Inbreeding has been difficult to quantify in wild populations because of incomplete parentage information. We applied and extended a recently developed framework for addressing this problem to infer inbreeding rates in Northern Spotted Owls (Strix occidentalis caurina) across the Pacific Northwest, USA. Using pedigrees from 14,187 Northern Spotted Owls, we inferred inbreeding rates for 14 types of matings among relatives that produce pedigree inbreeding coefficients of F = 0.25 or F = 0.125. Inbreeding was most common in the Washington Cascades, where an estimated 15% of individuals are inbred. Inbreeding was lowest in western Oregon (3.5%) and northern California (2.7%), and intermediate for the Olympic Peninsula of Washington (6.1%). Estimates from the Olympic Peninsula were likely underestimates because of small sample sizes and the presence of few pedigrees capable of resolving inbreeding events. Most inbreeding resulted from matings between full siblings or half siblings, although a high rate of inbreeding from mother–son pairs was identified in the Olympic Peninsula. Geographic variation in inbreeding rates may reflect population declines and bottlenecks that have been detected in prior investigations. We show that there is strong selection against inbred birds. Only 3 of 44 inbred birds were later identified as parents (6.8%), whereas 2,823 of 10,380 birds that represented a comparable cross section of the data were later seen as reproducing parents (27.2%). Habitat loss and competition with Barred Owls (S. varia) remain primary threats to Northern Spotted Owls. However, given the negative consequences of inbreeding, Spotted Owl populations in Washington with suitable habitat and manageable numbers of Barred Owls may benefit from translocations of individuals from Oregon and California to introduce new genetic variation and reduce future inbreeding events.

In 2007, the first miniature light-level geolocators were deployed on small landbirds, revolutionizing the study of migration. In this paper, we review studies that have used geolocators to track small landbirds with the goal of summarizing research themes and identifying remaining important gaps in understanding. We also highlight research and opportunities using 2 recently developed tracking technologies: archival GPS tags and automated radio-telemetry systems. In our review, we found that most (54%) geolocator studies focused on quantifying natural history of migration, such as identifying migration routes, nonbreeding range, and migration timing. Studies of behavioral ecology (20%) uncovered proximate drivers of movements, including en route habitat quality; that migration routes, but not timing, may be flexible in some species; and different age and sex classes show significant differences in migration strategy. Studies of the evolution of migration (9%) have illustrated that migration is a potential barrier to hybridizing species or subspecies, and some work has correlated gene polymorphisms and methylation patterns with migration behavior. Studies of migratory connectivity (11%) have shown that a moderate level of connectivity is common, although variability across and within species exists. Studies of seasonal interactions (7%) have found mixed results: in some cases, carryover effects have been identified; in other cases, carryover effects are buffered during intervening stages of the annual cycle. Archival GPS tags provide unprecedented precision in locations of nonbreeding sites and migration routes, and will continue to improve understanding of migration across large spatial scales. Automated radio-telemetry systems are revolutionizing our knowledge of migratory stopover biology, and have led to discoveries of previously unknown stopover behaviors. Together, these tracking technologies will continue to provide insight into small migratory landbird movements and contribute important information for conservation of this rapidly declining group.

Interspecific aggression is common in birds—Individuals regularly chase and attack members of other species. We analyzed cases of interspecific aggression between crows (American Crow [Corvus brachyrhynchos] and Northwestern Crow [C. caurinus]) and ravens (Common Raven [C. corax]) using ∼2,000 citizen scientist observations of interspecific aggression between crows and ravens from across North America. Crows and ravens may attack one another for nonadaptive reasons, because they compete with each other for food and space, or because they are nest predators of each other. We report 3 main results. First, although ravens are much larger than crows, crows chased and attacked ravens in ∼97% of observations. We observed this strong asymmetry in the direction of interspecific aggression throughout North America. Second, crow aggression toward ravens was most frequent during the crows' nesting season (∼March–May), but also occurred in other seasons. Third, crows typically attacked ravens in small groups, with relatively few observations of aggression in one-on-one contests. The pattern of seasonality in crow attacks on ravens suggests that nest predation by ravens is an important factor driving interspecific aggression from crows. Aggression also occurred outside the breeding season, particularly in the winter, indicating either that crows compete with ravens for resources at this time of year or that crows preemptively harass potential nest predators in advance of the breeding season. This study is an example of how citizen scientists can contribute to the study of behavioral interactions of birds at a continental scale.

