On the journey to wintering sites, most migratory birds alternate between flights and stopovers, where they rest and refuel. In contrast to the time-minimization strategy commonly assumed to drive the pre-breeding migration, birds are rather expected to follow an energy minimization during post-breeding migration. It is the cumulative duration of flights and stopovers that determines the total energy requirements and duration of the journey. Since migrating birds actually spend most of the time at stopovers sites, selection to minimize the amount of energy or time spent on migration is likely to operate on the effectiveness of stopover rest and refueling. Here, we address the relative contribution of factors acting on departure decisions from a stopover site during the post-breeding migration in a long-distance migratory songbird. When capture probability is low, it is impossible to measure fattening over the entire duration of the stopover. To get around this limitation, we use time since arrival (TSA) as a proxy for the progressive temporal change occurring in the internal state of an individual (i.e. rest, physiological recovery, and fuel loading) during the stopover. We develop a capture–recapture model to address the respective effects of estimated TSA and of weather conditions on departure probability. Using a 20-year dataset for Sedge Warblers (Acrocephalus schoenobaenus), we show that TSA served as a surrogate of the most important information that birds use when deciding to depart from a stopover site, while low humidity and rising atmospheric pressure only slightly increase daily departure probability. Hence, a bird would resume migration mainly according to the time it had to rest and refuel, and then fine-tuning departure decision according to weather conditions. The generality of these results needs to be assessed by applying this modeling framework to other migratory species and at sites or times with greater weather variability.

Divorce is widespread among species of birds and may either be an adaptive strategy to secure a better mate or territory or be a nonadaptive result of a failure to maintain the pairbond. I examined the causes and consequences for divorce in the Northern Flicker (Colaptes auratus), a migratory woodpecker with a high annual mortality rate. In a long-term population study of 1,793 breeding pairs over 17 years, the within-season divorce rate was 4.6% and the between-season divorce rate was 15.5%. Retained pairs within a season initiated their renest 5 days faster than divorced birds that had no greater fledgling production, suggesting that within-season divorce was making the best of a bad job with severe time constraints. Poor performance in the year prior to divorce was not strongly associated with divorce, and analysis of multiple breeding stages revealed that divorcing individuals in the subsequent year had later laying dates, smaller clutches, and fewer fledglings than retained pairs but no better performance than widowed individuals. Analyzing the data separately by sex showed that neither males nor females benefitted from divorce. Thus, there is a reproductive cost linked to finding a new partner per se, but no reproductive advantage associated with divorce. New mates after divorce were usually not older (not higher quality) than previous mates, so intrasexual competition was probably not driving partnership splits. The most plausible explanation seems to be a “bet-hedging” hypothesis in which birds re-pair rapidly in spring if their previous mate does not quickly arrive during spring migration. Divorce in Northern Flickers does not appear to be adaptive and future studies on arrival and interactions of individuals in spring will elucidate proximate constraints on relocating the previous partner.

Dickcissel (Spiza americana) males occupying territories in cropland sites produced songs that were less similar on average to other Dickcissel songs in their neighborhood than did Dickcissels living in grasslands, where conformity to the local vocal culture was higher. Further, Dickcissel vocal culture changed more quickly over time in cropland sites relative to grassland sites. These differences may have resulted from the lower site fidelity we observed in Dickcissel males in cropland sites relative to grassland sites. We expected this link with site fidelity because we hypothesized that conformity to local culture in Oscine songbirds and the persistence of culture over time and space are promoted by habitats that facilitate stable populations. In contrast, sites in which habitat features cause rapid population turnover provide more territory vacancies and so more opportunities for colonization. Colonization should drive cultural change, either through adult colonists importing foreign cultural variants or young colonists making errors as they learn the local song. This potential link between population turnover and cultural stability may apply to animal cultures more broadly and so may be a fruitful area for further research. Besides the link between site fidelity and cultural change over time, we also investigated the possibility that habitats with different levels of site fidelity might show differences in the spatial scale of song similarity. However, we found no evidence of such a difference. Finally, although our conclusions regarding conformity and change in vocal culture were based on many recorded songs, automated assessments of song similarity imprecisely estimated the overall degree of song similarity. Thus, we may have underestimated the strength of the effects of time and distance on song similarity.

