltmarsh Sparrows. Molt and departure timing also differed between the species, with Seaside Sparrows initiating molt and departing before same-sex Saltmarsh Sparrows. These observations support the hypotheses that breeding strategies can influence arrival timing and that reproductive investment can have carryover effects on molt and departure.

We used high precision computed tomography (CT) and traditional radiography to study the nasal conchae, complex structures within the nasal cavity that condition air via countercurrent heat exchange. Air conditioning in the conchae assists thermoregulation and water balance, both of which pose challenges for many birds. We hypothesized that hot and water-limited environments would select for larger or more complex conchae to maximize moisture recapture during exhalation and in turn cause the evolution of deeper and wider bills. We provide the first intraspecific comparison of concha size and structure in birds based on CT scans of 15 individuals and radiographs of 39 individuals of 2 subspecies of Song Sparrow (Melospiza melodia) that inhabit climatically distinct habitats. CT scans revealed that middle and rostral conchae filled the nasal cavities and had larger surface areas in individuals with larger nasal cavities. The subspecies that inhabits hot and dry coastal dunes (M. m. atlantica) had relatively larger conchae and greater overlap of middle and rostral conchae than a nearby inland subspecies that inhabits moister environments (M. m. melodia). Radiographs revealed deeper and wider nasal cavities in the dune-endemic subspecies, further indicating they have larger conchae. Locations of maximum complexity of both conchae were more distal in the dune endemic subspecies. These anatomical differences suggest current or past divergent selection pressures on conchae; the larger conchae in the dune subspecies may allow greater water recapture while exhaling. The conchae and external bill are nested structures that were positively related in size and play functionally related roles in thermoregulation, therefore suggesting phenotypic integration. We hypothesize that the typically deeper and wider bill of the dune subspecies has evolved, at least in part, to accommodate larger conchae.

Competition intensity depends on the number of competitors and the amount of resources available. Coexistence of potential competitors can be enabled through niche differentiation or high resource availability. Using diet analysis, we investigated which of these 2 mechanisms was in play for coexisting shorebirds at a major staging site in the northern Yellow Sea, China, during northward migration in 2011 and 2012. Competition for food at this site is expected to be intense, with an estimated 250,000 migratory shorebirds gathering annually to refuel over a short period. Great Knots (Calidris tenuirostris), Eurasian Oystercatchers (Haematopus ostralegus osculans), and Red Knots (C. canutus) selected mostly the bivalve Potamocorbula laevis, whereas Bar-tailed Godwits (Limosa lapponica) had a broader diet and showed selection for polychaetes, even though most of their biomass intake was of P. laevis. Although all of these shorebirds fed on P. laevis, they showed different size selection and used different feeding methods. Bar-tailed Godwits, Great Knots, and Red Knots mainly swallowed P. laevis whole and preferred medium-sized P. laevis with relatively high ratios of flesh content to shell mass. By contrast, Eurasian Oystercatchers stabbed open P. laevis, ingested only the flesh, and preferred large P. laevis that provided the highest energetic return per prey taken. Despite evidence of niche differentiation in prey selection, the diets between the numerically dominant Bar-tailed Godwits and Great Knots overlapped substantially. Their coexistence seems to be enabled by high resource availability rather than niche separation.