The Common Raven (Corvus corax) is widespread at high latitudes and a noted disperser to remote islands. We hypothesized that, given their dispersal abilities, Alaskan populations would be genetically similar and maintain genetic diversity across thousands of kilometers. We sampled 134 ravens from 9 areas in Alaska, including 6 populations we considered as mainland and 3 as island (Kodiak, Adak, and Attu islands). Using 8 microsatellite loci, we found that in most Alaskan populations gene flow is sufficient to counter population divergence due to genetic drift. Our dataset suggests that conservatively there are just 2 raven populations in Alaska (K = 2), although the second Aleutian population, from Adak Island, was significantly differentiated, too. The Attu Island population was quite different from the other sampled populations, and it was the only location that neither received immigrants nor sent emigrants to other sampled locations. In addition, a comparison of gene flow models using a Bayesian coalescent approach most strongly supported an Attu Island isolation model. Prior work suggested that the uniqueness of the Attu Island population is due to isolation in a glacial refugium during the last glacial maximum. Postglacial dispersal from the Alaskan mainland has not erased this signal, suggesting that there could be limits to the species' dispersal abilities (i.e. remote colonization is rare). The isolated Attu Island population underwent a substantial decline ∼4 Kya, which roughly coincides with the arrival of the ultimate invasive species, humans. Traditional cultural values suggest that any surmised effect of humans would likely be indirect rather than direct.

Predictable seasonal changes in resources are thought to drive the timing of annual animal migrations; however, we currently understand little about which environmental cues or resources are tracked by different migratory bird species across the planet. Understanding which environmental cues or resources birds track in multiple migratory systems is a prerequisite to developing generalizable conservation plans for migratory birds in a changing global environment. Within the New World, climatic differences experienced by Nearctic–Neotropical migratory (NNM; i.e. breed in North America and spend the nonbreeding period in the Neotropics) and Neotropical austral migratory (NAM; i.e. breed and spend the nonbreeding period wholly within South America) bird species suggest that their migratory strategies may be shaped by unique selective pressures. We used data gathered from individuals fitted with light-level geolocators to build species distribution models (SDMs) to test which environmental factors drive the migratory strategies of species in each system. To do so, we evaluated whether temperature, precipitation, and primary productivity (NDVI) were related to the seasonal distributions of an NNM (Eastern Kingbird [Tyrannus tyrannus]) and NAM species (Fork-tailed Flycatcher [T. savana]). Both Eastern Kingbird and Fork-tailed Flycatcher locations were positively correlated with high precipitation during their nonbreeding seasons. Eastern Kingbird locations were positively correlated with both NDVI and temperature during their breeding season and both pre- and post-breeding migrations. Fork-tailed Flycatcher locations were positively correlated with both temperature and precipitation during both migrations, but only temperature during the breeding season. The value of extending the application of geolocator data, such as in SDMs, is underscored by the finding that precipitation was such an important predictor of the nonbreeding distributions of both types of migrants, as it remains unclear how global climate change will affect wet–dry cycles in the tropics.

Despite increasing attention to geologically recent extinctions in the ocean, little information is available on natural distributions of extinct species before contact with humans. In the North Pacific, a large cormorant, Phalacrocorax perspicillatus (Spectacled/Pallas's Cormorant), was driven to extinction in the 19th century, probably due to overexploitation by humans. So far, no clear evidence has existed for the past presence of the cormorant species outside Bering Island, Commander Islands. We report fossil remains of P. perspicillatus from the upper Pleistocene (Marine Isotope Stage 5e, ∼120,000 yr ago) of Shiriya, northeast Japan. The occurrence is the first definitive record of the species outside Bering Island, and the first pre-Holocene record for the species, expanding the known geographic and temporal ranges of the species by ∼2,400 km and ∼120,000 yr, respectively. It indicates that the species had undergone a drastic range contraction or shift since the Pleistocene, probably before the first contact with humans. Available evidence indicates a drop of oceanic productivity near Shiriya in the Last Glacial Maximum (∼17,000 yr ago), which might have affected the local population of the cormorant species and possibly led to the species' local disappearance from this area. In any case, the population of P. perspicillatus on Bering Island at its first scientific discovery should be considered a relict.

