Dispersal is a ubiquitous behavior with important consequences for gene flow, demography, and conservation. Some birds engage in between-year breeding dispersal, but the factors shaping variation in this behavior are not well understood. In mid-continental grasslands, preliminary evidence suggested that Grasshopper Sparrows (Ammodramus savannarum) dispersed not only between seasons, but also within breeding seasons—an apparently uncommon avian behavior. We studied a population of Grasshopper Sparrows breeding in northeastern Kansas, USA, to document the spatial and temporal patterns of within-season breeding dispersal in an experimentally managed tallgrass prairie from 2013 to 2015. We combined color-band resighting, territory mapping, and radio telemetry to quantify changes in territory density, turnover of territorial males, and dispersal distances. Density of Grasshopper Sparrows varied seasonally in management-specific ways, simultaneously increasing and decreasing in watersheds that differed in management regime. Turnover was unexpectedly high, with over half of territorial males being replaced each month. We documented over a third of males dispersing up to ∼9 km between breeding attempts. Our study provides the first comprehensive description of the patterns of within-season breeding dispersal in a grassland songbird. Our results reveal the remarkable prevalence of within-season movement in this system and the relatively large distances over which birds disperse. Such mobility has important implications for survey design and habitat management, as birds select habitat at much larger spatial scales than is generally appreciated. These results also provide foundational information for tests of alternative hypotheses explaining the ecological and evolutionary basis for such movements.

Given that population dynamics of birds are known to be sensitive to high fledgling mortality, a comprehensive understanding of the environmental factors that drive variation in fledgling survival is essential to avian conservation. We quantified multiple aspects of the Wood Thrush postfledging period using breeding and radio-telemetry data collected over the course of 4 breeding seasons in southern Indiana, USA. First, we examined how drought, forest cover, and brood parasitism affected nestling body condition and brood size. Second, after controlling for the age-specific increase in survival, we used multimodel inference to examine how brood parasitism, drought, forest cover, nestling body condition, and nest vegetation structure influenced postfledging survival. Finally, we measured the relationship between these covariates and fledgling movements. Drought decreased cumulative survival probabilities, with the youngest age group (<4 days postfledging) being most affected; however, this relationship was dependent on the amount of mature forest cover. During non-drought years, fledgling survival was lower in study plots with a high proportion of mature forest cover. By contrast, postfledging survival during a drought year was higher in study plots with a high proportion of forest cover. This drought year was also associated with lower nestling body condition and brood size, and with delayed postfledging dispersal from natal territories. Our results suggest that while Wood Thrush postfledging survival is relatively insensitive to both nest vegetation structure and brood parasitism, forest cover and breeding-season precipitation interact to affect multiple aspects of the species' postfledging period.

Brood merging and conspecific brood parasitism are common in some waterfowl and New World quails. A previously unrecognized, unusual similarity between them—female-biased natal philopatry and local relatedness—may enable indirect inclusive fitness gains in brood parasitism and merging. New World quails offer a rare possibility to test the role of female-biased natal philopatry in brood parasitism, brood merging, and other aspects of sociality and to help clarify the evolution of these traits.

Many Arctic shorebird populations are declining, and quantifying adult survival and the effects of anthropogenic factors is a crucial step toward a better understanding of population dynamics. We used a recently developed, spatially explicit Cormack–Jolly–Seber model in a Bayesian framework to obtain broad-scale estimates of true annual survival rates for 6 species of shorebirds at 9 breeding sites across the North American Arctic in 2010–2014. We tested for effects of environmental and ecological variables, study site, nest fate, and sex on annual survival rates of each species in the spatially explicit framework, which allowed us to distinguish between effects of variables on site fidelity versus true survival. Our spatially explicit analysis produced estimates of true survival rates that were substantially higher than previously published estimates of apparent survival for most species, ranging from S = 0.72 to 0.98 across 5 species. However, survival was lower for the arcticola subspecies of Dunlin (Calidris alpina arcticola; S = 0.54), our only study taxon that migrates through the East Asian–Australasian Flyway. Like other species that use that flyway, arcticola Dunlin could be experiencing unsustainably low survival rates as a result of loss of migratory stopover habitat. Survival rates of our study species were not affected by timing of snowmelt or summer temperature, and only 2 species showed minor variation among study sites. Furthermore, although previous reproductive success, predator abundance, and the availability of alternative prey each affected survival of one species, no factors broadly affected survival across species. Overall, our findings of few effects of environmental or ecological variables suggest that annual survival rates of adult shorebirds are generally robust to conditions at Arctic breeding sites. Instead, conditions at migratory stopovers or overwintering sites might be driving adult survival rates and should be the focus of future studies.

