Island bird populations often provide exemplary cases of evolution based on historical colonization and diversification events. However, capturing contemporary cases of evolution is rare. Introductions of nonnative birds into the Hawaiian Islands, USA, provide numerous potential opportunities to assess evolutionary changes over a relatively short time frame. One introduced species, the Japanese Bush-Warbler (Cettia diphone), has a well-established history in the Hawaiian Islands, with a documented introduction in 1929 to the island of Oahu and natural colonization of the other main islands by 1997. We sampled 143 Japanese Bush-Warblers from 5 of the main Hawaiian Islands and amplified 12 microsatellite loci (9 were variable) and sequenced portions of the cytochrome b and cytochrome c oxidase subunit I (COI) genes to assess the genetic structure and potential original source of these populations. As predicted, genetic diversity, measured by allelic richness and private alleles, was greatest on Oahu (the original introduction site) and was significantly lower in birds on the islands farthest from Oahu. Accordingly, there was a clear isolation-by-distance effect, with highest F_ST values between island pairs farthest apart. The population on the westernmost island of Kauai appears to be diverging from the easternmost populations on the islands of Maui, Molokai, and Hawaii. The results provide a unique opportunity to document the microevolutionary process of genetic drift in action, and we speculate on the potential role of behavior in diversification.

Nest-switching is an important breeding strategy for multiple-brooded bird species. When deciding whether or not to switch nests for subsequent breeding attempts, pairs must weigh the costs and benefits of various factors related to the number of fledglings of the first breeding attempt, the likelihood of nest predation, and qualities of the nest environment, such as nest ectoparasites and the age of the nest. In this study, we analyzed the predictors and consequences of nest-switching behavior at 6 breeding sites of North American Barn Swallows (Hirundo rustica erythrogaster), where 60% of pairs that raised 2 broods within a season switched nests for a second breeding attempt. Pairs often reused existing (old) nests constructed during previous years, and pairs that settled in old nests for their first breeding attempt were the most likely to switch nests for a second breeding attempt. Contrary to previous studies, nest predation and nest ectoparasitism had no influence on whether or not pairs switched nests. Moreover, second breeding attempts overall had significantly more mites than first breeding attempts, but there was more variation in the change of mite intensities for those pairs that switched nests for a second breeding attempt compared to pairs that did not switch. Furthermore, pairs that switched from one old nest to another nest between breeding attempts decreased the time between first and second breeding attempts when compared to pairs that reused their first nest for a second breeding attempt. Because nest-switching led to greater fledging success for second breeding attempts compared to birds that reused their nests, our results suggest that switching between nests is an adaptive reproductive strategy for Barn Swallows.

Nest-site choice profoundly influences reproductive success and the survival of incubating adult birds. Asian Houbara (Chlamydotis macqueenii) nest in subtly contrasting habitats where the main cause of nest failure is predation. We examined nest-site selection across 3 semiarid shrub habitats that differed in vegetation structure and hypothesized that increased concealment would reduce nest predation. We quantified vegetation structure at 210 nests and 194 random control sites at 2 scales, the “nest area” (50 m radius, considering mean “shrub height” and mean “shrub frequency”) and “nest scrape” (2 m radius, considering a “concealment index”). Variance ratio tests showed that variance in both shrub height and concealment index was lower at nests than at random sites, indicating nonrandom selection. Analysis of the probability of nest occurrence for nest area indicated consistent selection of intermediate shrub heights (shrub height shrub height²) in the Astragalus, Salsola arbuscula, and S. rigida shrub assemblages (29.5–31.5 cm), although this was not supported statistically in S. rigida because the vegetation available was already similar to the optimal structure. Nest survival analysis, controlling for date, showed that shrub height (but not its quadratic term) in the nest area reduced nest predation rate. Females likely traded off nesting in even taller shrubs that may confer greater nest success against the ability to see approaching danger and thus to reduce the risk of being depredated themselves (head height during vigilance when incubating is ∼30 cm), given that we have no records of females being depredated on the nest. At the nest scrape, females strongly selected better-concealed locations, although the concealment index did not affect nest success. We suggest that concealing the scrape among shrubs may have other roles, such as thermoregulation.

