Although the ability to recognize related offspring is essential in the evolution of social behavior, the cues that birds use to identify their own offspring are not fully understood. The Cliff Swallow (Petrochelidon pyrrhonota) is a highly social species that nests in large colonies and exhibits a high incidence of both conspecific brood parasitism and extrapair fertilization, behaviors that can potentially lead to parents investing energy and resources in unrelated offspring, which reduces the parents' genetic fitness. Because parents continue to feed offspring after they leave the nest, parents also risk investing in unrelated offspring by misidentifying their own young after mobile, postfledging juveniles form crèches. Cliff Swallows possess a unique system of variable juvenile facial patterns, ranging from almost entirely black to almost entirely white. Interestingly, although these patterns are highly variable and distinctive in juveniles, they disappear upon maturation. We used image analysis to examine facial plumage of nestlings, and microsatellite genotyping to examine genetic relatedness among offspring within nests. We found substantial variation in facial plumage among juveniles and found a significant correlation between facial similarity and relatedness of nestlings. Genetically dissimilar juveniles in the same nest exhibited highly variable faces as measured by median pixel intensity. This variation in facial plumage may serve as a cue to allow birds to avoid misdirected parenting. We found no association between nestling relatedness and weight; this suggests that at least in the developmental period that we examined, parents may have not yet begun to use facial plumage or other cues to differentially provision offspring on the basis of genetic relatedness. If parents are able to use facial markings to distinguish between juveniles, they may do so at later stages of development, such as postfledging, to distinguish young raised in their own nest from others.

In addition to estimating survival probability of adult birds, estimating recruitment of new individuals into avian breeding populations is fundamental to understanding rates of population change. Notions about mean recruitment age can lead to erroneous conclusions about population projections if the probability of capture is ignored. We calculated the mean recruitment age of King Eiders (Somateria spectabilis) using two methods: (1) a naive estimate based strictly on observed age at first recapture of marked ducklings as nesting females; and (2) reversed capture histories, which incorporate probability of capture into estimates. From 1996 to 2009, we marked 2,390 King Eider ducklings, 53 of which were recaptured from 2007 to 2010 as females nesting at Karrak Lake, Nunavut, in Canada's Central Arctic region. The naive approach estimated mean (± 95% CL) recruitment age as 4.58 ± 0.42 yr, whereas reversed capture histories estimated mean recruitment age as 4.08 ± 0.34 yr. We illustrate the influence of recruitment age (range: 3–9 yr) on the predicted annual rate of population change. We fit numerous ecological covariates to test for cohort effects, phenology of vernal thaw, absolute and relative nesting phenology of mothers, maternal body size, density dependence, and relative clutch size on age-specific recruitment probability. There was good support for a negative effect of relative initiation date of nests that produced ducklings, and equivocal support for an additive negative influence of vernal thaw at the age that ducklings were recruited as breeders. We discuss the implications of variation in female recruitment age for King Eider population biology and fitness. More broadly, we reiterate previous advice (e.g., Pradel et al. 1997, Schwarz and Arnason 2000), against calculation of mean recruitment age from age at first capture, regardless of study species, particularly when detection probability of recruits is low.

Many species of insectivorous birds have contrasting plumage patches that are often displayed during foraging. Although such displays are widely hypothesized to flush potential prey and enhance foraging performance, experimental evidence that they function in this way was previously available only from redstarts (Myioborus spp.). I provide additional evidence from an experimental study of the Hooded Warbler (Setophaga citrina) in northwestern Pennsylvania, USA. Hooded Warblers regularly flick their tails while foraging, revealing large white spots on the outer tail feathers. I tested the function of the white tail spots with plumage manipulation experiments. At nests containing 5-day-old nestlings, the attending adults were captured and assigned to one of two treatment groups. For experimental birds, I temporarily darkened the white tail spots with a brown marking pen; for sham-darkened controls, I applied a comparable amount of ink to the dark inner rectrices. Experimental birds with darkened tails showed significantly reduced foraging performance compared with controls, and the decline in foraging performance was driven almost entirely by a decreased frequency of aerial prey attack. High-definition video recording at nests showed that plumage manipulation also altered the types of prey females delivered to nestlings; females with darkened tails delivered significantly fewer winged insects, and proportionally more insect larvae, than controls. However, the types of prey delivered by males were unaffected by plumage manipulation. Tail-flicking behavior codevelops with independent foraging in juveniles and is a significant positive predictor of juvenile foraging performance, even when the effects of juvenile age and activity are controlled statistically. Collectively, these results provide strong support for the hypothesis that white tail spots and tail-flicking behavior of Hooded Warblers function to flush visually oriented winged prey and enhance foraging performance. They also raise questions about sexual dimorphism in the tail pattern and its relationship to possible sex differences in foraging strategies.

