Using whole mitochondrial DNA sequences from 89 White-tailed Eagles (Haliaeetus albicilla) sampled from Iceland, Greenland, Norway, Denmark and Estonia between 1990 and 2018, we investigate the mitogenomic variation within and between countries. We show that there is a substantial population differentiation between the countries, reflecting similar major phylogeographic patterns obtained previously for the control region of the mitochondria, which suggested two main refugia during the last glacial period. Distinct mitogenomic lineages are observed within countries with divergence times exceeding the end of the last glacial period of the Ice Age. Deviations from neutrality indicate that these lineages have been maintained by natural selection and there is an excess of segregating amino acids in comparison with number of fixations suggesting a large load of deleterious mutations. The maintenance of the distinct mitogenic lineages within countries inflates our estimates of divergence times.

Both sexes of Whiskered Auklets (Aethia pygmaea) display the most elaborate feather ornaments of any seabird: a slender black forehead crest, and 3 bilaterally symmetrical pairs of white facial plumes (superorbital, suborbital, and auricular). We studied patterns of ornament variation in 796 banded individuals (147 of known sex, 254 of known age from 1 to 16 years) during 1992–2009 at Buldir Island (principally), and 3 other Aleutian Islands (Davidof, Ulak, and Egg) in Alaska, USA. As expected for socially selected traits, ornaments were more variable across individuals than anatomical traits in size but with only slightly male-biased sexual dimorphism. Body condition index increased from age 1 to 3 years but changed little thereafter. Even within birds ≥4 years old, ornament size was positively related to body condition index. Subadults (one-year-olds) had smaller ornaments than adults (age 2–16 years) but there was no further change in ornament size as adults aged and no evidence of senescence even in the oldest birds (>8 years old). Nonetheless, overall ornament size varied from year-to-year at Buldir and was correlated with indices of both ocean climate and auklet productivity in the preceding 2–5 years. From Buldir to Egg Island (1,266 km), the size of both anatomical and ornamental traits increased by 5–15% except for bill depth, which was largest in birds from Buldir and Egg at opposite ends of the Aleutian breeding range. This study is one of few to examine patterns of ornament variation in a long-lived, socially monogamous bird, even though such patterns are crucial to understanding the relationship between sexual selection and life history.

Migration consists of a sequence of small- to large-scale flights often separated by stopovers for refueling. Tradeoffs between minimizing migration time (more flights, shorter stopovers) and maximizing energy gain (fewer flights, longer stopovers) will affect overall migration timing. For example, some individuals make long-term stopovers in high-quality habitat that maximize energy gain (e.g., molt-migration), but movement to those habitats likely costs time. We used radio telemetry and blood plasma metabolite levels to examine physiological and behavioral tradeoffs between molt-migrant (birds molting at the molt stopover; n = 59) and post-molt (birds that presumably completed their molt elsewhere; n = 19) migrant Swainson's Thrushes (Catharus ustulatus) near Montreal, Canada. Molt-migration was a large time investment as the average stopover duration for molt-migrants was of 47 ± 9 days (∼13% of the entire annual cycle), almost twice as long as previously assumed from banding records, and far longer than stopovers of post-molting individuals (7 ± 2 days). Daily mortality rate during the molt stopover was similar to the average annual daily mortality rate. Molt-migrants' circadian rhythms closely matched light levels, whereas post-molting birds had irregular rhythms and averaged 1 hr greater activity per day than molt-migrants. Despite being less active, molt-migrants had similar refueling rates based on metabolite profiles. As compared with migrants that completed molt earlier, molt-migrants at this stopover site had slower subsequent migration rates. Thus, birds using long-term stopovers appeared to tradeoff energy (efficient refueling) for time (slower subsequent migration).

We investigated Dial's 2003 hypothesis that birds that rely more heavily on flight as their primary mode of locomotion and thus invest more in their forelimbs than hindlimbs will experience selection for smaller body sizes, greater altriciality, and more complex nests. To test this hypothesis, we examined the skeletons of over 2,000 individuals from 313 species representing the majority of avian families and all major branches of the avian tree. We used the lengths of the sternal keel and long bones of the wing relative to the lengths of the leg long bones as an index of relative locomotor investment. We found that locomotor investment was predicted by flight style, foraging method, and length of nestling period, supporting Dial's hypothesis. Soaring birds and birds with more acrobatic flight styles, birds whose foraging methods were heavily reliant upon flight, and birds whose young spent more time in the nest tended to invest more in their forelimbs relative to hindlimbs. Nest type and body size were not significant predictors of relative forelimb–hindlimb investment, however, suggesting that the relationships among flight style, locomotor investment, and life history are not as tightly intertwined as Dial originally hypothesized.