Juvenile prospecting, or exploratory behavior for gleaning information about areas or events, can have profound effects on the selection of future breeding habitat, particularly for birds with high site fidelity whose initial choice of breeding habitat could represent a lifetime investment in fitness. We present data from a 10-yr study of Piping Plovers (Charadrius melodus) on the Missouri River, USA, to determine whether postfledging, hatch-year (HY) prospecting movements inform second-year (SY) nest site choices. First, we compared the home ranges of HY and adult (after-hatch-year; AHY) plovers to investigate whether differences in habitat use existed between HY and AHY plovers. Second, we evaluated the effects of prospecting, nest density, reproductive success, and nesting and foraging habitat availability on SY plover nest site selection. Lastly, we evaluated the potential reproductive benefits of nesting in prospected areas by comparing the nest success and fledging success of SY plovers that nested within their HY prospecting range with the success of those that nested in presumably unexplored areas. Plover home ranges varied across ages and among years. Both HY and AHY plover home ranges were smaller when the proportion of foraging habitat in the home range was relatively high, compared with nonforaging habitat. Second-year plovers selected sandbars that they had prospected as HY birds more often than would have been expected if nest site selection were random. Second-year plovers also selected sandbars on which other plovers experienced higher average nesting success than on random nesting locations, but we found no evidence that individual SY birds that nested on prospected sandbars had higher reproductive success than individuals that nested in unexplored areas. Coupled with high adult site fidelity, affinity of plovers for sandbars where they experience relatively high average reproductive success could lead to long-term gains in fitness.

The strategies by which foraging predators decide when to redirect their gaze influence both prey detection rates and the prey's ability to detect and avoid predators. We applied statistical analyses that have been used to study neural decision-making for gaze redirection in primates to 3 species of predatory birds with different sizes, visual systems, habitats, and hunting behaviors: the Northern Goshawk (Accipiter gentilis), Cooper's Hawk (A. cooperii), and Red-tailed Hawk (Buteo jamaicensis). The timing of head saccades was measured during visual searches using field video recordings of foraging raptors, and during a variety of behaviors using a miniature camera mounted on the head of a Northern Goshawk. The resulting statistical distribution of latencies (time between successive head saccades) was compared to predictions from various models proposed to describe visual search strategies. Our results did not support models that assume a constant probability of gaze redirection per unit time, a constant time for “giving up” on the visual search, or an initial setup time before visual search initiation. Instead, our data were fit best by a log-normal distribution, consistent with the raptors stochastically changing their gaze direction on the basis of accumulated environmental information. Specifically, this suggests that saccade initiation arises from a neural computation based on detection of a threshold level of a dynamically updated decision signal that encodes noisy sensory data, similar to the processes inferred from previous studies of visual search strategies in primates. The only significant between-species difference we found was a slower mean gaze-redirection rate for 2 larger species compared to the Cooper's Hawk, even though the latter has hunting behavior and maneuverability similar to that of the Northern Goshawk. Head-saccade latencies measured for a Northern Goshawk during different behaviors showed that the bird changed gaze direction significantly less frequently, on average, while perched than while in motion.

Determining the ecosystem function of high-order predators is critical for evaluation of food web interactions. Insectivorous birds are abundant predators in many ecosystems yet because they forage upon small taxa, it remains largely unknown whether birds are providing ecosystem services in the form of pest control or disservices by preying upon predaceous arthropod species. We extracted DNA from noninvasive fecal samples of adult and nestling Western Bluebirds (Sialia mexicana) in California vineyards. Using universal arthropod-specific primers, we sequenced prey items via massively parallel sequencing on the Illumina MiSeq platform. Bluebirds consumed a broad diet comprising 66 unique arthropod species from 6 orders and 28 families. Aedes sp. (mosquitoes: Culicidae), a previously unknown prey, was the most common item recovered, occurring in 49.5% of the fecal samples. Ectoparasitic bird blowfly (Protocalliphora) DNA was found in 7% of adult and 11% of nestling samples, presenting clear evidence of active feeding by the avian hosts on adult or larval ectoparasites. Herbivorous insects, primarily from the orders Hemiptera and Lepidoptera, represented over half (56%) of the prey items in bluebird diets. Intraguild predation (consumption of predator or parasitoid arthropods) represented only 3% of adult and nestling dietary items. Diets of adults were significantly different from nestlings as were diets from birds sampled in different vineyard blocks. Sex, date, number of young, and individual bird (based on resampled individuals) were all insignificant factors that did not explain diet variability. Nestling age was a significant factor in explaining a small amount of the variability in dietary components. In addition, our analysis of subsampling larger fecal samples and processing them independently revealed highly dissimilar results in all 10 trials and we recommend avoiding this common methodology. Molecular scatology offers powerfully informative techniques that can reveal the ecosystem function and services provided by abundant yet cryptic avian foragers.

