Grebes (Aves: Podicipedidae) are a threatened family containing species that vary widely in demography. Podicipedidae includes several species that are either rare and confined to a single lake basin, or abundantly distributed across several continents. The most speciose genus, Podiceps, particularly the eared grebe lineage, best reflects this curious demographic pattern by representing the most abundant of extant grebes, several critically endangered species, and a recently extinct species. Here, we obtained genetic data from 3 mitochondrial markers to make phylogenetic and population genetic inferences about the eared grebe clade. Using DNA from tissue, feather, skin, and toe pads, our sampling encompassed all species and subspecies, including the extinct Colombian Grebe (Podiceps andinus) and migratory and resident populations of the North American Black-necked Grebe (P. nigricollis californicus). Bayesian inference yielded novel insights into the dynamics of this group, particularly the recent ecological isolation and incipient speciation of the Colombian and Junin (P. taczanowskii) grebes, as determined from limited genetic divergence and rapid evolution of plumage color and bill shape (elongation, deepening of the culmen). DNA barcode and cytochrome b distances supported these inferences. Population genetic and divergence time analyses further revealed that the abundance of the North American Black-necked Grebe is likely associated with mid-Pleistocene dispersal from South America followed by late Pleistocene expansion during a time when hypersaline lake habitat accommodated large populations of staging birds. In conclusion, the demographic variation among species in the eared grebe group can be explained by recent ecological speciation of both a sympatric and an allopatric nature. Future investigation is warranted to determine whether this pattern of speciation and associated rapid phenotypic divergence can be extended to other grebe taxa.

Observations that similar traits repeatedly evolve across independent taxa on islands—such as loss of flight in birds or reduction of body size in large mammals—suggest that deterministic processes, rather than drift, drive the convergent evolution of these traits. One such repeated pattern on islands that has received little attention is the evolution of entirely black coloration, or “melanism.” In several taxa, including birds, reptiles, and insects, melanism has evolved on islands despite the proximity of mainland populations with different colors. We explored the evolution of melanism on islands by testing how island size predicts the frequency of melanic birds in Monarcha castaneiventris obscurior, a subspecies of flycatcher from the Solomon Islands that is polymorphic for plumage color (i.e. melanic vs. chestnut-bellied). While accounting for potential spatial autocorrelation because some islands are <1 km apart, we found that island size predicted the frequency of melanic birds, with smaller islands having higher frequencies of melanism than larger islands. As with other traits that have repeatedly evolved on islands, this pattern suggests that melanism on small islands is likely mediated by deterministic processes like natural selection.

We used morphometrics, plumage color, and mitochondrial DNA to reconstruct the phylogeography of the Plain Wren (Cantorchilus modestus), an abundant bird species of low to middle elevations throughout Central America. Sampling the full geographic range of C. modestus, we uncovered significant phenotypic structure in size, shape, and plumage color that mirrored deep genetic divergences in mitochondrial DNA. This structure was mostly concordant with the 3 most broadly recognized subspecies of Plain Wren (C. m. modestus, C .m. zeledoni, and C. m. elutus), but a potentially undescribed taxon exists in the highlands of Central Belize. We also found evidence of a cryptic phylogeographic break on the Pacific Slope of Costa Rica, between the southernmost populations and the widespread northern populations. This latter discovery likely represents a significant expansion of the known range of the traditionally Panamanian subspecies C. m. elutus, corresponding to a transition between major ecoregions. Topology tests suggested that northern populations of the nominate taxon likely dispersed south into Lower Central America, with subsequent isolation and diversification. We also explored the possible role of adaptation in the phenotypic divergence of the “Canebrake Wren” (C. m. zeledoni) as a result of isolation in the more humid environments of the Caribbean Slope. Our data and analyses suggest that the Plain Wren may consist of multiple biological species that have been diverging since geophysical upheavals in the lower half of Central America approximately 2–3 mya.

Describing and then explaining individual behavior during migration can help us to understand why (on both proximate and ultimate levels) birds migrate; the altitude(s) at which migratory birds fly, for example, can have far-reaching consequences. However, to date, no fine-scale, full-flight altitude data have been available for small (<100 g) migratory birds. We tracked 7 Swainson's Thrushes (Catharus ustulatus) carrying altimeters from takeoff until landing or near-landing during 9 migratory flights. The average recorded flight altitude for the 9 flights was 673.0 ± 523.2 m (mean ± SD); average maximum flight altitude for the 9 flights was 1,199.5 ± 862.7 m (range: 319.2–2,744.5 m). Initial ascent rates (0.42 ± 0.15 m s⁻¹, n = 8) matched predictions; final descent rates were 0.55 ± 0.30 m s⁻¹ (n = 5). Contrary to expectations, the thrushes made numerous (9.33 ± 4.42), significant (>100 m) altitude adjustments during their flights (1.44 hr⁻¹), not including initial ascent and final descent. The repeated changes in flight altitude that we observed should cause these birds to use more energy than they would if they flew at or near a single altitude for several hours at a time. We speculate that these altitude modifications may result from variation in atmospheric conditions or from the birds descending toward anthropogenic light sources during the flights.