The polytypic species concept unites populations that theoretically could and would interbreed were the opportunity to arise. This concept places the burden of proof of reproductive incapability and species status on those claiming species or higher rank. Advances in our understanding of the nature of reproductive isolation, the genetics of speciation, the limited role of gene flow, the power of directional selection, and the dynamics of hybridization support a different null hypothesis for taxonomic decisions, one that places the burden of proof on ‘lumping' rather than on ‘splitting' taxa at the species level. Switching the burden of proof provides an improved conceptual basis for the recognition of many allopatric island taxa and subspecies groups that merit species status. Taxonomic revisions based on these advances predictably confirm that distinct sister populations once lumped as polytypic species are independent evolutionary lineages that exhibit essential reproductive isolation. Release from the concerns about hybridization also positions proposed species for timely taxonomic decisions. The stage is set to proactively redefine polytypic species to separate component species for the 21^st century. The improved species classification will better reflect phylogeny and evolutionary status, characterize biodiversity more accurately, guide improved sampling patterns of bird populations for systematic studies, and enable informed conservation decisions.

The Channel Islands off the coast of southern California host >50 species of terrestrial vertebrates with varying degrees of phenotypic differentiation. However, most organisms that breed on the Channel Islands remain unstudied with respect to genetic differentiation from mainland populations. By comparing patterns of genetic variation between the Channel Islands and the mainland, we aimed to further our understanding of the role that the Channel Islands have played in diversification of the North American biota. We evaluated long-standing, untested hypotheses regarding colonization patterns and evolutionary relationships among western populations of the Horned Lark (Eremophila alpestris), including the endemic Channel Island subspecies E. a. insularis. We also examined how many times Horned Larks have colonized the Channel Islands, whether the species exhibits asymmetrical patterns of gene flow between mainland and island populations, and whether E. a. strigata of the Pacific Northwest is closely related to the phenotypically similar, but geographically separated, island subspecies. We found that E. a. insularis is polyphyletic, which suggests either multiple colonization events from the mainland or incomplete lineage sorting of a large ancestral population. We also inferred higher rates of migration from the Channel Islands to the mainland, with E. a. strigata being closely related to individuals from the Channel Islands and coastal southern California. Moreover, ecological niche models for E. a. strigata identified suitable abiotic conditions in southern California and the Pacific Northwest during the Last Glacial Maximum, which suggests that E. a. strigata experienced a postglacial range shift in addition to a population bottleneck. Our results provide novel insight regarding the origins of the Channel Island avifauna and the evolutionary history of the Horned Lark in the western United States. Moreover, our findings suggest that Channel Island birds may be weakly differentiated from mainland populations despite phenotypic differences between recognized subspecies.