Temporal variation in avian brood parasite condition and reproduction seem to affect host–parasite dynamics. Few studies, however, consider dynamics from the perspective of the parasite. Here we examined how brood parasite body condition and reproductive output vary both seasonally and annually and investigate the resultant impacts on nest parasitism rates of a host species. In the breeding seasons of 2011 and 2012, we collected female Brown-headed Cowbirds (Molothrus ater) from Fort Hood, Texas, and conducted morphometric, phenotypic, and physiological measurements on carcasses. During the same period, we also monitored nests of Black-capped Vireos (Vireo atricapilla) and recorded parasitism occurrence. Based on an analysis of >400 cowbirds, we found that cowbird body condition was significantly lower in 2012 than in 2011. Fewer females developed follicles in 2012 and follicle development was substantially delayed. Further, nest parasitism rates were significantly lower in 2012 and vireo nest success was significantly higher. The substantial variation we observed in cowbird body condition, follicle development, and parasitism may be related to a record-breaking drought that occurred in 2011. Cowbirds appeared to suffer negative carryover effects from the drought, likely due to reduced food resources, although similar effects on vireos were not observed. Detrimental effects of drought on cowbirds but not on vireos may have significant implications for host–parasite dynamics under changing climate conditions.

Predation is a key factor in the nesting preferences of birds. Studies indicate that cavity-breeding birds prefer deeper nest sites, possibly because they are more safe from predation. We studied the Blue Tit (Cyanistes caeruleus), a cavity-breeding passerine, to test (1) whether nest-site depth affects breeding success and (2) whether potential effects of nest-site depth on breeding success are related to predation risk. We performed 2 experiments to separate effects of nest-box depth from potential effects of the quality of the breeding pair. In the first (free-choice) experiment, Blue Tits competed for scarce deep nest boxes that were provided well before nest-box choice, enabling an association between nest-box quality and bird quality. In the second (forced-choice) experiment, we randomly altered nest-box depth after Blue Tits had chosen a nest box, thus disconnecting the association between nest-box quality and bird quality. We found no evidence that the occurrence of signs of predation was related to nest-box depth. However, we did find clear positive effects of nest-box depth (1) on clutch size and hatching success throughout the study area and (2) on fledging success, the number of fledglings, and the overall probability of nest success, specifically in parts of the study area with high predation. We found no indication of independent effects of parental quality on breeding success. Parents also seemed to perceive the shallower boxes as more risky; in shallower boxes, nest thickness was decreased, irrespective of the local predation pressure during the free-choice experiment. Parents nesting in shallow boxes may have had lower breeding success because of (1) increased actual (but undetected) predation and (2) reduced reproductive investment by parents, based on the latter's experience with predation or an evolutionary response to past predation risk.

Theory suggests that animals should select resources on the basis of perceived fitness benefits, and avoid other available resources that may result in fitness costs. However, resource selection is both a scale- and a life-stage-specific process, in which the importance of different resources may vary among spatial scales of selection and phases of species' life cycles. We investigated multilevel resource selection and summer survival of Ruffed Grouse (Bonasa umbellus) in Maine, USA, to understand life-stage-specific resource selection and the cost of brood rearing. At the landscape level, male Ruffed Grouse selected areas with greater densities of woody stems taller than breast height, while females avoided these areas and selected areas with greater ground coverage by Rubus. Males further selected areas with greater stem densities at the local level, while females again avoided areas with greater stem densities and selected locally available areas with greater ground coverage by Rubus. Weekly survival rates (± SE) were reduced for females that were actively tending broods (S = 0.9393 ± 0.0294) when compared with nonreproductive individuals (both males and nonbrooded females; S = 0.9966 ± 0.0034). These differences in survival based on reproductive status were better supported than a difference associated solely with sex and resulted in a 69% chance that a female would survive raising a brood to 6 wk, compared with a 98% chance of survival for nonreproductive birds over the same 6 wk period. Our results suggest that male Ruffed Grouse selected resources that reduced their mortality risk, thereby investing in survival and their future reproductive potential, whereas brood-rearing females selected resources to benefit current reproduction and incurred a cost to their own survival in the process.

During migration, birds require stopover habitat to rest and refuel before resuming flight. While long-distance migratory flights represent a large energy investment, stopover accounts for roughly two-thirds of a bird's total energy expenditure during migration. Therefore, birds should minimize energy expenditure while also minimizing time and predation risk during stopover. To understand activity during migration, we recorded activity patterns (i.e. fine-scale movements associated with a range of behaviors) of 3 species, Red-eyed Vireo (Vireo olivaceus), Swainson's Thrush (Catharus ustulatus), and Wood Thrush (Hylocichla mustelina), at a stopover site along the northern coast of the Gulf of Mexico during autumn migration using automated radio telemetry. We found Red-eyed Vireos to be the most active and Swainson's Thrushes the least active. For each species, we used boosted regression trees to investigate associations between activity and factors known to influence bird behavior during stopover. While species differed, day of year and temperature were important predictors of activity for all species. Vireos were active early in the season, under light winds and warmer temperatures, and on evenings when winds were more favorable. Thrushes were more active as the season progressed and when temperatures were cooler. Thrush activity also differed between years, although thrushes increased activity later in the season during all years. Our results illustrate automated radio telemetry as a unique and valuable tool for understanding fine-scale behaviors of birds during stopover.