Determination of a bird's age or cohort is critical for studies on avian demography, occurrence patterns, behavior, and conservation management. Age designations have largely been developed in north-temperate regions and utilize calendar-based or seasonally based codes; however, in tropical regions and in the southern hemisphere, these coding systems have limited utility at best. To address these issues, we had previously devised the “WRP system,” based on the nomenclature of Humphrey and Parkes (H–P) and Howell et al., which defines molts in an evolutionary context applicable to birds globally. Here we refine and build upon core concepts and definitions of the WRP coding system, resolving key limitations that were identified during its first decade of use. The WRP system employs a three-letter alpha code in which each letter describes a different aspect of H–P terminology: the molt cycle (which informs a bird's age) and molt and plumage status within the cycle (each of which can also inform age). Here we recommend the continued use of most of the original (“core”) WRP coding while augmenting the system with an optional adjunct-code entry for comprehensiveness, clarity, and flexibility, and we clarify a few additional codes to cover less common molting and plumage strategies. For most users, from 7 to 13 core and 1 adjunct WRP code will be sufficient to describe all plumages and provide molt status and ages for demographic studies or other purposes. The revised WRP system is flexible enough to be adapted to the specific goals of programs while also providing core codes that can facilitate the comparison of avian age, molt, and plumage status on a global basis. We anticipate that our revised and standardized version of the WRP system will be easily adopted and could eventually replace calendar-based and seasonally based coding.

Understanding the functional significance of bird sounds can provide valuable insight into the behavior and how birds use habitat. We show that the Common Nighthawk (Chordeiles minor) wing-boom display is a territorial signal associated with the nest location that can be used to identify territorial habitat use. In other words, the Common Nighthawk wing-boom display can be considered analogous to song due to its potential function in territoriality. We captured, tagged, and tracked 21 male Common Nighthawks in northeastern Alberta to confirm the functional significance of the wing-boom display and describe Common Nighthawk territoriality. Mean wing-boom use density (hereafter “area”) size was 10.2 ha (SD = 11.7 ha). We found minimal overlap in the wing-boom area (5 of 15 neighboring male pairs, 0.2–4.5% overlap), suggesting the wing-boom display represents an exclusive territory. Comparison of wing-boom locations and random points within the wing-boom area confirmed that male Common Nighthawks select areas near the nest to perform wing-boom displays. There was high wing-boom area overlap for the same individual between years. Differences between years reflected shifts in nest location, suggesting that the wing-boom display is a good indicator of the nest location and territory. Future Common Nighthawk surveys should record the type of acoustic signal observed to differentiate territorial habitat use from other functions. Many taxa that produce non-vocal sounds as part of breeding displays could similarly benefit from a functional understanding to provide insight into habitat use.

We estimate hybridization rates among hummingbirds using nearly a million banding records from the United States and Canada. Annually from 2006 to 2019, an average of 44,600 individual hummingbirds and 14 hybrids were banded. Nearly all reports of hybrids come from localities west of the Mississippi, where multiple species breed in sympatry, whereas only Ruby-throated Hummingbird (Archilochus colubris) breeds east of the Mississippi. Adult male hybrids comprise 62% of all hybrids banded, a significantly greater fraction than “regular” adult males, which are 29% of all birds banded (excluding Ruby-throated Hummingbird). We infer that this excess of adult male hybrids is caused by ascertainment bias: banders more often misidentify female hybrids as parental species because females mostly lack species-specific showy sexual ornaments of male hummingbirds, making them harder to identify, rather than Haldane's rule of reduced survivorship of the heterogametic sex. Also influencing the apparent hybridization rate are banders, a few of whom seek out or avoid hybrids. After considering these biases, the data suggest that, in areas of the United States and Canada with >1 species, approximately 1 hummingbird in a thousand (0.1%) is an F1 hybrid.

Hummingbirds, a highly diverse avian family, are specialized vertebrate pollinators that feed upon carbohydrate-rich nectar to fuel their fast metabolism while consuming invertebrates to obtain protein. Previous work has found that morphologically diverse hummingbird communities exhibit higher diet specialization on floral resources than morphologically similar hummingbird communities. Due to the difficulties of studying avian diets, we have little understanding whether hummingbirds show similar patterns with their invertebrate prey. Here, we use DNA metabarcoding to analyze floral and invertebrate diets of 3 species of sympatric North American hummingbirds. We collected fecal samples from 89 Anna's (Calypte anna), 39 Black-chinned (Archilochus alexandri), and 29 Calliope (Selasphorus calliope) hummingbirds in urban and rural localities as well as across an elevational gradient from sea level to 2,500 meters above sea level in California, USA. We found hummingbirds showed high dietary overlap in both invertebrate and plant resources, with few invertebrate and plant families common to most individuals and many families found in only a few individuals. Chironomidae was the most common invertebrate family across all species, and Rosaceae and Orobanchaceae were the most common plant families. Anna's Hummingbirds had significantly higher invertebrate diet diversity than Black-chinned Hummingbirds when found at the same sites, but we found no difference in plant diet diversity among any of the 3 species. Hummingbirds in urban sites had higher plant diet diversity than in rural sites, but we found no effect of elevation on dietary richness. Our study shows how DNA metabarcoding can be used to non-invasively investigate previously unknown life-histories of well-studied birds, lending insight to community structure, function, and evolution.