Across many taxa, clinal variation in body size has been observed to follow Bergmann's rule, which predicts larger body size in colder climates. For more than a century, this pattern has typically been ascribed to selection for large body size in cold winter climates. Here, in spatially distributed observational data from 30 populations of House Sparrow (Passer domesticus) introduced into Australia and New Zealand, we show that this relationship appears to be explained by a negative relationship with high temperatures during the breeding season. Our results suggest that higher temperatures during the breeding season could reduce body size through developmental plasticity, which should be considered in combination with or as an alternative to selection. Our findings would predict that a hotter climate during breeding could drive significant changes in morphology among populations (and potentially within populations as well, if climate varies temporally across a breeding season). This idea, and our support for it, could account for much of the variation in body size that drives the well-observed patterns first described by Bergmann, and that are still largely attributed to selection on adult body size during cold winters. Understanding the mechanisms behind any climate-dependent developmental plasticity could prove useful for understanding how endotherms may be affected by climate change in the future.

To anticipate future breeding conditions in spatiotemporally dynamic environments, any proximate aspect of breeding is expected to be causally associated with environmental cues or constraints. Photoperiod, ambient temperature, and food are considered to be among the most important environmental determinants of breeding performance. However, the importance of these determinants in the expression of nest size has not previously been investigated experimentally in laboratory conditions. We report that captive Blue Tits (Cyanistes caeruleus) and Great Tits (Parus major) did not adjust the size of their cavity nest to experimental changes in photoperiod or ambient-temperature regimes. Results did not change when we controlled for first-egg date and clutch size. In addition, captive birds fed ad libitum did not build larger nests than free-ranging breeders in standardized nest boxes. Thus, we report no evidence that nest size per se is proximately influenced by commonly studied environmental factors like photoperiod, temperature, food abundance, or the level of investment in egg production—factors that can all pose energetic challenges to breeding birds. However, we do not exclude the possibility that, in natural conditions, energy- or resource-associated constraints might influence aspects of nest design other than nest size (e.g., rare nest components) or that constraints on nest building might only be revealed when birds need to build substantially larger nests (i.e. in larger cavities). Therefore, additional experimental work will be required to determine which are the proximate determinants of nest design in cavity breeders.

Conservation of small populations is often based on limited data from spatially and temporally restricted studies, resulting in management actions based on an incomplete assessment of the population drivers. If fluctuations in abundance are related to changes in weather, proper management is especially important, because extreme weather events could disproportionately affect population abundance. Conservation assessments, especially for vulnerable populations, are aided by a knowledge of how extreme events influence population status and trends. Although important for conservation efforts, data may be limited for small or vulnerable populations. Integrated population models maximize information from various sources of data to yield population estimates that fully incorporate uncertainty from multiple data sources while allowing for the explicit incorporation of environmental covariates of interest. Our goal was to assess the relative influence of population drivers for the Lesser Prairie-Chicken (Tympanuchus pallidicinctus) in the core of its range, western and southern Kansas, USA. We used data from roadside lek count surveys, nest monitoring surveys, and survival data from telemetry monitoring combined with climate (Palmer drought severity index) data in an integrated population model. Our results indicate that variability in population growth rate was most influenced by variability in juvenile survival. The Palmer drought severity index had no measurable direct effects on adult survival or mean number of offspring per female; however, there were declines in population growth rate following severe drought. Because declines in population growth rate occurred at a broad spatial scale, declines in response to drought were likely due to decreases in chick and juvenile survival rather than emigration outside of the study area. Overall, our model highlights the importance of accounting for environmental and demographic sources of variability, and provides a thorough method for simultaneously evaluating population demography in response to long-term climate effects.

