Forest soils in many areas of the world are becoming increasingly acidified, in part because of atmospheric deposition of strong acids produced by the burning of fossil fuels. Soil acidification and subsequent calcium depletion of forest soils in eastern North America have the potential to affect forest songbird populations negatively by reducing the availability of calcium-rich food items that are critical for reproduction. Nonetheless, little experimental evidence exists for the purported relationship between soil calcium and avian reproduction. Our objective was to use an experimental and observational study to determine how forest songbird-habitat quality is related to soil calcium availability. We experimentally elevated soil calcium using limestone sand and observed a 1.8-fold increase in Ovenbird (Seiurus aurocapilla) territory density, larger clutch sizes and more nests, but no effects on egg characteristics. The observational study of 14 forested sites in central Pennsylvania likewise showed positive relationships between natural soil calcium levels and Ovenbird territory density, clutch size, and nest density. Again, no relationships existed between soil calcium and eggshell characteristics. Snails are a critical calcium source for many breeding birds, and we suspect that snails are the link between soils and birds because snail abundance increased with liming and was positively correlated with soil calcium. We conclude that Ovenbird habitat quality is related to soil calcium, that birds on our sites were calcium-limited, and that reduced soil calcium could play an important role in bird declines in acidified forests.

Understanding the connectivity between breeding and nonbreeding populations of migratory birds is fundamental to our knowledge of biological phenomena such as population dynamics and dispersal. Moreover, our ability to quantify migratory connectivity has inevitable consequences for both conservation and management of species that utilize distinct geographic locations. Technology is rapidly advancing our ability to track birds throughout the annual cycle and to collect data on the degree of connectivity among breeding and nonbreeding populations. We combined two direct methods, mark—recapture (n = 17) and geolocation (n = 6), to estimate the migratory connectivity of breeding and nonbreeding populations of Gray Catbirds (Dumetella carolinensis). Data from geolocators show that birds breeding in the Mid-Atlantic overwinter in both Cuba and southern Florida. Mark-recapture data supported our geolocator results but also provided a broader spatial perspective by documenting that Mid-Atlantic and Midwestern populations occupy distinct geographic localities during the nonbreeding period. This research underscores the importance of geolocators, as well as other tools, to advance our understanding of migratory connectivity. Finally, our results highlight the potential value of U.S. Geological Survey (USGS) Bird Banding Laboratory mark-recapture data, which are often underutilized in ornithological research.

Guira Cuckoos (Guira guira) are cooperative breeders with joint nests, where several breeding and nonbreeding males and females remain in a cohesive unit through repeated breeding attempts within a single territory. We used nine microsatellite markers to analyze parentage and relatedness in a population of Guira Cuckoos in central Brazil, comprising 225 progeny from 51 breeding attempts. The Guira Cuckoos presented a variety of mating patterns, polygynandry and monogamy being the most common. We found low levels of extragroup fertilization, and cobreeding males and females within groups shared reproduction to some extent. Relatedness among group members varied. In some groups, adult males were more related to each other than expected by chance and, overall, males within groups were genetically more similar than background genetic relatedness. In addition, for some of the groups, males were more genetically similar than expected by chance in different years, which indicates some degree of male philopatry or possible joint dispersal by male kin. Male Guira Cuckoos were more likely to breed closer to their natal territories than females, a pattern of dispersal commonly found in birds. We also found that nonbreeding adult males had a higher number of nondescendent kin (chicks and embryos) in the nest than expected from background genetic relatedness, which implies that possible indirect reproductive benefits may have been a significant factor in the evolution of this breeding system.

Social mimicry was postulated as a possible evolutionary mechanism that would produce convergence in the characteristics of species that interact extensively and, thus, act as an opposing force to competition and character displacement. The examples for this hypothesis were drawn mostly from visual resemblances among bird species that flock together. We evaluated plumage mimicry among groups of birds, asking 31 raters to score the resemblances among 22 sets of putative mimicry postulated in the past four decades. The resemblance between the putative model and the mimic (if there was no one model hypothesized, we considered both species to be possible models for each other) was compared with that between the model and (1) a species closely related to the mimic and (2) another species that only occasionally associated with the mixed-species groups but lived in the same habitat. We found significant support for 14 of the cases. Our results emphasize the importance of assessing similarity by using multiple raters and that several mechanisms may drive phenotypic resemblance, some reflecting phylogenetic inertia or habitat constraints. Nevertheless, these mechanisms may be accentuated in groups of birds that interact extensively.