Investigamos la hipótesis de Dial del 2003 de que las aves que dependen más del vuelo como su modo principal de locomoción y, por lo tanto, invierten más en sus extremidades anteriores que posteriores, experimentarán una selección para tamaños corporales más pequeños, crías más altriciales y nidos más complejos. Para evaluar esta hipótesis, examinamos los esqueletos de más de 2.000 individuos de 313 especies que representan la mayoría de las familias de aves y todas las ramas principales del árbol aviar. Usamos las longitudes de la quilla esternal y de los huesos largos del ala en relación con las longitudes de los huesos largos de la pierna como un índice de inversión locomotora relativa. Encontramos que la inversión locomotora fue predicha por el estilo de vuelo, el método de forrajeo y la duración del período de cría, lo que respalda la hipótesis de Dial. Las aves planeadoras y las aves con estilos de vuelo más acrobáticos, las aves cuyos métodos de forrajeo dependen en gran medida del vuelo y las aves cuyos juveniles pasan más tiempo en el nido tendieron a invertir más en sus extremidades anteriores que en las posteriores. Sin embargo, el tipo de nido y el tamaño del cuerpo no fueron predictores significativos de la inversión relativa entre las extremidades anteriores y posteriores, lo que sugiere que las relaciones entre el estilo de vuelo, la inversión locomotora y la historia de vida no están tan estrechamente entrelazadas como hipotetizó originalmente Dial.

Correct assessment of species limits and phylogenetic relationships is a prerequisite for studies in ecology and evolution. Even in well-studied groups such as birds, species delimitation often remains controversial. Traditional avian taxonomy is usually based on morphology, which might be misleading because of the contingent nature of evolutionary diversification. The sand plover complex (genus Charadrius) may be such an example wherein 2 Lesser Sand Plover C. mongolus subspecies groups have been proposed to comprise 2 species. We use genome-wide data of 765K SNPs to show that the widely accepted taxonomic treatment of this sand plover complex appears to be a paraphyletic grouping, with two Lesser Sand Plover subspecies groups found not to be each other's closest relatives, and with the mongolus subspecies group being the sister taxon of Greater Sand Plover C. leschenaultii. Based on genomic and acoustic analyses, we propose a three-way split of the Sand Plover complex into the Siberian Sand Plover C. mongolus, Tibetan Sand Plover C. atrifrons, and Greater Sand Plover C. leschenaultii. The similar sizes of the Siberian and Tibetan Sand plovers may be the result of niche conservatism coupled with rapid morphological and ecological differentiation in the Greater Sand Plover. Gene flow between the non-sister Tibetan and Greater Sand plovers might have happened in phases of secondary contact as a consequence of climate-driven range expansions. We call for further studies of the Sand Plover complex, and suggest that speciation with intermittent gene flow is more common in birds than currently acknowledged.

Mixed-species flocks presumably provide birds with antipredator and foraging benefits. The foraging benefits hypothesis predicts that a reduction in arthropod abundance will trigger flocking activity; however, flocking activity may also be influenced by the difficulty of detecting arthropods, a seldom explored possibility. We found that environmental traits (temperature and foliage density) combined with arthropod abundance explained arthropod detection by birds in the Yungas foothill forest of NW Argentina. Prey detection was inversely related to ambient temperature and foliage density while positively associated with arthropod abundance. Based on this result, we built a structural equation model using a latent proxy variable for arthropod detectability, arthropod crypsis, integrating ambient temperature, foliage density, and proportion of immature arthropods. This model allowed us to compare the relative importance of arthropod abundance and the difficulty in detecting prey items as predictors of flocking propensity. After 2 yr of studying 129 mixed-species flocks, 1,351 bird foraging sequences, and 25,591 arthropod captures, we found that the flocking propensity of birds was only significantly correlated with arthropod detectability and not with arthropod abundance. Flocking propensity peaked when the arthropod community was comprised of proportionately more immature and non-flying arthropods, the temperature was low, and the foliage cover was denser; all factors are contributing to a low arthropod detectability. Finally, we evaluated whether joining mixed-species flocks provided foraging benefits such as increased foraging efficiency. Individuals benefited from joining flocks by an average increase of their prey-capture attempt rate of 40%, while the search rate increased by 16%. Our results add a new perspective on the drivers of mixed-species flocking by showing that the capacity to find prey items may have a more significant effect than prey abundance per se.