The Common Yellowthroat (Geothlypis trichas) exhibits widespread geographic variation in plumage, morphology, migratory behavior, and song. In addition, researchers recently found evidence that the Common Yellowthroat has three genetically distinct groups across its North American range: eastern, western, and southwestern. These groups are more genetically similar to other Geothlypis species than to each other, which suggests relatively long-term isolation. I hypothesized that geographic variation in song behavior should reflect these genetic differences. To test this hypothesis, I examined spatial patterns of variation in both note types and acoustic characteristics of song. Consistent with the hypothesis, I found significant differences among the three groups, particularly in frequency measures, internote duration, notes per phrase, and note elaborateness. Within the eastern and western groups, I also found significant song differences among historically recognized subspecies. When comparing western and eastern subspecies, I found different latitudinal trends, even though subspecies found at similar latitudes that exhibit similar migratory behavior might be expected to have similar song characteristics. Two possible explanations for this lack of convergence are (1) stochastic changes in song in isolated populations and (2) nonlatitudinal dissimilarities in habitat, including transmission properties or effects on morphological evolution, that drive song divergence. Without excluding other explanations, I found evidence of an effect of morphological divergence: Subspecies with larger bills sang songs with lower frequencies. Overall, the geographic variation in the songs of the Common Yellowthroat demonstrates that multiple evolutionary processes interact to shape birdsong, and that the importance of each of these processes and their interactions varies among populations.

Information can enhance fitness, and the ways in which organisms acquire and use information are of heightened interest in ecological studies today. Migratory birds, as long-distance travelers of the globe, depend on rapid access to accurate information and thus provide particularly interesting study subjects for cognitive ecology. Yet, questions regarding how migratory birds collect information and make decisions en route remain to be answered. Here, we review the current status of this field of study and focus our attention on social learning (broadly defined as the use of inadvertently produced social information) as an important cognitive mechanism that can operate across taxonomic boundaries. We argue that social learning is critical to accelerate resource acquisition while reducing risks and uncertainties during migration. We put forward eight testable predictions in relation to when increased use of social information might be expected. Finally, we argue that migrant communities at stopover sites may serve as additional sources of information where transient associations with others may have important and long-lasting benefits.

A growing collection of mtDNA genetic information from waterfowl species across North America suggests that larger-bodied cavity-nesting species exhibit greater levels of population differentiation than smaller-bodied congeners. Although little is known about nest-cavity availability for these species, one hypothesis to explain differences in population structure is reduced dispersal tendency of larger-bodied cavity-nesting species due to limited abundance of large cavities. To investigate this hypothesis, we examined population structure of three cavity-nesting waterfowl species distributed across much of North America: Barrow's Goldeneye (Bucephala islandica), Common Goldeneye (B. clangula), and Bufflehead (B. albeola). We compared patterns of population structure using both variation in mtDNA control-region sequences and band-recovery data for the same species and geographic regions. Results were highly congruent between data types, showing structured population patterns for Barrow's and Common Goldeneye but not for Bufflehead. Consistent with our prediction, the smallest cavity-nesting species, the Bufflehead, exhibited the lowest level of population differentiation due to increased dispersal and gene flow. Results provide evidence for discrete Old and New World populations of Common Goldeneye and for differentiation of regional groups of both goldeneye species in Alaska, the Pacific Northwest, and the eastern coast of North America. Results presented here will aid management objectives that require an understanding of population delineation and migratory connectivity between breeding and wintering areas. Comparative studies such as this one highlight factors that may drive patterns of genetic diversity and population trends.

The degree to which local song structure reflects the singer's population of origin is a long-standing and contentious issue. Young songbirds that settle to breed outside their natal song-dialect area may learn to produce nonnatal dialect by hearing and memorizing these songs during juvenile dispersal. We quantified adult singing rates in a population of Mountain White-crowned Sparrows (Zonotrichia leucophrys oriantha) in the Sierra Nevada in 2 yr, and compared them with the arrival schedule of juveniles not hatched on the study area. Our results show that opportunities for juveniles to learn new (nonnatal) song types become highly limited during the premigratory dispersal phase, because adult singing rates are very low by the time nonlocal juveniles begin to arrive. Thus, if individuals learn nonnatal songs during dispersal, they must do so hearing relatively few repetitions of the unfamiliar dialect.