For cavity-nesting birds, tree cavities often represent a critical, defendable, and limiting resource that is frequently under intense interspecific competition. However, the dynamics of interspecific conflict resolution, especially between species of similar size, are often unclear. We experimentally tested aggression and nest-defense behaviors in Gouldian Finches (Erythrura gouldiae) and Long-tailed Finches (Poephila acuticauda), two sympatric, cavity-nesting estrildid finches that are very similar in size, ecology, and nest-site requirements. Mounted taxidermie models of conspecific and heterospecific nest-site competitors (black and red Gouldian Finch morph and Long-tailed Finch models), as well as a control noncompetitor (Black-chinned Honeyeater [Melithreptus gularis]), were presented to Gouldian and Long-tailed finches. These two competing species differed in their overall responses to simulated intruders and in their relative aggression toward conspecific and heterospecific intruders. Long-tailed Finches reacted more quickly to models, approached closer, and were more likely to attack models (i.e., make physical contact) than Gouldian Finches, which suggests that Long-tailed Finches are intrinsically more aggressive. In addition, Long-tailed Finches were more aggressive toward Gouldian Finches than toward conspecific models. By contrast, Gouldian Finches were more aggressive to conspecific models and avoided approaching Long-tailed Finch models. Male Gouldian Finches were particularly aggressive toward conspecifics, and red head-color morphs were more aggressive than black morphs. These results suggest that the outcomes of competitive asymmetries within and between these species are driven by differences in aggression. Together with the substantial overlap in nest-site use, the Long-tailed Finch's aggressive domination of limited nest sites may lead to competitive exclusion of the endangered Gouldian Finch.

The natural range extension and colonization of a new continent by a bird species is rare, and even more rarely is it observed and documented. In 1980, six pairs of Barn Swallows were found breeding in Argentina within the species' historical wintering range, and this South American population has since grown to thousands of pairs. We explored the genetic context and consequences of this natural trans-hemispheric colonization event via comparisons among the South American population and two North American populations. We tested for evidence of a genetic founder event by assessing allelic diversity at eight microsatellite loci and haplotype diversity of mitochondrial ND2 sequences. Contrary to our expectations, the recently established South American breeding population showed no evidence of a founder effect, with no difference in heterozygosity, allelic diversity, haplotype diversity, or population differentiation in comparison to the large North American populations. The genetic similarity of these populations suggests that this long-distance colonization event was not associated with a strong demographic bottleneck, perhaps because the South American population has been augmented by ongoing immigration from North America.

The experimentally supported and prevailing opinion is that blood sampling has few to no long-term effects on survival of birds when conducted properly, and blood sampling has become a vital addition to the toolbox of many ornithologists. However, many of the studies that concluded that blood sampling had negligible effects on birds used approaches that did not account for temporary emigration and probability of capture. To date, the only study to have done so found that blood sampling had a strong negative effect on survival. We conducted a mark-recapture analysis of 8 years of banding and bleeding data on Eastern Kingbirds (Tyrannus tyrannus) to determine whether survival was negatively influenced by blood sampling. Our analyses adjusted for temporary emigration and probability of recapture and accounted for (1) transitions between the bled and the nonbled state and (2) a change in protocol roughly midway through the study that resulted in a change from single to often multiple (and larger) draws of blood per year from single individuals. We found that survival rates of nonbled (0.61) and bled (0.67) males were statistically indistinguishable and that bled females had a higher probability of survival than nonbled females (0.68 and 0.58, respectively). The change to larger and more frequent blood samples was also not associated with a change in survival. Our data show that when accepted protocols were followed, blood sampling had no detectable influence on the survival of adult Eastern Kingbirds. Whether this applies generally awaits analyses using similarly rigorous methods on other species.

