Disjunct distributions within a species are of great interest in systematics and biogeography. This separation can function as a barrier to gene flow when the distance among populations exceeds the dispersal capacity of individuals, and depending on the duration of the barrier, it may eventually lead to speciation. Here, we describe patterns of geographic differentiation of 2 disjunct populations of Diglossa brunneiventris separated by ∼1,000 km along the Andes. Diglossa brunneiventris vuilleumieriis isolated in northern Colombia, while Diglossa brunneiventris brunneiventris has a seemingly continuous distribution across Peru, Bolivia, and Chile. We sequenced mitochondrial and nuclear DNA of the 2 D. brunneiventris subspecies to evaluate whether they form a monophyletic clade, while including the other 3 species within the carbonaria complex (D. gloriosa, D. humeralis, and D. carbonaria). We also constructed ecological niche models for each D. brunneiventris subspecies to compare their climatic niches. We found that when using all available molecular data, the 2 D. brunneiventris subspecies are not sister lineages. In fact, each subspecies is more closely related to other species in the carbonaria complex. Our niche modeling analyses showed that the subspecies are occupying almost entirely different climatic niches. An additional and not expected result was that the carbonaria complex might encompass more cryptic species than previously considered. We suggest reevaluating the taxonomic status of these brunneiventris populations, especially the northern subspecies, given its highly restricted range and potential threatened status.

Phylogeographic structure within high-latitude North American birds is likely shaped by a history of isolation in refugia during Pleistocene glaciations. Previous studies of individual species have come to diverse conclusions regarding the number and location of likely refugia, but no studies have explicitly tested for biogeographic concordance in a comparative phylogeographic framework. Here we use a hierarchical approximate Bayesian computation analysis of mitochondrial DNA sequences from 653 individuals of 6 bird species that are currently co-distributed in the boreal forest of North America to test for biogeographic congruence. We find support for congruent phylogeographic patterns across species, with shallow divergence dating to the Holocene within each species. Combining genetic results with paleodistribution modeling, we propose that these species shared a single Pleistocene refugium south of the ice sheets in eastern North America. Additionally, we assess modern geographic genetic structure within species, focusing on Newfoundland and disjunct high-elevation populations at the southern periphery of ranges. We find evidence for a “periphery effect” in some species with significant genetic structure among peripheral populations and between peripheral and central populations. Our results suggest that reduced gene flow among peripheral populations, rather than discordant biogeographic histories, can explain the small differences in genetic structure and levels of genetic diversity among co-distributed boreal forest birds.

Ecological traits related to pace of life, such as foraging strategies and activity levels, influence daily energy expenditure (DEE) and can affect fitness. A fast pace of life tends to be supported by high-energy aerobic activity and is positively correlated with high DEE and basal and maximal metabolic rates in some endotherms. Given that maximal capacities for exercise and thermogenesis are both functions of aerobic muscle output and are often positively correlated with each other, high-energy aerobic lifestyles might be associated with high aerobic capacities, which would be expected to produce high thermogenic capacities as a side effect. We tested whether the high-energy aerial insectivore lifestyle in swallows is correlated with elevated basal and maximal thermogenic metabolic rates. We measured basal (BMR) and summit (M_sum = maximum cold-induced metabolic rate) metabolic rates in 6 species of swallows (Hirundinidae) and combined these data with literature data for additional swallows (n = 10 for BMR; n = 8 for M_sum) and non-aerial insectivore birds (n = 215 for BMR; n = 64 for M_sum) to address the hypothesis that swallows have higher BMR and M_sum than non-aerial insectivores. BMR in swallows was significantly higher than for non-aerial insectivore birds for phylogenetically adjusted analyses after correcting for body mass and region of origin (tropical vs. temperate). In contrast, M_sum did not differ significantly between swallows and non-aerial insectivores. Thermogenic scope (M_sum – BMR), however, was lower in tropical non-aerial insectivore birds compared with tropical swallows and temperate birds. This suggests that the aerial insectivore lifestyle elevates maintenance costs, but maximum thermogenic capacities are not clearly upregulated, despite tropical swallows having higher thermogenic scope than tropical non-aerial insectivores. These data suggest that the high-energy aerial insectivore lifestyle does not produce strong thermogenic side effects in swallows.