Research is increasingly addressing the evolution and use of sexually selected traits in females. One strong area of interest is the display of intrasexual aggression and how female ornaments are used in intrasexual competition. One particular type of ornament focused on for its use in intrasexual aggression in both sexes is the melanin pigmented ornaments. Over the course of 2 breeding seasons in a southeastern Mississippi population of cardinals, we assessed brightness of the melanin face mask plumage ornament in female Northern Cardinals (Cardinalis cardinalis) and compared it to behavioral responses during intraspecific simulated nest intrusions (SNIs). All females responded to the SNI, but face mask brightness did not co-vary with the level of aggression shown. Our findings do not support earlier work suggesting that the face mask in female Northern Cardinals is an indicator of aggression at the nest. Potentially, differences in behavior and environmental variables between populations of cardinals could be factors in this difference and deserve further assessment. Future research should investigate other populations of cardinals to fully assess the communicative malleability of this ornament type and different selective pressures on female ornamentation and behavior.

The gut microbiota is a large and diverse community of microorganisms that provides many beneficial functions to the animal host; however, any change in the host's external or internal environment can affect microbiota composition. Migratory passerines arriving at stopover sites show highly variable microbiotas, which is likely reflective of the widely different habitats and foods utilized by migrants prior to arrival. If the previous environmental conditions led to the observed initial variability, then the microbiotas of birds should become more similar through stopover when migrants are in the same habitat and able to utilize similar resources. During spring 2014, migratory Swainson's Thrush (Catharus ustulatus), Wood Thrush (Hylocichla mustelina), and Gray Catbird (Dumetella carolinensis) were captured at a site in southwest Louisiana, USA, (1) upon arrival after crossing the Gulf of Mexico and (2) ≥1 day later during stopover. Fecal samples were collected and the microbial communities in them were analyzed using next-generation sequencing. The microbiotas of the majority of birds showed distinct shifts in community composition and became more similar during stopover, with birds stopping at the site for longer periods showing more pronounced changes in their microbiotas. These results are consistent with the hypothesis that local food-resource availability heavily influences the microbiotas of passerines; however, it is likely that gut remodulation during stopover after having crossed an ecological barrier also played a role.

Information is key to successful migration. Yet, how much information is learned and stored over the course of migration relative to other phases of the annual cycle is still unknown. We assessed seasonal variation in long-term memory formation indirectly by measuring the expression of cAMP response element binding protein (CREB) in hippocampal samples collected from migratory songbirds, Veery (Catharus fuscescens) and Wood Thrush (Hylocichla mustelina) at different life-history events (fall stopover, spring stopover, breeding season) during their first year of life. Consistent with our prediction, CREB expression was low during autumn (first) migration relative to levels in conspecifics during the first breeding season. These results suggest that young migratory birds may store relatively little spatial information during migration, particularly until they reach their wintering area and their global navigational map is more fully developed.

Seabirds inhabiting large, multispecies colonies face intraspecific and interspecific competition for prey and this often results in foraging strategies that partition resources. Here, we identified mechanisms that facilitate partitioning of resources between 2 congeneric tropical seabirds, Great Frigatebirds (Fregata minor) and Lesser Frigatebirds (F. ariel), for which traditional research methods have documented high levels of resource overlap. Stable isotope analysis (SIA) indicated that throughout the breeding cycle, male and female Great Frigatebirds consumed prey with higher δ¹⁵N compared to male Lesser Frigatebirds. This trend was not significant when comparing δ¹⁵N values of male and female Great Frigatebirds to female Lesser Frigatebirds. During the breeding period, GPS tracking and SIA indicated considerable spatial overlap among species and sexes. This contrasted with SIA of samples that provide insight into nonbreeding resource acquisition because these indicated that male Great Frigatebirds and male Lesser Frigatebirds had lower δ¹³C values than females of each species, signifying greater use of offshore foraging grounds by males of both species. Together these results suggest that body size differences influence trophic position of the prey consumed. Furthermore, central-place foraging constraints, and spatially unpredictable resource distribution, limit potential for spatial differences in foraging strategies when breeding. By contrast, spatial distribution of foraging differs during the nonbreeding period as the requirement for central-place foraging is lifted.