Woodcreepers (Dendrocolaptinae) represent a remarkably uniform group of brownish birds that move by hitching up tree trunks as they forage for arthropod prey. Despite these superficial similarities, we were able to uniquely differentiate the niches of all 13 species north of Manaus by integrating morphological traits (e.g., mass and bill size) with behavioral traits (e.g., sociality, stratum use, and foraging maneuvers). The 5 ant-following (myrmecophilous) species, with their larger bodies and heavier bills, were morphologically distinct from the 7 species that join mixed-species flocks. A combination of vertical stratum, mass, and bill length further distinguished among mixed-flocking species. Two canopy species—the solitary Dendrexetastes rufigula and the mixed-flocking Lepidocolaptes albolineatus—consistently foraged at higher strata than other species. For the remaining mixed-flocking species, the largest 3 species differed significantly by mass, whereas the smallest 3 species, which overlapped broadly in mass, were uniquely distinguished by bill length. The 5 ant-following species differed in their degree of specialization on ant swarms, from facultative (Hylexetastes perrotii) to obligate (Dendrocincla merula). The ant-followers also showed nearly discrete mass distributions that essentially differed by Hutchinsonian 1:1.3 ratios, which likely allows them to maintain interspecific dominance hierarchies at the front of raiding army ant swarms. The behaviors we quantified (sociality, vertical strata, and myrmecophily), together with morphology (mass and bill size), separated all 13 species. We speculate that niche partitioning and competitive exclusion allow each woodcreeper to uniquely access invertebrate prey, permitting coexistence and contributing to high alpha diversity at our study site.

Understanding species' responses to temperature via behavior, and the factors affecting the extent of behavioral responses, is a critical and timely endeavor given the rapid pace at which the climate is changing. The young of altricial songbirds are particularly sensitive to temperature, and parents may modulate temperatures at nests via selection of nest sites, albeit to a largely unknown extent. We examined whether sagebrush-obligate songbirds, that reproduce within an open ecosystem with wide temperature fluctuations and span a range of body sizes, selected their nest sites on the basis of temperature. We further investigated whether nest predation risk and ambient conditions modulated temperature-based choices. We placed temperature loggers at nest sites and in unused but available nest niches and nest shrubs along a known predation-risk gradient and used nearby weather stations to determine ambient temperatures. The two smaller-bodied birds, Brewer's Sparrow (Spizella breweri) and Sagebrush Sparrow (Artemisiospiza nevadensis), selected nest shrubs and niches that were warmer and less variable relative to unused sites whereas the larger bodied species, Sage Thrashers (Oreoscoptes montanus), did not. Brewer's Sparrows and Sage Thrashers dampened selection for warmer nest sites when temperatures experienced during the nest-site prospecting period were warmer. None of the three species altered nest-site selection with respect to temperature in response to ambient temperature variability or our index of nest predation risk. The microhabitat characteristics that most influenced temperatures at nests varied across species. Our results suggest that songbirds can modulate temperatures at nests to some extent, and such responses can vary depending on the conditions experienced prior to nest initiation. Responses also varied across species, likely reflecting different physiological tolerances. The extent to which breeding birds will be able to continue to proximately influence temperature via nest-site choices likely will depend on the extent and rate of future climatic shifts.

Behavioral flexibility of individuals is essential if organisms are to ultimately adapt to climate change. As environmental conditions, such as precipitation patterns become increasingly variable, fine-scale spatiotemporal flexibility in space use may allow for individuals to track resources during periods of adverse or atypical conditions. Individual behavioral flexibility is observable over short timeframes and can therefore be used to assess resilience of a species to projected shifts in climate. The goal of our study was to determine if and how individuals modified their space and habitat use in response to rainfall-driven changes in resources throughout a period of atypical seasonal rainfall patterns. We used radio telemetry to estimate home ranges of nonbreeding Swainson's Warblers (Limnothlypis swainsonii) in 2 time frames (bi-seasonally and bi-weekly) in Jamaica during dry and wet periods. We measured habitat structure and food (leaf litter arthropod) availability within each home range to determine possible predictors of space use change. Individuals modified the area and/or location of their home ranges with changes in precipitation, and those occupying more open habitats had greater changes in home range area as seasonal rainfall increased. As food increased following rain, individuals constricted their home ranges (bi-weekly) or shifted spatially (bi-seasonally) to a novel area with greater food availability. This suggests individuals are able to rapidly respond to how their environment changes, presumably adjusting to trade-offs between home range size and resource availability. This flexibility may be a key behavioral component in enduring long-term increasingly unpredictable environmental variability and may have population-level consequences. These responses are, however, mediated by habitat, suggesting the ability to respond to variable or poor conditions is not homogeneous across a population.