Migratory birds engage in 2 periods of endurance flight annually as they travel between summer breeding and overwintering grounds, and such endurance flights likely incur oxidative costs. These costs may differ between fall and spring migration, especially for females who must prepare for breeding and egg laying in spring. The objective of this study of a migratory bird was to test proposed hypotheses about how key components of the female's antioxidant system differ in response to flight training in the fall and spring and to dietary antioxidant supplementation. We hand raised female European Starlings (Sturnus vulgaris) and fed them either a diet supplemented with dietary anthocyanins or a diet without added anthocyanins. We flew females in a wind tunnel for 15 days during fall and spring migration seasons and measured over time oxidative lipid damage (d-ROMs) and 3 components of the antioxidant system: nonenzymatic antioxidant capacity (OXY), uric acid, and glutathione peroxidase (GPx) activity. Prior to flight training, OXY and oxidative damage were lower in females during spring compared with fall, and females fed a low-antioxidant diet had consistently higher circulating uric acid. GPx activity decreased more in spring immediately after a long-duration flight. Females fed a high-antioxidant diet had a greater decrease in OXY after the 15-day flight training. Flight-trained females had higher circulating uric acid than untrained females immediately after the longest-duration flight and decreased GPx activity after the 15-day flight training. In sum, females upregulated enzymatic and nonenzymatic endogenous antioxidants in spring, and females fed a diet with less antioxidants appear to compensate by increasing circulating uric acid. Our findings emphasize the important role of dietary antioxidants for birds during migration, and similar flights in fall and spring likely represent distinct oxidative challenges in the life history of female birds.

Do birds detect and respond to forces acting on feathers through filoplumes, which appear to be unique mechanosensory feathers? If filoplumes function as sensors, their morphology should covary with the morphology of their companion feather to better detect feather movements and position. We explore covariation in filoplumes and primaries across 5 species of birds that vary in body size, molt strategy, and the functional life span of their primaries (Green-winged Teal [Anas crecca], Ring-billed Gull [Larus delawarensis], Turkey Vulture [Cathartes aura], Red-tailed Hawk [Buteo jamaicensis], and Red-winged Blackbird [Agelaius phoeniceus]). Filoplumes never extended beyond the coverts and inserted immediately adjacent to the base of their companion primaries, positioning them to detect subtle changes in feather vibration or movement. Far more variation in filoplume number and morphology was due to species differences than to individuals or position in the wing. Across species, filoplume length and number increased with calamus length of primaries. In the 2 species with growing primaries, the number and length of filoplumes were only weakly associated with molting primaries, suggesting that filoplumes were not replaced when their companion primary was replaced. Further, filoplumes associated with a growing primary were not replaced synchronously, leaving others to sense primary position and movement. Finally, filoplume number and length were greatest in Red-tailed Hawks, a species that carries individual feathers for multiple years, but links between filoplume morphology and molt strategy await broader comparative studies. Taken together, the morphology of filoplumes and their replacement schedule relative to their associated primary suggests that they are sensors, capable of detecting subtle differences in the position and movement of their companion feathers.

The family Thamnophilidae is a species-rich Neotropical radiation of passerine birds. Current classification of its 235 species is mostly based on morphological similarities, but recent studies integrating comprehensive phenotypic and phylogenetic data have redefined taxonomic limits of several taxa. Here, we assess generic relationships of Herpsilochmus, Sakesphorus, Thamnophilus, Biatas, and Dysithamnus using DNA sequences from the mitochondrion, nuclear exons, and ultraconserved elements, with further attention to interspecific relationships within Herpsilochmus. We show that Herpsilochmus and Sakesphorus are not monophyletic. We resolve Herpsilochmus sellowi as a deep-branch sister to the monotypic genus Biatas and Sakesphorus cristatus as sister to a clade comprising Herpsilochmus sensu stricto and Dysithamnus. These results are consistent across loci, obtained via concatenation and coalescent-based analyses, and supported by likelihood-ratio tests of the distribution of our sampled coalescent histories. The phenotypic distinctiveness of both H. sellowi and Biatas argues against merging them into a single genus. Because no generic name is available for H. sellowi, we describe a monotypic genus. The polyphyly of Sakesphorus warrants recognition of the available generic name Sakesphoroides for the distinctive and monotypic S. cristatus. Furthermore, we recover 6 well-supported species groups within Herpsilochmus sensu stricto. Within the context of the family as a whole, the ubiquity of long terminal branches representing monotypic genera points to extinction events among ancestors of these lineages. We suggest that retention of ancestral characters or random genetic drift coupled with extensive extinction could explain the high degree of morphological and ecological similarity across these taxa, but we highlight the potential role of the environment in driving adaptive phenotypic convergence. Finally, our results send a cautionary message against the blind use of phylogenies containing imputed data based on taxonomy due to the increasingly frequent mismatches between traditional taxonomic classification and molecular phylogenies.