Partial migration—a strategy in which some individuals are resident and others are migratory within the same population—is widespread among avian species and could play an important transitional role in the evolution of migratory behavior. Nevertheless, detailed movement data are unavailable for most partial migrant species. We examined migration strategies of the American Crow (Corvus brachyrhynchos), a partially migratory species that overwinters in large communal roosts, from which some birds migrate north to breed. We used a combination of satellite telemetry, isotopic signatures (δ²H), and molecular markers (33 microsatellites) to describe and characterize the migratory movements of individuals from overwintering roosts on the east coast (Utica, New York) and west coast (Davis, California) of the United States. We collected 11,951 data points from 18 satellite-tagged individuals between 2014 and 2018, among which 14 (77.8%) were migratory (8 of 11 and 6 of 7 birds on the west and east coasts, respectively). Migration distances were 280–1,095 km and 177–793 km on the west and east coasts, respectively. Individual birds were consistent in their migratory behavior across years, and breeding-site fidelity was high: both migratory and resident birds returned to the same location in the breeding season of each year. Both isotopic signatures and molecular markers could generally differentiate residents from long-distance migrants (i.e. those breeding at latitudes >3.5°N of the resident populations), but they did not consistently differentiate residents from migrants with shorter migration distances. Overall, these data on the migratory movements of American Crows and the proportion of migrants in their roosts can serve as a baseline against which to test predictions about how partial migrants will respond to environmental alterations such as climate change and urbanization.

Marine birds in Alaska, USA, have been monitored systematically for more than 4 decades, and yet it remains unclear why some populations have increased while others have declined. We analyzed the population dynamics of 5 seabird species—Black-legged (Rissa tridactyla) and Red-legged kittiwakes (R. brevirostris), Common (Uria aalge) and Thick-billed murres (U. lomvia), and Tufted Puffins (Fratercula cirrhata)—across 4 decades in Alaska. We tested hypotheses that each species' carrying capacity varied continuously through time with climate cycles and/or in response to habitat covariates. Using an information-theoretic approach, we evaluated competing candidate stochastic growth models of each species' annual rate of change, incorporating various environmental covariates. The North Pacific Index and Pacific Decadal Oscillation were the most important climatic covariates across the whole of Alaska, where the former generally was negatively related to rates of population change, and the latter positively related. Across the 40-yr time series, we found slight decreases in zooplankton (i.e. krill) concentrations across the Gulf of Alaska, and significant increases in sea surface temperature across the Aleutian Islands. Kittiwakes showed the greatest level of sensitivity to these 2 environmental changes. Our results provide evidence that deteriorating secondary productivity (i.e. euphausiids) has contributed to declines of Black-legged Kittiwakes in the Gulf of Alaska. In contrast, the carrying capacity of murres has increased across the state, even in regions affected by warming waters and reduced productivity. These results suggest that kittiwakes act as indicators of detrimental impacts of climatic variability, whereas murres demonstrate resilience to such environmental change. Identifying the ecological factors that explain seabird population dynamics is necessary to understand the implications of climate and environmental change for long-term marine ecosystem dynamics.

Three North American passerines are known to perform transoceanic flights during their fall migration, with open-water flights ranging in length from 1,700 to 3,400 km. However, little is known about within-population variation of these flights. From 2013 to 2017, I used geolocators to study variation in the fall migratory track of the Bobolink (Dolichonyx oryzivorus) from a population that bred in agricultural grasslands of Vermont, USA. Thirteen of seventeen birds took transoceanic flights during fall migration, ranging in length from 1,098 to 3,536 km (mean ± SD = 1,969 ± 640); five of these flights were nonstop from North to South America. Bobolinks initiated transoceanic flights between Nova Scotia, Canada, and the central coast of North Carolina, USA, an orthodromic distance of 2,500 km. Mean arrival to 12° latitude in South America was October 17 ± 17 days (range: September 26–November 26). Neither sex, morphological measurements, nor age significantly explained variation in nonstop flight length.