We used stable-isotope (δD) measurements of primary feathers to demonstrate that Northern Rough-winged Swallows (Stelgidopteryx serripennis) that breed in western North America migrate to the monsoon region of northwest Mexico for their annual postbreeding molt. Both adults and juveniles replace all their primaries in late summer. As expected, in samples of adults and fledglings collected at a northern breeding locality in eastern Washington, primary δD values of adults were of southern origin, whereas those of fledglings, whose feathers were grown in the nest, were consistent with their northern origin. Similarly, some young birds collected as they were replacing primaries in northwest Mexico had a southern signature in newly replaced primaries but a northern signature in their yet unreplaced juvenile primaries, which indicates that they were molt migrants from the north. Feather OD values increased from P1 to P9 in breeding adults collected in upstate New York. In the eastern United States, both adults and juveniles replace their primaries as they migrate southward in the fall. These isotopic results are consistent with the observation that Northern Rough-winged Swallows are halfway through their primary molt when they reach the Gulf Coast, where they pause in coastal marshes to finish the primary molt before crossing the Gulf of Mexico. The striking difference in molt and migration schedules between eastern and western populations of Northern Rough-winged Swallows suggests further investigations of their species status be conducted in the region where these populations make contact.

We used light-level archival geolocators to track five Veeries (Catharus fuscescens) for one annual cycle as they migrated from Delaware to South America and back. Southward migration commenced in late August and September 2009. Veeries arrived at wintering sites from 2 November to 2 December 2009. All birds initially wintered in the Amazon basin south of the Amazon River in Mato Grosso, Para, and Amazonas states, Brazil. All birds showed intratropical migration to a second winter site between 7 January and 7 March 2010. These second winter sites were in the interior, northern, and southern periphery of the Amazon basin and the Orinoco River headwaters, Venezuela. Northward migration commenced in mid-April 2010, and the Veeries returned to Delaware between 29 April and 20 May 2010. Despite variation in migratory patterns, several stopover regions were visited by multiple individuals: the coasts of the Carolinas, the coast of the Gulf of Mexico, Cuba and Jamaica, the north coast of Colombia and Venezuela, and southern Venezuela. Our spatial and temporal data necessitates that the Veery winter range include the entire Amazon basin and the headwaters of the Orinoco River, as well as two disjunct regions in Mérida state, Venezuela, and São Paulo state, Brazil. We hypothesize that Veeries initially settled in lowland forest and that intratropical migration was prompted by the ecological factors associated with the seasonal flood pulse of Amazonian rivers. If so, the Veery may be threatened by the recent, unprecedented proposed alteration of Amazonian lowland forests.

Cross-seasonal studies of avian movement establish links between geographically distinct wintering, breeding, and migratory stopover locations, or assess site fidelity and movement between distinct phases of the annual cycle. Far fewer studies have investigated individual movement patterns within and among seasons over an annual cycle. Within western Oregon's Willamette Valley throughout 2007, we quantified intra- and interseasonal movement patterns, fidelity (regional and local), and migratory patterns of 37 radiomarked Wilson's Snipe (Gallinago delicata) to elucidate residency in a region of breeding- and wintering-range overlap. Telemetry revealed complex regional population structure, including winter residents (74%), winter transients (14%), summer residents (9%), and one year-round resident breeder (3%). Results indicated a lack of connectivity between winter and summer capture populations, some evidence of partial migration, and between-season fidelity to the region (winter-resident return; subsequent fall). Across seasons, the extent of movements and use of multiple wetland sites suggested that Wilson's Snipe were capable of exploratory movements but more regularly perceived local and fine-scale segments of the landscape as connected. Movements differed significantly by season and residency; individuals exhibited contracted movements during late winter and more expansive movements during precipitation-limited periods (late spring, summer, fall). Mean home-range size was 3.5 ± 0.93 km² (100% minimum convex polygon [MCP]) and 1.6 ± 0.42 km² (95% fixed kernel) and did not vary by sex; however, home range varied markedly by season (range of 100% MCPs: 1.04–7.56 km²). The results highlight the need to consider seasonal and interspecific differences in shorebird life histories and space-use requirements when developing regional wetland conservation plans.