Vocalizations in birds play a significant role in species and mate recognition as well as sexual selection. Geographic variation in vocalization is well studied in male songbirds but largely unexplored in seabirds and in females. We investigated variation in male and female agonistic and advertising calls between 4 populations of Little Penguins (Eudyptula minor) in South Australia. We also determined whether call similarity was better explained by the geographic distances between the colonies, by microhabitat variation, or by variation in the physical characteristics of the individuals. Further, we used playback experiments testing male and female responses to determine the biological importance of geographic call variation. Both agonistic and advertising calls differed between individuals and sexes, with males producing calls at higher frequencies than females. Our results also reveal significant variation in agonistic calls across the colonies, best explained by variation in microhabitat. However, resident birds did not discriminate between calls originating from different colonies. The behavioral patterns are discussed in relation to gene flow and population differentiation.

Predation can shape community structure, so understanding the diet of predators is an important aspect of ecology. Stable isotope analysis using Bayesian mixing models is a potentially powerful method of estimating diet, but results are often ambiguous. A commonly cited advantage of Bayesian mixing models is the ability to include informative priors, which can improve precision and accuracy of mixing model results. However, factors such as a large number of potential prey and high amounts of variation and correlation among isotopic signatures of prey can lead to imprecise estimate of diet when Bayesian mixing models are used. In this study, we tested the efficacy of using Bayesian mixing models for stable isotopes to estimate the diet of Arctic Peregrine Falcon (Falco peregrinus tundrius) nestlings in Nunavut, Canada, consuming a diversity of terrestrial and marine prey. In addition to stable isotopes, we also estimated diet composition by monitoring peregrine nests with motion-sensitive cameras. Stable isotope analysis was conducted using blood plasma samples collected weekly from nestlings and tissue samples from all prey groups they consumed. Uninformed mixing models, based on stable isotopes alone, had wide credible intervals around diet estimates, which indicated lemmings (Lemmus trimucronatus and Dicrostonyx groenlandicus) were the main contributor to diets. In contrast, diet estimated with motion-sensitive cameras had high precision and indicated that insectivorous birds were the dominant prey consumed. When informative priors from motion-sensitive camera data were included in Bayesian mixing models, resulting diet estimates had narrow credible intervals and generally reflected the priors. We conclude that with our data stable isotope analysis alone is inaccurate for monitoring the diet of Arctic Peregrine Falcons, but motion-sensitive cameras at nest sites provide a viable alternative method.

Embryonic heart rates have the potential to provide great insight into physiological variation and ontogenic status in early development. The availability of a relatively inexpensive and portable piece of equipment, the Buddy digital egg monitor (Vetronic Services, Abbotskerswell, Devon, UK), provides an opportunity to measure embryonic heart rate noninvasively in the field. Here, we demonstrate the application of this equipment in the climatically harsh Australian outback. We characterize variation in embryonic heart rate in the Zebra Finch (Taeniopygia guttata) with respect to a range of abiotic and biotic variables. Heart rate increased throughout embryonic development and was positively correlated with ambient temperature. There was a strong effect of the nest of origin, but no clear effect of laying order or egg size, on embryonic heart rate. Our results demonstrate the sensitivity of embryonic heart rate to environmental conditions and/or natal origin. We review other studies that have used the digital egg monitor, and, in discussing our own results, identify areas of avian biology that could benefit from embryonic heart rate measurements in the future.

Migration and the breeding season are periods in which birds face many physical and energetic challenges that may cause changes in stress levels and body condition. Gray Catbirds (Dumetella carolinensis) and Song Sparrows (Melospiza melodia) were captured in 3 consecutive seasons—spring migration, the summer breeding period, and fall migration—in 2014 at a long-term bird-banding station located on the south shore of Lake Ontario to determine patterns of chronic stress and physiological condition related to seasonal shifts in their annual cycles. The heterophil/lymphocyte (H/L) ratio was used to assess chronic stress; and plasma metabolite concentrations (triglyceride and uric acid), along with a scaled body mass index, were used as indicators of the physiological condition and fuel utilization of Gray Catbirds and Song Sparrows. Plasma triglycerides were highest during migration periods for Gray Catbirds, while plasma uric acid was greater in spring and summer than in fall, indicating a seasonal shift in diet, possibly related to seasonal fruit availability. Gray Catbirds had elevated H/L ratios during spring, which suggests that spring migration is a period of modest stress for this species, and this spring stress seemed to be predominantly associated with earlier migration passage through the area. Our results suggest that physiological and immune function can vary across seasons in these species, perhaps in parallel with factors related to food availability at stopover or breeding sites during different times of the year.