The “good-genes” hypothesis to explain female extra-pair mating states that females benefit from this behavior by acquiring better genes for their offspring. Despite extensive research, results are mixed, and the predictions of the good-genes hypothesis have been met in fewer than half of published papers. One possible explanation for this lack of consensus is that the benefits of extra-pair copulation are context-dependent. Here we use chick size, the probability of fledging, and telomeres, the protective caps of chromosomes, as markers for individual quality. Telomere length (TelL) integrates across many stressors and covaries with probability of survival and reproductive success. To test whether benefits to extra-pair (EP) matings are context-dependent we look at the telomere length of extra-pair and within-pair offspring (EPO and WPO, respectively) reared either in experimentally enlarged broods or in broods left at their natural size. We predicted that EPO would have longer telomeres than WPO, and that this difference would be more pronounced among nutritionally limited nestlings reared in enlarged broods. Contrary to our predictions, EP status did not predict chick size or TelL of nestlings reared in either treatment group. EPO from enlarged broods had a higher probability of fledging than similarly reared WPO, but this effect was only seen after a separate analysis per group and not in the full model. Even though these results give mixed support to the good-genes hypothesis they also highlight the difficulty in choosing the proper metric and context.

The Bahia Tapaculo (Eleoscytalopus psychopompus) is a rare taxon endemic to a narrow strip of lowland Atlantic Forest in the coast of Bahia, Brazil. Its phenotypic distinction from its sister species, the White-breasted Tapaculo (E. indigoticus), has been considered doubtful because the supposed diagnostic plumage characters were proposed on the basis of only 3 specimens (the type series) and there were no vocal data for the Bahia Tapaculo. Given that it is classified as endangered globally according to IUCN criteria, the definition of the taxonomic status of the Bahia Tapaculo is fundamental for the adoption of effective conservation measures. We conducted analyses of plumage, morphometrics, and vocalizations of both Bahia and White-breasted tapaculos to test for the phenotypic distinction between them. We found that their songs differ significantly in 2 parameters, pace (no overlap) and frequency (slight overlap); and that their calls (both monosyllabic and multisyllabic calls) differ in multiple parameters, including note shape and structure. Plumage color and pattern differ in 4 aspects, including a prominent one, barred vs. unbarred flanks. We found a difference in wing:tail ratio, with no overlap, revealing that these taxa are proportioned differently, with the Bahia Tapaculo tending to have longer wings and shorter tail. These differences, besides the genetic distance and reciprocal monophyly, are of the same or greater magnitude than those found between several sister taxa accepted as biological species in the family Rhinocryptidae, thus supporting biological species status for the Bahia Tapaculo.

The adaptive significance of avian egg shape is a long-standing problem in biology. For many years, it was widely believed that the pyriform shape of the Common Murre (Uria aalge) egg allowed it to either “spin like a top” or “roll in an arc,” thereby reducing its risk of rolling off the breeding ledge. There is no evidence in support of either mechanism. Two recent alternative hypotheses suggest that a pyriform egg confers mechanical strength and minimizes the risk of dirt contamination of the blunt end. We present a new hypothesis: that the Common Murre egg's pyriform shape confers stability on the breeding ledge, thus reducing the chance that it will begin to roll. We tested this hypothesis by measuring the stability of Common Murre and Razorbill (Alca torda) eggs of different shapes on slopes of 20°, 30°, and 40° above the horizontal. Common Murre eggs were more stable, and easier to stabilize, than the more elliptical Razorbill eggs. Within Common Murre eggs, more pyriform eggs were more stable. From a fitness perspective, the stability of the Common Murre egg on a slope seems likely to confer an advantage and thus may be a strong force of natural selection favoring the pyriform shape.

Growth is a key life history trait that is closely related to individual fitness. In altricial birds, growth is restricted to a relatively short period, and depends primarily on the amount or quality of food and hence on parental care. Obligate brood parasites do not care for their own offspring but impose this burden on other species (hosts). As many brood parasites exploit various host species, their progeny are expected to receive different levels of parental care. Parasite growth has thus often been explored in the context of host parenting abilities and only rarely with respect to its sex specificity. Here, we fill this gap in knowledge and explore sex differences in the growth of Common Cuckoo (Cuculus canorus) nestlings reared by 2 warbler hosts in the genus Acrocephalus. As adult Common Cuckoo males are 5–16% heavier than females, we assumed that nestlings would also differ in size and thus in growth performance. To test this assumption, we used a nonlinear mixed effects modeling approach to fit an overall logistic curve across all nestling masses and ages. We chose the logistic growth model over its alternatives because it is one of the most used models for birds and it is suitable for the growth of Common Cuckoo nestlings. We found that both sexes exhibited similar mass after hatching and grew at a similar rate. Nevertheless, males reached ∼10% higher asymptotic mass than females, while fledging at a similar age as females. These findings imply that male Common Cuckoo nestlings may have higher needs than female nestlings; however, this still awaits proper testing.