We compared support for 3 hypotheses that might explain observed morphological variation among islands of 4 species of Caribbean land birds: ecological release from competition and predation pressure, predation pressure from 1 novel predator species (small Indian mongoose, Herpestes auropunctatus), and climate. We measured wing chord, tarsus length, bill length, and mass of Bananaquits (Coereba flaveola), Black-faced Grassquits (Tiaris bicolor), Lesser Antillean Bullfinches (Loxigilla noctis), and Common Ground Doves (Columbina passerina) in Grenada, 2015–2017, and combined these measures with data from 23 other Caribbean islands collated from academic papers and researchers, for a total sample size of 6,518 individuals. We found the strongest support for the ecological release hypothesis, but each of our hypotheses received some support, suggesting that ecological release from competition, predation pressure from mongoose, and climate may all interact to influence morphological adaptations of birds to local conditions in the Caribbean.

Mixed-species flocks constitute community modules that can help test mechanisms driving changes to community composition across environmental gradients. Here, we examined elevational patterns of flock diversity (species richness, taxonomic diversity, species, and guild composition) and asked if these patterns were reflections of the full bird community at a given elevation (open-membership hypothesis), or if they were instead structured by environmental variables. We surveyed both the overall avian community and mixed-species flocks across an undisturbed elevational gradient (∼1,350–3,550 m) in the Bolivian Andes. We then tested for the role of temperature (a surrogate for abiotic stress), resource diversity (arthropods, fruits), and foraging niche diversity (vegetation vertical complexity) in structuring these patterns. Patterns for the overall and flocking communities were similar, supporting our open-membership hypothesis that Andean flocks represent dynamic, unstructured aggregations. Membership openness and the resulting flock composition, however, also varied with elevation in response to temperature and vegetation complexity. We found a mid-elevation peak in flock species richness, size, and Shannon's diversity at ∼2,300 m. The transition of flocking behavior toward a more open-membership system at this elevation may explain a similar peak in the proportion of insectivores joining flocks. At high elevations, increasing abiotic stress and decreasing fruit diversity led more generalist, gregarious tanagers (Thraupidae) to join flocks, resulting in larger yet more even flocks alongside a loss of vegetation structure. At lower elevations, flock species richness increased with greater vegetation complexity, but a greater diversity of foraging niches resulted in flocks that were more segregated into separate canopy and understory sub-types. This segregation likely results from increased costs of interspecific competition and activity matching (i.e., constraints on movement and foraging rate) for insectivores. Mid-elevation flocks (∼2,300 m) seemed, therefore, to benefit from both the open-membership composition of high-elevation flocks and the high vegetation complexity of mid- and low-elevation forests.

Aridland breeding bird communities of the United States are among the most vulnerable to drought, with many species showing significant population declines associated with decreasing precipitation and increasing temperature. Individual breeding bird species have varied responses to drought which suggests complex responses to changes in water availability. Here, we evaluated the influence of water deficit, an integrative metric of drought stress, on breeding bird communities within 3 distinct aridland habitat types: riparian, pinyon-juniper, and sagebrush shrubland. We used 12 years of breeding bird survey data from 11 national parks and monuments in the Northern Colorado Plateau Inventory and Monitoring Network (NCPN). We used a multivariate community-level approach to test for the effect of annual water deficit on the bird communities in the 3 habitats. We found that bird communities responded to water deficit in all 3 habitat types, and 70% of the 30 species–habitat combinations show significant relationships between density and variation in water deficit. Our analyses revealed that the direction and magnitude of species responses to water deficit were habitat-dependent. The habitat-specific responses we observed suggest that the adaptive capacity of some species depends on the habitat in which they occur, a pattern only elucidated with our habitat-based approach. The direction and magnitude of the relationships between predicted densities and annual water deficit can be used to predict the relative sensitivity of species within habitat climate changes. These results provide the first attempt to determine how the indirect effect of changes in water availability might affect aridland breeding birds in distinct habitat types. Linking breeding bird density to annual water deficit may be valuable for predicting changes in species persistence and distribution in response to climate change.