Consideration of ecological scale is fundamental to understanding and managing avian population growth and decline. Empirically driven models for population dynamics and demographic processes across multiple spatial scales can be powerful tools to help guide conservation actions. Integrated population models (IPMs) provide a framework for better parameter estimation by unifying multiple sources of data (e.g., count and demographic data). Hierarchical structure within such models that include random effects allow for varying degrees of data sharing across different spatiotemporal scales. We developed an IPM to investigate Greater Sage-Grouse (Centrocercus urophasianus) on the border of California and Nevada, known as the Bi-State Distinct Population Segment. Our analysis integrated 13 years of lek count data (n > 2,000) and intensive telemetry (VHF and GPS; n > 350 individuals) data across 6 subpopulations. Specifically, we identified the most parsimonious models among varying random effects and density-dependent terms for each population vital rate (e.g., nest survival). Using a joint likelihood process, we integrated the lek count data with the demographic models to estimate apparent abundance and refine vital rate parameter estimates. To investigate effects of climatic conditions, we extended the model to fit a precipitation covariate for instantaneous rate of change (r). At a metapopulation extent (i.e. Bi-State), annual population rate of change λ (e^r) did not favor an overall increasing or decreasing trend through the time series. However, annual changes in λ were driven by changes in precipitation (one-year lag effect). At subpopulation extents, we identified substantial variation in λ and demographic rates. One subpopulation clearly decoupled from the trend at the metapopulation extent and exhibited relatively high risk of extinction as a result of low egg fertility. These findings can inform localized, targeted management actions for specific areas, and status of the species for the larger Bi-State.

Certain vocal signals can be difficult or challenging to produce due to constraints on vocal mechanics. In the case of birdsong, both body size and beak morphology have been shown to constrain spectral and temporal song parameters. Therefore, variation within a clade in both these morphological traits could exert an important effect in vocal evolution and diversity. However, given the allometric relationship between body size and beak size, it is not always easy to separate the effects they may have. Additionally, different habitats impose different sources of sound attenuation and degradation, and birdsong seems to be adapted to minimize habitat-specific sound degradation. Here, we analyze vocal variation in relation to both body size and beak morphology in 13 of 14 species in the “blue” clade of the Cardinalidae, controlling for habitat characteristics. On one hand, we found the predicted negative correlation of body size with song frequencies, but no effect of body size on temporal variables. On the other hand, we found no relationship between different beak dimensions (corrected by body size) and song frequencies, but we found that beak length was correlated negatively with note rate and positively with the duration of the interval between notes. Similar results have been found previously, but mostly in species with trilled songs, in which notes are equal and produced at a constant rate. Our results show that different morphological variables, even if strongly correlated, can exert differential constraints in a complex behavioral trait such as song. A better understanding of the relevance of these proximate factors in shaping vocal evolution is fundamental to understanding the causes of avian vocal diversity.

We used the prevalence of avian haemosporidian blood parasites (Plasmodium and Haemoproteus), quantified by PCR screening and DNA sequencing, to reevaluate a previously described negative relationship, based on microscopic examination of blood smears, between the prevalence of parasites in an avian host population and the length of the embryo development period of the host species. Analyses were restricted to small land birds in continental areas of the Americas. Prevalence determined by DNA methods averaged 22.5% and did not differ between tropical and temperate latitudes. Compared across taxonomic families or subfamilies, parasitism levels obtained by microscopic examination of blood smears and by DNA methods were strongly correlated and decreased, similarly in tropical and temperate latitudes, with increasing length of the embryo development period. Understanding the association of reduced haemosporidian infection with prolonged embryo development remains a fundamental challenge that emphasizes a potential role for defense against pathogens in the evolution of avian life histories.