We compared the primary molt of the 4 species of skuas and jaegers (Stercorariidae) that breed in the Northern Hemisphere: Long-tailed Jaeger (Stercorarius longicaudus), Parasitic Jaeger (S. parasiticus), Pomarine Jaeger (S. pomarinus), and Great Skua (S. skua). We analyzed primary molt data of 1,573 individuals of multiple age classes, mostly collected from photographs taken at sea but also from museum specimens and beached individuals. Whereas molt duration generally increased with species' size, molt duration in Parasitic and Pomarine jaegers was surprisingly similar given their size difference. Larger species started primary molt earlier and showed more overlap with postbreeding migration, such that there was complete overlap in Great Skua but no overlap in Long-tailed Jaeger. Within jaeger species, the first primary molt cycle took longer than later molt cycles. We suggest that, unlike birds in their first primary molt cycle, birds in their second or subsequent primary molt cycles are time-constrained to complete primary molt before the onset of prebreeding long-distance migration. By contrast, molt duration did not differ between age classes of Great Skuas. Adult Great Skuas may have overcome the time constraint by completely overlapping molt and postbreeding migration. Molt-migration overlap is generally rare in birds but may be feasible for Great Skuas given their shorter migration distance and low migration speed.

The pace of life history events varies with latitude in many taxa. In birds, especially, life history events tend to be protracted in the tropics. This pattern is likely the result of reduced seasonality in resources and changes in trade-offs associated with risk of mortality. In general, animals invest more in reproduction per attempt and less in adult survival at higher latitudes. Feather growth is a major investment that birds make in their own survival, but geographic patterns of feather growth have received very little study. Evidence from separate studies of molt timing in individual species suggests that high-latitude species complete molt faster than tropical birds. Within species, feathers that are grown more quickly tend to show lower structural integrity than feathers grown more slowly, but seasonality of resources may place temporal constraints on the pace of feather growth. I hypothesized that increased seasonality of resources or decreased investment in adult survival in relation to reproduction would result in an increase in feather growth rate with latitude within species of birds. I tested this hypothesis and alternatives using ptilochronology methods to measure the growth rates of rectrices from museum specimens of 4 species of broadly distributed, resident Neotropical birds across their ranges. I compared these feather growth rates to latitude, climatic suitability, bioclimatic variables, sex, and body mass between and among species. I found that feather growth rate consistently increased with latitude in all 4 species. My results confirmed previous findings that feather growth rate increases with mass in a nonlinear fashion among species; but I found no consistent relationship between body mass and feather growth rate within species. I discuss these results in the context of life history theory and propose 3 potential mechanistic explanations for the relationship between feather growth rate and latitude within species, as (1) a programmed response to increasingly seasonal resources, (2) a decreased investment in adult survival, or (3) fixed to some other factor that may vary with latitude, such as basal metabolic rate.

Comparative studies of nest predation and identification of nest predators promote understanding of the selective environment that shapes avian life histories. Due to the low diversity of native mammalian and reptilian predators on oceanic islands, insular forest birds are assumed to incur lower nest predation rates than related continental species. We studied correlates of nest predation in insular and continental subspecies of Rufous Fantail (Rhipidura rufifrons) found on the island of Rota and in eastern Australia. Overall, daily survival rate (DSR) was similar between study sites, but egg stage DSR (laying and incubation) was higher in Australia than on Rota while nestling stage DSR was higher on Rota than in Australia. DSR was negatively related to nest age in Australia and the magnitude of this relationship varied by year. On Rota, DSR was higher in the nestling stage than during the egg stage and also higher on our study plot where Mariana Crows (Corvus kubaryi)—the principal nest predator—were less common. Although climate variables did not predict DSR at either site, in Australia, lace monitors (Varanus varius) were more likely to prey upon nests on days without rain. Lace monitors also tended to prey upon nests late in the nestling stage, which likely contributed to the decline of DSR with age. Our results suggest that life history variation between continental and insular birds may be explained, in part, by differences in age-dependent DSR due to the reduced diversity of certain predator guilds on oceanic islands. Therefore, consideration of the nest predator community and age-dependent nest predation risk could help explain additional life history variation in comparative studies.