Late arrival on the breeding grounds has been associated with reduced fitness in migratory birds. Because migratory stopover accounts for the majority of time spent on migration, how individuals respond to both exogenous and endogenous factors associated with stopover will influence the pace of migration and arrival timing on the breeding grounds. Among the most important endogenous factors, body condition (i.e. fat stores) is expected to influence both movement behavior and departure decisions of individual migrants during stopover. In a novel experiment, we manipulated the condition of after-second-year (adult) male American Redstarts (Setophaga ruticilla) captured during spring migratory stopover and radio-tracked all individuals after release using a passive automated telemetry array and conventional hand tracking. After accounting for the positive effect of tailwind and capture date on departure probability, we demonstrated that body condition influenced both the movement behavior of individuals and stopover duration. Individual movement decreased throughout the stopover period at a rate of 0.7 km day⁻¹, supporting the notion that birds progressively limit their movements as stopover progresses. However, treatment birds whose condition was experimentally reduced (i.e. food-restricted) exhibited ∼3.4 times greater daily movement rates, and control birds were 4 times more likely to depart on a given day than treatment birds. These results underscore the importance of energetic condition in mediating the movement and stopover dynamics of migratory birds. Further, despite their capacity to make behavioral adjustments, our results suggest migratory songbirds that experience decreases in body condition during migration (which could result from inclement weather or low-quality stopover habitat) are likely to be delayed in their arrival on the breeding grounds, which could lower reproductive performance and ultimately have population-level ramifications.

Understanding intrinsic (physiological) and extrinsic (e.g., temperature) causes of variation in embryonic development time (incubation period) is important because they can have different impacts on individual quality. Robert Ricklefs and colleagues have argued that longer incubation periods result primarily from intrinsic physiological programs that increase individual quality and adult survival. They claim that incubation periods are largely invariant and that extrinsic factors like temperature have little impact. We have argued that adult survival may be a cause rather than a consequence of much of the variation in embryonic development time. A reduction in extrinsic sources of annual adult mortality (e.g., migration, predation, nonbreeding-season mortality) favors reduced parental effort during incubation to minimize costs to future reproduction and survival. Reduced parental effort, in turn, manifests as cooler average egg temperatures that yield longer incubation periods. Ricklefs and colleagues mischaracterized our hypothesis and deconstructed their own incorrect version, while also making some incorrect statements. We show that reevaluation of previous evidence provided by this group actually supports a role of egg temperature for the variation in incubation periods. We also summarize other observational and experimental evidence that incubation periods are not invariant and that egg temperature has a strong causal influence on variation within and among species. In fact, egg temperature explains ∼60% of the difference in incubation periods among species. The remaining ∼40% reflects intrinsic physiological programs and other factors, potentially providing intrinsic benefits. Ultimately, annual adult mortality explains substantial variation in parental effort and egg temperature, and the latter strongly explains variation in incubation periods. Both intrinsic programs and extrinsic temperature effects need to be considered in attempts to understand incubation strategies.

We describe a distinctive new species of antbird (Passeriformes: Thamnophilidae) from humid montane forest (1,340–1,670 m above sea level) of the Cordillera Azul, San Martín Region, Peru. Plumage, voice, and molecular evidence distinguish this species from its sister taxon Myrmoderus ferrugineus (Ferruginous-backed Antbird), which is found in lowland Amazonian rainforests of the Guiana Shield and Madeira-Tapajós interfluvium. The new species is presently known only from one ridge in the Cordillera Azul, and therefore we recommend further fieldwork to better estimate its distribution and population size.

Geographic variation in acoustic signals can be important in species divergence, especially the maintenance of prezygotic barriers to gene flow. Furthermore, selective pressures on acoustic signals likely vary both across geographic distances and among vocalizations used in different behavioral contexts. We described the call repertoire of 5 subspecies of Marsh Wren (Cistothorus palustris) in eastern North America and tested for variation in both the acoustic structure and likelihood of production of each call type at 3 functional–ecological levels: subspecies identity, migratory pattern, and habitat type. Three of the 7 described call types exhibited acoustic variation best explained by either migratory pattern or habitat type. These calls were used principally in courtship–territorial patrol contexts, whereas 4 calls that did not exhibit geographic variation were used in agonistic interactions. How often a call is used in a population may be indicative of the behavior or breeding phenology associated with that vocalization. We found that 4 calls varied in how commonly they were produced among the subspecies and/or habitat types. We also described and quantified the degree to which these 5 subspecies produce calls in association with song—a little reported, but possibly more widespread, behavior in birds. Marsh Wrens commonly embedded 3 call types into song in a nonrandom pattern. This behavior was more common in freshwater-marsh populations than in saltmarsh populations, and we discuss several possible functions for call-song associations. Overall, when geographic variation in call structure occurred, it was most commonly explained by differences in habitat type and, therefore, may be indicative of local adaptations that could limit gene flow across environments.