Physical and environmental barriers drive evolutionary diversification by limiting gene flow among populations. Rivers are barriers to gene flow in birds and other vertebrates, but differences in ecological conditions among sites also can affect the genetic structure of populations. The Magdalena River Valley (MRV) of Colombia, the largest South American river valley west of the Andes, is an appropriate location in which to test for the joint role of physical and ecological barriers because the river separates populations to the east and west, and a marked precipitation gradient leads to the occurrence of dry forests in the south and wet forests in the north. We conducted phylogeographic and population genetic analyses using mitochondrial DNA sequences of 4 avian species (Xiphorhynchus susurrans, Mionectes oleagineus, Leptopogon amaurocephalus, and Eucometis penicillata) sampled across 15 localities along the MRV. We found no spatial genetic structure in any of the species in the MRV region, and thus failed to find evidence for the role of the river or of the precipitation gradient as a driver of population differentiation. Gene flow across the river may be facilitated by its narrow headwaters and may occur as a consequence of river dynamics reducing its effectiveness as a barrier. Also, the dispersal abilities of the species may allow them to overcome the potential barrier that the river represents. As for the ecological barrier, we did not find that it caused any effect on the populations that we studied, probably because climatic differences were not strong enough to have led to population isolation along the MRV; alternatively, it is possible that the time elapsed since the origin of the precipititation gradient has been insufficient to lead to detectable effects on population structure. Our results contrast with work in other regions, which has shown marked genetic structure of vertebrate populations across major Neotropical rivers.

Recently, there have been contradictory results regarding relationships among 3 Australian parrot species known commonly as the Northern (Platycercus venustus), Pale-headed (P. adscitus), and Eastern rosella (P. eximius). An early phylogenetic analysis of the rosella genus Platycercus found that P. adscitus and P. eximius, 2 species putatively forming a hybrid zone, were sister taxa, while the geographically isolated P. venustus was in turn their sister taxon. However, a recent reexamination using multilocus sequence data found that evolutionary relationships differed depending on marker choice. Mitochondrial DNA (mtDNA) supported the earlier finding, but nuclear sequence data reflected an alternative relationship aligning P. adscitus and P. venustus as sister taxa. Two hypotheses were proposed to reconcile the discordance: (1) incomplete lineage sorting, and (2) mtDNA introgression from P. adscitus into populations of P. eximius. To clarify the relationships among these 3 species and to distinguish between the proposed hypotheses, restriction site-associated DNA (RAD) sequencing data from 23 individuals (∼4 individuals per subspecies) were examined. Species trees were generated based on datasets either excluding loci with missing data (393 loci) or including them (5 nonoverlapping subsets of 1,000 loci). Findings strongly supported a sister relationship between P. adscitus and P. venustus (posterior probabilities of 1 at all nodes), suggesting that the discordant patterns previously identified may have been the result of mitochondrial introgression and/or capture. This and other potential explanations are discussed.

Most landbirds migrate at night and typically make landfall in suitable stopover habitat before dawn. However, when birds find themselves over large water bodies at dawn, they must continue flying into the day and either finish crossing the water body and land on the far shore or backtrack to the near (i.e. first encountered) shore to land. Their collective decisions will influence how migrants are distributed among shoreline stopover habitats on either side of the water crossing. We studied birds during 4 spring migration seasons from 2010 to 2013 in the Great Lakes region, USA. We used 3 weather surveillance radars to observe migrating landbirds' behavior at dawn and subsequent terrestrial distributions during stopover. Mean flight heights over land and water were higher and mean flight directions were more oriented toward the closest shore at dawn when compared to peak migration earlier in the night. The wider the lake crossing, the higher that birds along the lakeshore flew at dawn. Seasonal mean stopover densities of migrants on land within 3 km of shorelines were 48% higher on the near shores of lakes (based on the seasonal mean direction of migration) than on the far shores. There was a moderate positive correlation (r = 0.584, P < 0.001, n = 358) between the seasonal mean density of birds aloft over water at dawn and the stopover density of birds in adjacent shorelines. Thus, birds over the water at dawn may tend to return to the near shore of the Great Lakes for stopover rather than continuing across the water. As a result, shoreline habitats on the near shores of lakes harbor greater densities of migrants and are thus critical stopover sites for migrating landbirds in the Great Lakes region.