The Pacific Basin, by virtue of its vastness and its complex aeroscape, provides unique opportunities to address questions about the behavioral and physiological capabilities and mechanisms through which birds can complete spectacular flights. No longer is the Pacific seen just as a formidable barrier between terrestrial habitats in the north and the south, but rather as a gateway for specialized species, such as shorebirds, to make a living on hemispherically distributed seasonal resources. This recent change in perspective is dramatic, and the research that underpins it has presented new opportunities to learn about phenomena that often challenge a sense of normal. Ancient Polynesians were aware of the seasonal passage of shorebirds and other landbirds over the Pacific Ocean, incorporating these observations into their navigational “tool kit” as they explored and colonized the Pacific. Some ten centuries later, systematic visual observations and tracking technology have revealed much about movement of these shorebirds, especially the enormity of their individual nonstop flights. This invites a broad suite of questions, often requiring comparative studies with bird migration across other ocean basins, or across continents. For example, how do birds manage many days of nonstop exercise apparently without sleep? What mechanisms explain birds acting as if they possess a Global Positioning System? How do such extreme migrations evolve? Through advances in both theory and tracking technology, biologists are poised to greatly expand the horizons of movement ecology as we know it. In this integrative review, we present a series of intriguing questions about trans-Pacific migrant shorebirds and summarize recent advances in knowledge about migratory behavior operating at temporal scales ranging from immediate decisions during a single flight, to adaptive learning throughout a lifetime, to evolutionary development of migratory pathways. Recent advances in this realm should stimulate future research across the globe and across a broad array of disciplines.

Ecological disturbance is a key agent shaping the spatial and temporal landscape of food availability. In forests of western North America, disturbance from fire can lead to resource pulses of deadwood-associated arthropods that provide important prey for woodpeckers. Although the foraging strategies among woodpecker species often demonstrate pronounced differences, little is known about the ways in which woodpeckers exploit and partition prey in disturbed areas. In this study, we employed DNA metabarcoding to characterize and compare the arthropod diets of 4 woodpecker species in Washington and California, USA—Black-backed Woodpecker (Picoides arcticus), Hairy Woodpecker (Dryobates villosus), Northern Flicker (Colaptes auratus), and White-headed Woodpecker (Dryobates albolarvatus)—primarily using nestling fecal samples from burned forests 1–13 years postfire. Successful sequencing from 78 samples revealed the presence of over 600 operational taxonomic units (OTUs) spanning 32 arthropod orders. The nestling diets of two species in particular—Northern Flicker and Black-backed Woodpecker—proved to be much broader than previous observational studies suggest. Northern Flicker nestlings demonstrated significantly higher diet diversity compared to other focal species, all of which displayed considerable overlap in diversity. Wood-boring beetles, which colonize dead and dying trees after fire, were particularly important diet items for Black-backed, Hairy, and White-headed woodpeckers. Diet composition differed among species, and diets showed limited differences between newer (≤5 yr) and older (>5 yr) postfire forests. Our results show mixed evidence for dietary resource partitioning, with three of the four focal species exhibiting relatively high diet overlap, perhaps due to the pulsed subsidy of deadwood-associated arthropods in burned forests. Woodpeckers are frequently used as management indicator species for forest health, and our study provides one of the first applications of DNA metabarcoding to build a more complete picture of woodpecker diets.

The extent to which interspecific competition structures species interactions and coexistence within communities, and the relevant mechanisms, are still debated. We focus on New World wood warblers (Parulidae), beginning with Robert MacArthur's iconic 1958 paper in which he shows how subtle foraging behaviors, purportedly linked to dietary differences, within spruce trees contribute to the coexistence of 5 spruce-woods warbler species. MacArthur coined the phrase “resource partitioning”, and profoundly impacted the field of Ecology for subsequent decades in diverse ways. To understand what MacArthur got right and what he missed, we reviewed both ecological and evolutionary approaches to questions of the origin and coexistence of competing species in the context of diet. We argue that an important, underappreciated, mechanism of competition among coexisting migratory warbler species, particularly in winter, is diffuse exploitation competition, based in part on our own studies of warbler diets in relation to foraging behavior, substrate use, bird morphology, and other traits. Our review and synthesis of interspecific competition and coexistence in warblers have important consequences, including our questioning of the importance and effectiveness of resource partitioning in birds. We also suggest a novel hypothesis for the success of warblers today in the Caribbean and other habitats, beginning with their relatively recent adaptive radiation and the ecological opportunity on Caribbean islands.