Niche theory predicts that, to avoid interspecific competitive exclusion, ecologically similar bird species in sympatry should reduce habitat use overlap, especially when shared resources are limiting in an environment. Dust-bathing sites represent a distinctive behavior-specific habitat, differing from other habitats as they are used for short durations and infrequently. However, we have little understanding of selection, potential interspecific overlap, and partitioning of dust-bathing sites among sympatric species. In this study, we examined overlap and selection of dust-bathing sites of 3 sympatric montane galliform species on the Qinghai-Tibet Plateau: Blood Pheasant (Ithaginis cruentus), Szechenyi's Partridge (Tetraophasis szechenyii), and White Eared-Pheasant (Crossoptilon crossoptilon). We identified dust-bathing scrapes and measured habitat characteristics of dust-bathing sites along 10 transects established in Gexigou National Nature Reserve, Sichuan Province, China, from April to August in 2015. In total, we identified 105 dust-bathing scrapes and established 58 dust-bathing site quadrats. More than one-third of dust-bathing scrapes and ∼20% of dust-bathing sites were used by multiple species. The 3 species showed no significant differences in the dimensions or soil particle size distributions of their dust-bathing scrapes, nor did we find that habitat features of dust-bathing sites differed significantly among the 3 species (all pairwise overlap indices >0.6). All 3 species preferred surrounding fir forests with dense tree and shrub strata, good concealment that could provide sufficient protection from predators and precipitation, and sparse grass cover with suitable substrates containing finer soil particles for better dust-bathing. Our results suggest that the 3 sympatric galliform species had similar habitat requirements that led to overlap in selection for dust-bathing sites. We recommend further monitoring of temporal use of dust-bathing sites by the 3 galliform species to test potential temporal partitioning mechanisms that may minimize competition between heterospecific and conspecific individuals.

Understanding and quantifying demographic parameters that influence population growth is necessary to effectively maintain population persistence in the presence of recent changes in climate and land use. We used a combination of capture–recapture and harvest information from a 42 yr dataset to create an integrated population model that estimates population abundance, annual survival, and per capita recruitment in Northern Bobwhite (Colinus virginianus). We evaluated the associations of commonly reported population metrics (age ratios from harvests, annual survival, and recruitment) to population growth rate using correlation analysis. We also modeled the effects of population abundance, breeding-season temperature, breeding-season precipitation, fall minimum temperature, and fall precipitation on recruitment. Recruitment averaged 1.55 (range: 0.82–2.21) and was most strongly correlated with population growth (r = 0.78, 95% confidence interval [CI]: 0.63–0.88), followed by annual survival (r = 0.38, 95% CI: 0.09–0.61). We found a weak correlation between fall age ratios and population growth rate (r = 0.11, 95% CI: −0.19 to 0.40). Recruitment exhibited weak, nonlinear, negative density dependence (β₁ = −0.13, 95% credible intervals [CRI]: −0.17 to −0.08 and β₂ = −0.03, 95% CRI: −0.07 to 0.004). Breeding-season maximum temperature and breeding-season precipitation had a significant, positive effect on recruitment (β₃ = 0.15, 95% CRI: 0.11–0.19 and β₄ = 0.10, 95% CRI: 0.05–0.16, respectively). Fall minimum temperature and fall precipitation both had negative but uncertain effects on recruitment (β₅ = −0.03, 95% CRI: −0.08 to 0.01 and β₆ = −0.02, 95% CRI: −0.07 to 0.01, respectively) but less so compared to weather in the breeding season. Our results illustrate that recruitment (1) can be estimated without data on fecundity and chick survival by using integrated population models and (2) is highly correlated with population growth in Northern Bobwhites. We encourage natural resource managers to focus on actions that increase Northern Bobwhite recruitment.

The time of day that nestlings fledge from a nest is thought to be shaped by predation risk and energetics. To minimize predation risk, fledging is predicted to start as early in the day as possible so that nestlings can maximize time outside the nest to find a safe place to stay before nightfall. Fledging times are predicted to be tightly grouped and to not be affected by the number of nestlings, given that all nestlings are responding to the same relative risk of predation. Conversely, energetic considerations predict that fledging time of day should vary so that nestlings can maximize energy intake before having to forage for themselves. However, data to evaluate the relative importance of these drivers in grassland birds are scarce because of the difficulty of observing nestlings as they fledge. We used nest surveillance video from 178 nests to evaluate how the initiation and duration of fledging varied among 7 grassland passerine species, as well as by the number of nestlings in the nest and fledging date. Fledging initiation varied most strongly by species, with some effects of date. Across species, the median start time of fledging was 4.55 hr after sunrise. Fledging before the solstice started ∼30 min earlier compared to fledging at or after the solstice. Fledging duration increased with number of nestlings in the nest and was spread over >1 day in 21% of nests. While our results primarily supported the hypothesis that fledging is motivated by energetic considerations, additional data on basic life history traits and behavior will be needed to fully understand how fledging grassland birds balance energetics against predation risk.