The manner in which individual life history traits respond to the environment and to each other, and how these traits combine to form overall patterns of life history variation, remains poorly characterized in wild populations. We monitored breeding Dark-eyed Juncos (Junco hyemalis) across a 700-m elevational range. We compared breeding season length, temporal patterns of breeding activity, adult body size, clutch size, brood size, nestling quality, and nest mortality among elevations. We also compared environmental measures across the studied elevations to determine whether abiotic factors explained life history trait variation. We used 12 microsatellite loci to test for genetic differentiation in populations at different elevations. Finally, we constructed a computer simulation to evaluate the combined effects of observed variation in life history traits. We found differences among elevations in breeding season length and in patterns of reproductive timing, which did not match each other and which were not explained solely by abiotic factors. We found no differences among elevations in adult body size, clutch size, brood size, or nestling quality. Nest mortality increased significantly with elevation. Genetic differentiation was too low to define distinct subpopulations. The simulation suggested that differences in mortality, in combination with differences in breeding season length, contributed to substantial differences in reproductive success among elevations. Thus, although individual life history traits showed little evidence of variation in response to the environment or to each other and little genetic differentiation, variation in breeding season length and in nest mortality were potential drivers of substantial elevational variation in overall life history in this system. These results demonstrate that individual life history traits may vary substantially in their patterns of variation, and that some life history traits may have disproportionate effects on overall life history.

The search for explanations of the well-documented positive relationship between latitude and avian clutch size has created the expectation that tropical birds should balance their smaller clutch sizes with relatively high survival probabilities. So far, efforts to detect a latitudinal gradient in survival have found no statistical support, leading to the hypothesis that a gradient may be present in the survival of juveniles alone. Such a gradient could be masked by the data on adults when field records make no distinction between ages. We aimed to (1) assess the effect of age on survival of tropical birds by estimating age-specific annual apparent survival probabilities for a set of 40 passerine understory species from the central Brazilian Amazon and (2) test the hypothesis of a latitudinal gradient in adult survival with a meta-analysis of tropical and temperate-zone forest passerine survival probabilities at study areas from Peru to Alaska. We estimated age-specific survival using a hierarchical, multispecies Cormack-Jolly-Seber (CJS) model that treats species-specific parameters as random effects. To extend our analysis to data on birds of unknown age at the time of banding, we developed a novel CJS model with a mixture component for the survival of birds of unknown age. We found a strong effect of age on survival at our site, with juveniles having lower survival than adults. The meta-analysis of 342 survival estimates from 175 species and a latitude span of >60 degrees revealed a negative effect of latitude on survival, which supports the widely accepted hypothesis that, on average, tropical birds have higher annual survival than their temperate counterparts. We conclude that there is no need for an alternative latitudinal trend in juvenile survival to account for the general trend in clutch size.

Research on bird song has contributed to important advances in diverse biological fields from neurobiology to conservation biology. Bird song has traditionally been studied as an elaborate male trait, but female song is also widespread in both temperate and tropical species and likely evolved in the early ancestors of modern songbirds. However, female song is underrepresented in biological collections compared to male song, and we lack documentation of female songs for most songbird species. Better documentation of female bird song is necessary for an understanding of the prevalence, regulation, function, evolution, and conservation applications of avian vocalizations. Therefore, we call on all researchers to disseminate their observations of female bird song, and to spread the word among other researchers, students, field technicians, and citizen scientists that many female songbirds sing. To this end, we provide resources for disseminating recordings and written documentation of female song, including best practices for documentation, venues for archiving and publishing, and our citizen science project, the Female Bird Song Project. We especially appeal to researchers studying marked populations who can accurately assess sex-specific singing behavior. Documenting female song across many species and geographic regions is a major endeavor. By working collectively, we can make the greatest progress toward applying the resultant knowledge to a wide variety of fields.