DNA metabarcoding is a molecular technique frequently used to characterize diet composition of insectivorous birds. However, results are sensitive to methodological decisions made during sample processing, with primer selection being one of the most critical. The most frequently used DNA metabarcoding primer set for avian insectivores is ZBJ. However, recent studies have found that ZBJ produces significant biases in prey classification that likely influence our understanding of foraging ecology. A new primer set, ANML, has shown promise for characterizing insectivorous bat diets with fewer taxonomic biases than ZBJ, but ANML has not yet been used to study insectivorous birds. Here, we evaluate the ANML primer set for use in metabarcoding of avian insectivore diets through comparison with the more commonly used ZBJ primer set. Fecal samples were collected from both adult and nestling Purple Martins (Progne subis subis) at 2 sites in the USA and 1 site in Canada to maximize variation in diet composition and to determine if primer selection impacts our understanding of diet variation among sites. In total, we detected 71 arthropod prey species, 39 families, and 10 orders. Of these, 40 species were uniquely detected by ANML, whereas only 11 were uniquely detected by ZBJ. We were able to classify 54.8% of exact sequence variants from ANML libraries to species compared to 33.3% from ZBJ libraries. We found that ANML outperformed ZBJ for PCR efficacy, taxonomic coverage, and specificity of classification, but that using both primer sets together produced the most comprehensive characterizations of diet composition. Significant variation in both alpha- and beta-diversity between sites was found using each primer set separately and in combination. To our knowledge, this is the first published metabarcoding study using ANML primers to describe avian diet, and also the first to directly compare results returned by ANML and ZBJ primer sets.

Learned vocalizations play a key role in parrot social dynamics and vocal dialects have been documented for several mainland species, but to date no studies of geographically structured call variation in parrot species have examined the role of isolation on islands. In a study of the Brown-throated Parakeet (Eupsittula pertinax), which inhabits 5 small Caribbean islands as well as the adjacent mainland, we found that the contact calls of island and mainland parakeets show divergence in vocal characters as well as in call variability. We assessed call variation using 3 approaches: frequency measurements, spectrogram cross-correlation (SPCC) analyses, and call duration measurements. Island parakeets' calls were longer and had lower mean frequencies, and calls from different islands were distinguishable from each other as well as from mainland calls using measures derived from the SPCCs. In addition, we measured call variability at 2 different levels—within-location and within-individual. We found calls to be more variable for island parakeets for SPCC and duration measures, but less variable for frequency measures. The observed call differentiation among locations may be due to drift, whereas the lower frequency of island calls could either be a response to the windy environment on the islands or a consequence of the island subspecies' larger body sizes. We also hypothesize that the isolation of parakeet populations on small islands may have resulted in reduced selection for local call convergence, allowing island parakeets to produce more variable calls. We suggest that due to poor signal transmission in the windy island environment, selective pressures may favor variability in more easily perceived call features (like call duration) rather than more subtle features, like frequency shifts. Experimental tests are required to determine whether observed call patterns translate into similarly structured patterns in the responses to vocal variants.

Identifying the composition of avian diets is a critical step in characterizing the roles of birds within ecosystems. However, because birds are a diverse taxonomic group with equally diverse dietary habits, gaining an accurate and thorough understanding of avian diet can be difficult. In addition to overcoming the inherent difficulties of studying birds, the field is advancing rapidly, and researchers are challenged with a myriad of methods to study avian diet, a task that has only become more difficult with the introduction of laboratory techniques to dietary studies. Because methodology drives inference, it is important that researchers are aware of the capabilities and limitations of each method to ensure the results of their study are interpreted correctly. However, few reviews exist which detail each of the traditional and laboratory techniques used in dietary studies, with even fewer framing these methods through a bird-specific lens. Here, we discuss the strengths and limitations of morphological prey identification, DNA-based techniques, stable isotope analysis, and the tracing of dietary biomolecules throughout food webs. We identify areas of improvement for each method, provide instances in which the combination of techniques can yield the most comprehensive findings, introduce potential avenues for combining results from each technique within a unified framework, and present recommendations for the future focus of avian dietary research.

Since the early 1990s, aerial insectivorous birds have shown serious population declines in North America, but it is not clear if factors common to all species within this guild account for these declines. Among sympatric swallows, population trends differ, and this may be due to differences in ecology operating throughout the annual cycle. Although these species all feed on aerial insects, prey taxa can differ tremendously in their “aeroecology” and use by swallows. We examined the potential for dietary differences among three species of swallows, Barn Swallow (Hirundo rustica), Cliff Swallow (Petrochelidon pyrrhonota), and Tree Swallow (Tachycineta bicolor), breeding sympatrically in southern Ontario, Canada. Potential interspecific differences in nestling diet were examined using two endogenous biomarkers, DNA barcoding of nestling feces and stable isotope analysis (δ²H, δ¹³C, δ¹⁵N) of nestling feathers. We found evidence for differences in dietary sources of provisioned young where Barn Swallows provisioned more terrestrial-based prey, Cliff Swallows provisioned an intermediate diet, and Tree Swallows the most aquatic-emergent insect diet. We suggest this information may help to identify potential factors contributing to differential declines of aerial insectivores operating on the breeding grounds, including diet quality.