Both natural and experimentally manipulated brood size can influence the competition dynamics among siblings in a nest and alter the environment during early development in birds. Brood size affects a variety of life-history and fitness-related traits, but relatively little is known about the mechanisms that might mediate these effects. There is accumulating evidence that early-life environments can influence adult phenotypes though epigenetic mechanisms such as variation in DNA methylation. Here, we profile DNA methylation in nestling Zebra Finches (Taeniopygia guttata) raised in naturally variable and experimentally manipulated brood sizes. We found that (1) natal brood size is significantly, positively correlated with percent DNA methylation in blood; and (2) individuals in manipulated broods experience significantly more demethylation events across early development than individuals from broods that remain the same size. Any manipulation of brood size creates fluctuations in early developmental conditions, potentially explaining why the frequency of demethylation events in these treatments was higher than in control broods. We also found that the specific loci that lost and gained methylation across early development differed between individuals in enlarged and reduced broods, which may reflect the different developmental pressures imposed by the different manipulations. Although the phenotypic consequences of reduced levels of methylation are yet to be elucidated, our findings support the hypothesis that brood size is associated with the prevalence and pattern of DNA methylation in wild birds.

The occurrence of molt during migration, known as “molt-migration,” has increasingly received attention across many avian taxa since first being described in waterfowl in the 1960s. However, despite the many different types of molt stages and strategies, most, if not all, uses of the term “molt-migration” apply to the definitive prebasic molt of flight feathers in post-breeding adults, whereas fewer studies address migration for body-feather molts. Here, we argue that the current definition of molt-migration, as applied, is limited in focus relative to the diverse ways in which it can manifest in avian populations. We suggest a new, broader definition of molt-migration and highlight examples of molt-migration as traditionally defined, and the many examples that have not been defined as such. We propose a new, 2-tiered typology for defining different forms of molt-migration, based on (1) its progression relative to stationary portions of the annual cycle, and (2) the stage of molt involved. In order to advance our understanding of the ecology and evolution of this increasingly documented phenomenon and apply this knowledge to conservation and management, avian researchers must begin to utilize a common framework for describing molt-migration in its various forms.

Hybrid zones provide a key natural context within which to study the barriers between incipient species. In some avian hybrid zones, there is indirect evidence of selection against hybrid offspring, yet the source of that selection is often unclear. We examined the frequency distribution of hybrids between Myrtle Warblers (Setophaga coronata coronata) and Audubon's Warblers (S. c. auduboni), using data to quantify—for the first time at a genomic scale—the composition of hybrids in this hybrid zone. We sampled birds during the breeding season and during fall migration and compared the frequencies of hybrids of different sex and age classes. Specifically, we tested for evidence of early-generation hybrids being significantly under- or over-represented in any of these classes, as would be expected if hybrids have lower or higher fitness than non-hybrids. We found that the genomic composition of birds in the hybrid zone spans the full ancestry spectrum. Across all our sampling periods, we found an excess of birds that had more Audubon's ancestry, with a stronger bias toward Audubon's ancestry in fall migrants than in breeding birds, consistent with asymmetric introgression. Notably, we did not find any differences in hybrid frequencies between juvenile and adult age classes or between males and females. Therefore, our results do not support large differences in viability between male and female hybrids or between different age classes of hybrids.

We describe a new species of the genus Oreotrochilus from the southwestern Andes of Ecuador. The new species is most similar in adult male plumage to O. stolzmanni and O. chimborazo. However, male and female show unique combinations of plumage characters that are likely to act as social signals. Phylogenetic analyses based on mitochondrial DNA indicate that this new taxon is closely related to O. stolzmanni and O. melanogaster, whereas genetic distances and preliminary comparisons of vocalizations suggest a sister relationship with O. stolzmanni. The geographic distribution of the new species seems to be restricted to cordillera Chilla-Tioloma-Fierro Urcu, in the southwestern highlands of Ecuador, an area historically poorly explored by ornithologists. Thus, based on its restricted distribution, apparently low population size, and lack of protection of its habitat, we evaluate it as critically endangered.