Energy acquisition and storage are important for survival and fecundity of birds during resource-limited periods such as spring migration. Plasma-lipid metabolites (i.e. triglyceride [TRIG], β-hydroxybutyrate [BOHB]) have been used to index changes in lipid stores and, thus, have utility for assessing foraging habitat quality during migration. However, such an index may be affected by energetic maintenance costs, diet, and other factors, and further validation under experimental conditions is needed to understand potential sources of variation and verify existing indices. We evaluated a plasma-lipid metabolite index using 30 female and 28 male wild Lesser Scaup (Aythya affinis; hereafter scaup) held in short-term captivity (∼24 hr) during spring migration. Similar to previous observational studies, BOHB was negatively associated and TRIG was positively associated with mass change (R² = 0.68). BOHB estimates were nearly identical to those published on free-living scaup, but TRIG estimates differed from free-living scaup and varied by sex, with females having a greater rate of predicted mass change than captive and free-living males. Our results suggest TRIG may be a better measure of energy income than deposition because lipid deposition likely varies with energetic maintenance costs, stress, and underlying physiological processes while TRIG relates primarily to energy income. In contrast, BOHB was a reliable predictor of negative mass change across sexes. The sex-based differences in apparent lipid deposition rates warrant further research before a generalizable model is advisable for comparing mass change predictions across studies. However, if predictions are standardized, this technique is generally robust to variations in energy income vs. lipid deposition across sexes. Accordingly, our evaluation provides verification for the utility of plasma-lipid metabolites as an indicator of foraging habitat quality during migration.

Incubation length and hatching asynchrony are integral elements of the evolved reproductive strategies of birds. We examined intra- and interpopulation variation in both traits for Eastern Kingbird (Tyrannus tyrannus) populations from New York (NY), Kansas (KS), and Oregon (OR) and found that both incubation length and hatching asynchrony were not repeatable among females, after controlling for a repeatable trait, clutch size. Instead, incubation length and clutch size were influenced by ambient temperature and precipitation. Incubation length exhibited the same median (15 days) and range (13–17 days) at all sites. Model selection results indicated that incubation periods for the smallest and largest clutches were longer in NY than KS when rain was frequent throughout incubation, in replacement nests, and likely when ambient temperatures were low during egg-laying. Full hatching usually required 2 days (but up to 3), with synchronous hatching associated with small clutch sizes, short incubation periods, frequent rain during the egg-laying period, and low ambient temperatures during the first half of incubation. Nestling starvation was uncommon (5–9% of nestlings monitored) and not associated with greater hatching asynchrony. These results indicate that while clutch size, a repeatable female trait, contributed to variation in incubation length and hatching asynchrony in Eastern Kingbirds, weather was a greater source of variation, especially for incubation length.

Most birds that show geographic variation in their songs discriminate between local and foreign songs, which may help them avoid unnecessary conflicts with vagrant individuals or similar-sounding congeners. However, some species respond equally to foreign and local songs, which may be useful if foreign individuals present territorial threats or if there are no sympatric congeners to avoid. Species without sympatric congeners are not commonly tested in playback studies, but they offer an opportunity to see how song variation and recognition unfolds when the pressure to avoid similar congeners is absent. Here, we use Verdins (Auriparus flaviceps), a monotypic genus of songbird with no confamilials in North America, to explore song variation and recognition in a species living without close relatives. We assessed geographic variation in song across the Verdin range and conducted a playback experiment using exemplars from 2 acoustically divergent and geographically distant regions as treatments. We found significant geographic variation in song that mapped well onto ecologically distinct desert regions. We found that Verdins had stronger vocal responses to local-sounding songs, but had equal movement responses to local-sounding and foreign songs. These results are similar to results found in other species without sympatric congeners and provide an example of a species that investigates acoustically divergent conspecific songs, despite recognizing salient differences in those songs.

Within animal hybrid zones, parasites may determine competitive outcomes between host species and thus affect hybridization dynamics. We addressed this hypothesis by evaluating haemosporidian prevalence and community composition in a rapidly moving hybrid zone between Black-capped Chickadees (Poecile atricapillus) and Carolina Chickadees (P. carolinensis). Using molecular methods, we screened for haemosporidians in multiple chickadee populations across the hybrid zone and investigated whether parasite prevalence varied as a function of admixture among these birds. We identified 36 parasite lineages from 3 haemosporidian genera (Plasmodium, Haemoproteus, and Leucocytozoon) but found no genera or lineages more likely to infect any particular chickadee taxon. Instead, haemosporidian prevalence varied across sites and seasons: Leucocytozoon was more prevalent during chickadees' breeding season, whereas Haemoproteus prevalence peaked during nonbreeding periods. Leucocytozoon infected proportionally fewer birds at the leading edge of the hybrid zone than near its center. However, haemosporidian communities were similar among chickadee populations, and evidence for parasite exchanges between chickadee taxa was lacking. These results underscore the complexity of bird–parasite relationships and suggest that haemosporidians are unlikely to play a major role in the ongoing movement of this hybrid zone.