Factors that affect the survival of nestling birds can drive the dynamics of populations and the evolution of life histories. Such factors may include a variety of intrinsic, biotic, or abiotic factors that vary in magnitude or effect over ecological time scales. Consequently, estimates of the influence of such factors on nest success may differ when estimated from short- vs. long-term studies. We tested this possibility by estimating the influence of 2 intrinsic factors (female age and inbreeding coefficient), 2 abiotic factors (rainfall and temperature), and 3 biotic factors (breeding density, parasitism rate, and brood parasitism) previously found to predict success (≥1 fledged young) in a 39 yr study of a Song Sparrow (Melospiza melodia) population on Mandarte Island, British Columbia, Canada. We then compared those estimates to others obtained from parallel models based on the the first 13 yr, middle 13 yr, and last 13 yr of the study. Over the entire study, nest success increased in females from ages 1 to 3 yr but declined thereafter. Nest success also declined as female inbreeding, rainfall during nesting, and breeding density increased. We further observed that nests parasitized by Brown-headed Cowbirds (Molothrus ater) or located in areas subject to high parasitism rates failed more often than unparasitized nests or those in areas with fewer parasitized nests. By contrast, the only factors identified as influencing nest success in all three 13 yr study periods examined were female age and population density, whereas the effects of inbreeding, brood parasitism, and rainfall varied temporally. Our results indicate that precise estimates of the effects of intrinsic, abiotic, and biotic drivers on nest success may require long-term studies, particularly for influential factors that occur episodically.

Evolutionary theory suggests that natural selection should favor the ability of animals to modify their offspring sex ratios when the fitness benefits of producing one sex over the other vary in relation to environmental conditions. The Saltmarsh Sparrow (Ammodramus caudacutus) is an extreme habitat specialist, with breeding behavior influenced by the tidal cycle. The challenges of nesting in the harsh environment of salt marshes and the unique promiscuous mating system of the Saltmarsh Sparrow provide a context for offspring sex ratio manipulation. We investigated adaptive sex ratio manipulation in this system across multiple sites and years using a mixed modeling approach. We collected data on nest initiation and nestling survival from 370 nests of 210 females during 2011–2015 in 4 marshes in the northeastern United States. Using molecular techniques, we determined the sex of 990 offspring and characterized variation in site- and population-level sex ratios. Using binomial linear mixed-effects models, we tested the influence of environmental, temporal, and maternal factors on offspring sex ratios. Across years and sites, we found an even offspring sex ratio of 1.03:1, with an alternating pattern of interannual variation between male and female bias at both the population and site level. The fluctuating sex ratio mirrored that of the adult sex ratio in the preceding year. Sex ratios did not vary as a function of timing of breeding within the breeding season or in relation to tidal flooding. Offspring sex was also independent of female condition at the time of nest initiation. Taken together, our findings suggest that female Saltmarsh Sparrows do not manipulate the sex ratio of their clutches in relation to environmental, temporal, or maternal factors. Our finding of a 1:1 offspring sex ratio and interannual variation in offspring and adult sex ratios in a wild bird population is more consistent with the predictions of Fisher (1930) than with those of Trivers and Willard (1973).

Avian fossils give us a unique opportunity to assess changes through time in species diversity and distributions. We report a previously unstudied collection of ∼500 bird fossils from Banana Hole, New Providence Island, Commonwealth of The Bahamas. Based on comparisons with fossil sites of known age on Abaco, the species composition of the Banana Hole fossils suggests a late Pleistocene rather than Holocene age for the site, although this remains uncertain because of the inability to date the fossils radiometrically. The specimens represent 49 species (45 resident, 4 migratory), 25 of which had not been recorded before as fossils from Banana Hole. Among the 45 resident species, 4 are extinct and 17 others are extirpated from New Providence. Combining our data with those previously compiled from Banana Hole, 52 resident species of birds now are known from this site, among which 6 (12%) are extinct (a hawk [Buteo quadratus], eagle [Titanohierax gloveralleni], caracara [Caracara creightoni], barn-owl [Tyto pollens], thick-knee [Burhinus nanus], and snipe [Gallinago kakuki]), 18 (35%) are extirpated on New Providence but still live elsewhere, and 28 (54%) still occur on New Providence. The modern diversity and distribution of Bahamian birds reflects interrelated changes in climate, island size and isolation, and habitat during the Pleistocene–Holocene Transition (15,000 to 9,000 yr ago) as well as species lost since human arrival ∼1,000 yr ago.