Nestlings of birds breeding in closed nests compete for food by both vocal begging and competing for a position near the nest entrance (locomotor activity). The latter strategy can be particularly effective in nests with long entrance tunnels, where nestlings can intercept feedings from incoming parents. However, little is known about whether the intensity of locomotor activity is related to food availability in the same way as the intensity of vocal begging. We hand-fed Bank Swallow nestlings (Riparia riparia) in the laboratory under four feeding regimes: without restriction (AL), or facing different levels of food restriction (in order of increasing degree: FR1, FR2, FR3). Compared with group AL, the intensity of vocal begging was elevated in group FR1 and did not increase further in the FR3 group, whereas only the FR3 nestlings showed higher locomotor activity (quantified as the frequency of crawling to the nest tunnel). The FR3 group was also the only group where reduced food intake slowed wing development. We conclude that vocal begging and locomotor activity were modulated independently by the level of food intake. We hypothesize that in young Bank Swallow nestlings locomotor activity is a more effective but also more expensive means of soliciting food than vocal begging and therefore is used only by the neediest of nestlings (e.g., those with impeded wing development). Our results illustrate the interplay between different components of complex begging displays serving to convey information about individual needs and competition with nestmates.

To maximize fitness, breeding adults may respond to environmental processes by adjusting their progeny's sex ratios. R. A. Fisher in 1930 hypothesized that frequency-dependent selection would result in equal investment in sons and daughters over the long term, yielding a balanced sex ratio if the costs of raising a son and daughter are equal. Diverse hypotheses have tried to explain population and brood-by-brood deviations from this mean as well as annual variation by focusing on adult sex ratios, resources, abiotic conditions, and female and male quality. We collected data in 2002–2010 to explore population-level variation in nestling sex ratios in 2 migratory grassland songbird species: the Bobolink (Dolichonyx oryzivorus) and Savannah Sparrow (Passerculus sandwichensis). These species differ in migratory strategy (long-distance vs. short-distance), and morphological dimorphism. Fisher's hypothesis was rejected for Savannah Sparrows (n = 684 nestlings; 39% male) but not rejected for Bobolinks (n = 390 nestlings; 53.8% male). No relationship was found between nestling and adult sex ratios measured in the same year. In descriptive analyses at the brood level, male and female body size and age, and ecological conditions (temperature and precipitation) failed to predict nestling sex ratios. Although male nestlings were heavier than female nestlings and resource availability changed through the season, these factors did not influence sex ratios relative to female body size or seasonality. For Savannah Sparrows, larger broods tended to be male-biased. While we were otherwise not able to explain deviation in offspring sex ratio for Savannah Sparrows, our results suggest that the ecological and evolutionary pressures that affect sex ratios may be both species- and population-specific.

Sandpipers and allies (Scolopacidae) show an astounding diversity in mating and parental care strategies. Comparative studies have tried to interpret this variation in terms of phylogenetic constraints and ecological shaping factors. In such analyses, mating and parental care systems are necessarily discretely classified at the species level. The few available descriptive studies on breeding strategies of the Sanderling (Calidris alba) came to variable conclusions, which, in turn, were inconsistently used in these comparative studies. We provide empirical data on mating and parental care patterns in Sanderlings studied during six summers in northeast Greenland. In 135 clutches, we determined parental care from incubation profiles using temperature loggers and confirmed that both uniparental incubation by both sexes and biparental incubation (45 and 90 clutches, respectively) occurred. We used microsatellite-based parentage analyses to describe the degree of extrapair mating. In 48 completely assayed families, we found 6 cases of polygamy (4 cases of polyandry, 2 cases of polygyny) that involved both uniparental and biparental clutches. This implies substantial variation in the patterns of mating and parental care, defying categorical assignments even at the local level. We conclude that the classification of mating strategy and parental care pattern for the Sanderling has been rather coarse, and that comparative analyses have not taken the observed intrapopulation variability into account. Because sandpipers show such variable reproductive behavior, between and within species, more detailed descriptive studies using parentage analyses are required to revisit previous statements about the intensity of sexual selection, including sexual size dimorphism, in shorebirds. In view of the great variability, methods of comparison will need elaboration too.