Interspecific interactions, such as aggressive territorial behaviors, can be important in establishing range boundaries, especially in hybrid zones where 2 divergent taxa meet and interbreed. Further, differences in the aggressive responses among hybridizing taxa can be important in moving hybrid zones, especially when there are strong asymmetries in their aggressive interactions. To determine how between-taxon interactions may be contributing to hybrid zone movement between 2 species of sapsucker, we studied an aspect of territory maintenance: aggression toward a territory intruder between species. Using a series of field experiments involving playback and taxidermic mounts, we tested whether Red-naped (Sphyrapicus nuchalis) and Red-breasted (S. ruber) sapsucker differ in their levels of aggressive behavior, thereby influencing the dynamic nature of the hybrid zone between these species. We found that 2 of the 3 aggressive response variables we measured differed between the 2 species. Red-naped Sapsuckers showed higher levels of aggression, as measured by overall acoustic aggression and the time it took a bird to reach its closest position to a taxidermic mount. These differences, however, were opposite of our prediction that Red-breasted Sapsuckers would show a greater aggressive response, thus contributing to the expansion of Red-breasted Sapsuckers and movement of the hybrid zone. Despite these overall differences between species, we found no significant differences between sapsuckers in sympatry. Further, we argue that acoustic aggression may not be a good proxy for physical aggression, and may instead represent a difference in communication methods between species. We suggest that species asymmetries in territorial aggressive responses do not cause hybrid zone movement in this system. Nevertheless, our work highlights the importance of comprehensive studies of hybrid zones in assessing the mechanisms that may be important in maintaining species boundaries between hybridizing taxa.

Building upon the rich legacies of bioacoustics and animal communication, soundscape ecology represents a new perspective through which ecologists can use the acoustic properties of ecosystems to understand the complex interactions of organisms, geophysical dynamics, and human activities. In this paper, we focus on the potential benefits of a soundscape approach for enhancing ornithological research and of ornithological perspectives for advancing the nascent field of soundscape ecology. We first summarize 4 major grounding principles of soundscape ecology in relation to avian ecology, evolution, and behavior. We then propose 3 research objectives that we envision as future directions for soundscape ecology: development of (1) soundscape metrics and interpretation, (2) understanding of soundscape drivers, and (3) soundscape-based disturbance indicators. Ornithological contributions can help advance the field of soundscape ecology to obtain these research objectives across various spatial, temporal, and organizational scales. Detailed ornithological knowledge can aid in the improvement of soundscape databases, interpretation of soundscape metrics, and validation of soundscape theories. Such contributions should also invite input from other taxonomic-group specialists, further enriching soundscape ecology. Reciprocally, soundscape approaches can enrich ornithology by offering an acoustic-based theoretical framework grounded in a broad ecological context, hosting soundscape collections from diverse ecosystems, and advancing acoustic methodologies. This paper is intended to stimulate further discussion and collaboration between ornithologists, soundscape ecologists, and any researchers studying sound in an ecological context in order to enhance research in these important domains of ecology.