Cooperative breeding behavior in birds ranges from inducible to obligate strategies and has evolved across diverse taxa, in species that display a wide range of social and reproductive behavior. It is often thought to evolve when independent breeding is constrained, and cooperation increases fitness. Yet many systems show variable, even maladaptive, fitness effects. This observation, together with the wide range in the form and frequency of cooperative breeding, raises the question of how the recurrent appearance of cooperative breeding and its extensive variation across species—from inducible to obligate—can be explained. Here, we take a proximate perspective on the evolution of cooperative breeding to argue that cooperative strategies are delineated by the history of prior adaptations and emerge through the rearrangement of preexisting neuroendocrine mechanisms underlying social, dispersal, and parental behaviors. Natural selection sorts among the resultant variants to alter regulation of cooperation, producing stabilization through either greater developmental entrenchment or greater reliance on environmental cues. Thus, species showing inducible cooperative behavior may be at a transitory stage in this process. To assess this possibility, we first evaluate evidence that the components of cooperative breeding are common across taxa. Then, we review the neuroendocrine mechanisms that regulate the pathways underlying cooperative strategies. Finally, we assess the evidence for neuroendocrine linkages during development that may channel coexpression of some components of cooperative breeding and facilitate its evolution. We conclude that understanding the mechanistic bases of the behaviors comprising cooperative breeding strategies may provide novel insight into the recurrent emergence of this strategy across disparate environments and avian taxa.

The timing of events in birds' annual cycles is important to understanding life history evolution and response to global climate change. Molt timing is often measured as an index of the sum of grown feather proportion or mass within the primary flight feathers. The distribution of these molt data over time has proven difficult to model with standard linear models. The parameters of interest are at change points in model fit over time, and so least-squares regression models that assume molt is linear violate the assumption of even variance. This has led to the introduction of other nonparametric models to estimate molt parameters. Hinge models directly estimate changes in model fit and have been used in many systems to find change points in data distributions. Here, we apply a hinge model to molt timing, through the introduction of a double-hinge (DH) threshold model. We then examine its performance in comparison to current models using simulated and empirical data. Our results suggest that the Underhill–Zucchini (UZ) and Pimm models perform well under many circumstances and appear to outperform the DH model in datasets with high variance. The DH model outperforms the UZ model at low sample sizes of birds in active molt and shorter molt durations and provides more realistic confidence intervals at smaller sample sizes. The DH model provides a novel addition to the toolkit for estimating molt phenology, expanding the conditions under which molt can accurately be estimated.

LAY SUMMARY

More than one-third of the bird species found in the Caribbean are endemic to a set of neighboring islands or a single island. However, we have little knowledge of the evolutionary history of the Caribbean avifauna, and the lack of phylogenetic studies limits our understanding of the extent of endemism in the region. The Sharp-shinned Hawk (Accipiter striatus) occurs widely across the Americas and includes 3 endemic Caribbean taxa: venator on Puerto Rico, striatus on Hispaniola, and fringilloides on Cuba. These island populations have undergone extreme declines presumably due to ecosystem changes caused by anthropogenic factors, as well as due to severe hurricanes. Sharp-shinned Hawks, in general, and Caribbean Sharp-shinned Hawks, in particular, have not been placed in a modern phylogenetic context. However, the island taxa have historically been presumed to have some ongoing gene flow with mainland populations. Here we sequenced ultraconserved elements (UCEs) and their flanking regions from 38 samples, focusing on Caribbean taxa. Using a combination of UCEs, mitochondrial genome sequences, and single-nucleotide polymorphisms, we investigated the phylogenetic relationships among Caribbean lineages and their relationships to mainland taxa. We found that Caribbean Sharp-shinned Hawks are reciprocally monophyletic in all datasets with regard to mainland populations and among island taxa (with no shared mtDNA haplotypes) and that divergence in the NADH dehydrogenase 2 gene (ND2) between these mainland and island groups averaged 1.83%. Furthermore, sparse non-negative matrix factorization (sNMF) analysis indicated that Hispaniola, Puerto Rico, and mainland samples each form separate populations with limited admixture. We argue that our findings are consistent with the recognition of the 3 resident Caribbean populations as species-level taxa because nuclear and mitochondrial genetic data indicate reciprocal monophyly and have species-level divergences, there is no sharing of mitochondrial haplotypes among or between island taxa and those on the mainland; and they are diagnosable by plumage.