A developing organism faces a dilemma: whether to allocate available resources to building its body structures (growth) or to the development of its immune system. The outcome of this tradeoff is likely to be modified by parasites. We manipulated the abundance of ectoparasitic Hen Fleas (Ceratophyllus gallinae) on nestling Blue Tits (Cyanistes caeruleus) by microwaving nests and subsequently adding 200 Hen Fleas (15 infested nests) or not (16 reduced-infestation nests). In addition, we manipulated the host nestlings' food resources by supplementary feeding 10–15% of daily energy needs to half the nestlings in a nest during the key developmental period (days 2–12). Feather growth (tail and wing length) and hematocrit were reduced by the presence of Hen Fleas, indicating negative effects on nestling development. In comparison to the control nestlings, food-supplemented nestlings aged 16 days were larger (tarsus, residual body mass), but only in reduced-infestation nests (interaction between both treatments). Body size of fed male offspring increased in relation to that of females, but only in the absence of ectoparasites. We hypothesized that supplemented resources are allocated to immune defense when ectoparasites are present, but humoral immune function (total immunoglobulin concentration) and cell-mediated immune defense (phytohemagglutinin response) were not affected by either treatment. Either the nestlings allocated additional resources away from growth (into an unknown developmental component) when parasites were abundant, or the ectoparasites preferentially fed on supplementary-fed host nestlings and thereby equalized the development of soma and immune defense of nestlings despite provision of additional resources.

Interpreting habitat quality requires an understanding of the consequences of habitat use because bird density alone may not reflect habitat quality. Although habitat use can be readily quantified, subsequent effects remain elusive for most species, especially for migrants on the wintering grounds. Wintering Henslow's Sparrow (Ammodramus henslowii) densities in Longleaf Pine (Pinus palustris) savannas are fire-dependent, with highest bird densities in the first winter after a fire, but reduced density in subsequent winters without fire. We determined habitat characteristics associated with variation in bird density in the first and second winters following fire and then followed birds' diet, condition, and survival in 10 plots over two winters. Habitat structure and seed energy availability, but not plant species composition, were equally important for explaining bird density, except for two outlier plots with extremely high seed production and fewer birds than expected. Henslow's Sparrows consumed a variety of seeds and arthropods, which indicates that they are generalists capable of responding to fire-mediated changes in seed composition. Body condition decreased in midwinter, associated with a decrease of arthropods in the birds' diet. Although Henslow's Sparrow densities responded strongly to ephemeral savanna conditions, consequences of habitat use were negligible because survival and three measures of body condition did not vary with years since fire or among age and sex classes. These aspects of the winter ecology of sexually monochromatic Henslow's Sparrows are consistent with tenets of the ideal free distribution, whereby differences in bird density between habitats result in equal fitness.

Understanding the role of green space in urban—suburban landscapes is becoming critical for bird conservation because of rampant habitat loss and conversion. Although not natural habitat, golf courses could play a role in bird conservation if they support breeding populations of some native species, yet scientists remain skeptical. In 2003–2009, we measured reproduction of Eastern Bluebirds (Siala sialis) in Virginia on golf courses and surrounding reference habitats, of the type that would have been present had golf courses not been developed on these sites (e.g., recreational parks, cemeteries, agriculture land, and college campus). We monitored >650 nest boxes and 2,255 nest attempts (n = 1,363 golf course, n = 892 reference site). We used an information-theoretic modeling approach to evaluate whether conditions on golf courses affected timing of breeding, investment, or nest productivity compared with nearby reference sites. We found that Eastern Bluebirds breeding on golf courses reproduced as well as those breeding in other disturbed habitats. Habitat type had no effect on initial reproductive investment, including date of clutch initiation or clutch size ( = 4 eggs). During incubation and hatching, eggs in nests on golf courses had higher hatching rates (80%) and brood sizes ( = 4.0 nestlings brood^-1) than nests on reference sites (75% hatching rate; = 3.8 nestlings brood^-1). Mortality of older nestlings was also lower on golf courses and, on average, golf course nests produced 0.3 more fledglings than nests on reference sites. Thus, within a matrix of human-dominated habitats, golf courses may support productive populations of some avian species that can tolerate moderate levels of disturbance, like Eastern Bluebirds.