In many avian species, a male's colorful feather ornaments are known to signal individual quality and can serve as the basis for mate choice. Fewer studies have assessed whether this is true for females, who also can display plumage ornaments. We investigated whether breast and crown plumage reflectance in female Prothonotary Warblers (Protonotaria citrea) was correlated with annual reproductive success and nestling provisioning rate. We collected 160 feather samples from 112 nesting female warblers across 5 breeding seasons (2010–2014) in eastern Virginia, USA, and measured their reflectance. Because these data were from a population that was part of a long-term study, we knew the exact age and annual reproductive success of most individuals in all study years. We found a significant positive relationship between crown feather carotenoid content and the number of young fledged, after accounting for the effects of nest initiation date and female age, both of which are known to significantly influence annual reproductive success. There was also some support for an interaction between age and crown feather carotenoid content, such that older females had a stronger relationship between crown carotenoids and reproductive success compared with younger birds. We also had parental provisioning data for some females and found that individuals with higher breast feather carotenoid content provisioned their young more often than females with lower breast feather carotenoid content, and that this was particularly true when males provisioned young less often than average. These results indicate that female plumage is correlated with individual quality and may be used as a signal by males to select higher-quality females. However, more study is needed, and we cannot rule out the role of ornaments in other social contexts such as female–female competition.

Recent evidence suggests that blood sampling may negatively affect avian survival, which runs counter to the prevailing assumption that blood sampling, if performed properly, has little effect on survival in birds. Here, we take advantage of a long-term dataset (1986–2009) documenting the breeding biology of a population of Field Sparrows (Spizella pusilla) in northeastern Pennsylvania, USA, to assess the influence of blood sampling on sparrow survival. This study included 2 periods when blood sampling was not performed, interspersed with a period when blood was sampled. We examined the potential influence of blood sampling on annual apparent survival using Cormack-Jolly-Seber models in Program MARK. We also examined the influence of blood sampling on within-season site fidelity, as determined by whether birds remained on territory following collection of a blood sample. Our results suggest that apparent survival was actually higher for both sampled males and females in comparison to individuals captured during the same time interval but not sampled for blood. Further, apparent survival varied considerably over the years of the study (ranging from 0.27 to 0.59 in males and from 0.17 to 0.45 in females), and males consistently exhibited higher apparent survival than females. Finally, our results suggest that either (1) sampled birds had higher within-season site fidelity (males) or (2) blood sampling did not affect site fidelity (females). Blood sampling in our population of Field Sparrows did not appear to have any measurable short- or long-term effects on survival.

Many animals use cues from other species to gather information about foraging opportunities, but engaging in such behavior requires balancing the potential benefits with the risks of increased competition. Neotropical birds that follow army ants are an appropriate system within which to study such eavesdropping behavior. Mixed-species groups of these understory birds congregate around ant swarms, where they forage on insects flushed by the ants. Obligate ant-followers have specialized behaviors to track ant swarms and may serve as information sources for facultative ant-followers. Owing to strong dominance hierarchies among species, however, the value of the information likely depends on the competitive relationship between the information provider and the eavesdropper. I investigated whether ant-following birds preferentially followed the songs of some antbird species over others, testing the prediction that the vocalizations of small (subordinate) ant-following species would attract more individuals than the vocalizations of large (dominant) species. I conducted playback experiments that broadcast songs of the small White-cheeked Antbird (Gymnopithys leucaspis), the larger Reddish-winged Bare-eye (Phlegopsis erythoptera), and a control that does not attend ant swarms (the Peruvian Warbling-Antbird [Hypocnemis peruviana]). I also observed birds at swarms to investigate additional factors that may affect information utility. Although overall response rates to playback were low (as expected in this low-density setting), heterospecific individuals responded significantly more often to playback of songs of the White-cheeked Antbird compared with those of both the Reddish-winged Bare-eye and the control. Observations at swarms did not show significant differences in abundance or vocalization rate between species, so this difference in response rate may be explained by the White-cheeked Antbird's favorable combination of subordinate status and far-carrying song. These 2 factors (competition and cue prominence) are also important determinants of information utility in analogous systems involving other taxa. These dynamics of interspecific information use by ant-following birds may provide insights into the general mechanisms involved in the formation and maintenance of mixed-species foraging aggregations.