Common Cuckoos (Cuculus canorus) are obligate brood parasites that lay their eggs in nests of other species and use these hosts to raise their parasitic offspring. Two key adaptations that increase their reproductive success are (1) the capacity for cuckoos to lay large numbers of eggs and thereby parasitize many nests per year, and (2) the ability of cuckoo eggs to hatch before those of hosts, enabling cuckoo nestlings to evict host eggs and eliminate competition for food. Producing more eggs is generally associated with reduced investment of nutrients and energy reserves per egg, which in turn is associated with shorter incubation periods both within and between species. We hypothesized that Common Cuckoos deposit reduced energy reserves into their eggs than do their hosts to facilitate both (1) and (2). To test these hypotheses, we compared the concentration of yolk lipids (per wet yolk mass) between eggs of 3 cuckoo gentes and their respective host species: Great Reed Warblers (Acrocephalus arundinaceus), Common Redstarts (Phoenicurus phoenicurus), and Reed Warblers (A. scirpaceus). Yolk lipids provide the bulk of energy required for embryonic development and can also serve structural and cell-signalling functions. As a general pattern, cuckoo eggs contained a lower concentration of energy-reserve lipids than eggs of their respective hosts, but not structural or cell-signalling lipids. When controlling for their heavier eggs and yolks, Common Cuckoo eggs had an estimated lower amount of energy reserve lipids for their size than host eggs. Our findings suggest a potential role of yolk lipid composition in facilitating (1) and (2) and advocate the need for further research in this area. We also highlight the potential problems of using either concentration or total yolk mass alone to compare maternal investment across taxa in comparative studies.

Avian reproductive strategies have been hypothesized to vary with elevation. Shorter breeding seasons due to harsh environmental conditions, and potentially higher predation risks, may reduce clutch sizes at higher elevations, which in some species leads to increased parental care and offspring survival. However, this phenotypically plastic and potentially adaptive response has been documented only in a handful of species in the Northern Hemisphere. For the first time in a southern temperate ecosystem, we studied whether the breeding strategy of a secondary cavity-nester varied along an elevational gradient in Andean temperate forests, Chile. We installed 240 nest-boxes at 260–1,115 m elevation and monitored the breeding activity of 162 nests of Thorn-tailed Rayaditos (Aphrastura spinicauda) over 2 seasons (2010–2012). We included 50 nests from a third season only for recording clutch size and nestlings per clutch. As predicted, the breeding season was shorter in highland forests than in lower elevations, by 28% and 55% over the 2 successive seasons. Although timing of egg laying (1 egg every second day) and incubation period (average = 15 days) did not vary with elevation, we found smaller clutch sizes (average = 4.1 vs. 4.5) and fewer nestlings per clutch (average = 3.5 vs. 4.2) at higher elevations. The extent of parental care, expressed as the duration of the nestling period, was slightly but significantly greater in highland than in lowland forests (22.2 vs. 21.6 days). Despite the longer nestling period at higher elevations, nesting success was lower at high elevations, mainly because of nest predation. Our findings suggest that Thorn-tailed Rayaditos may change to a slower reproductive strategy along elevational gradients. Yet these changes do not appear to compensate for the increased predation rates at higher elevations, calling into question the potential adaptive significance of this strategy.

The role of Quaternary glacial–interglacial intervals in shaping the diversity and distribution of Neotropical species has been the focus of considerable research. The Neotropics sustain the highest passerine diversity on Earth, but little is known about this region's historical biogeography based on fossils. To assess how passerine species were affected by Quaternary climate fluctuations, we identified 625 late Pleistocene fossils (individual fossilized bones) from the now arid and faunally depauperate Talara Tar Seeps in northwestern Peru. Of the 21 passerine species identified, only 2 likely live at the site now; the remaining 19 species require more mesic conditions. Species identified included members of the Thamnophilidae (antbirds), Melanopareiidae (crescentchests), Tyrannidae (flycatchers), Hirundinidae (swallows), Mimidae (mockingbirds), Thraupidae (seedeaters, “finches”), Emberizidae (sparrows), and Icteridae (blackbirds). Nearly half of the individual fossils and 8 of the 21 species were icterids, including 3 extinct species (1 previously described, 2 new). The late Pleistocene passerine community at Talara, which was nonanalog to any modern community, suggests that the site once supported savanna, grasslands, and forests during the last glacial interval, which are absent near Talara today. Quaternary climate change and the collapse of the community of large mammals had a major influence on the community composition and the geographic ranges of passerine species in northwestern Peru.

Conditions during development, including nutrition and immune challenges, can have long-lasting effects on subsequent physiological parameters. Many of these organizational effects are mediated by hormones, including corticosterone (CORT), a hormone involved in nutrient mobilization, immune response modulation, and responses to stressors. While the effects of exogenous CORT during development on later-life physiology have demonstrated the important role this hormone plays in developmental plasticity, relatively less attention has been paid to other factors capable of driving a CORT-mediated cascade of phenotypic effects. We tested the effect of nonpathogenic immune challenges during different stages of development and used levels of CORT in feathers (CORT_f) to investigate whether CORT levels during development (when the feathers were grown) could predict variation in traits at adulthood. Mallards (Anas platyrhynchos) exhibit a variety of traits as adults that could be influenced by CORT, including a carotenoid-pigmented beak used in mate choice and melanin-based patterning in the speculum of the wing that has no known signaling function. We found that nonpathogenic immune challenges during development had no detectable effects on duckling CORT_f, which suggests that more robust immune challenges are required to create a sustained increase in circulating CORT levels. However, duckling CORT_f predicted multiple traits at adulthood: higher levels of CORT_f were associated with increases in circulating carotenoid levels, degree of carotenoid-based beak coloration, and mass gain. We failed to demonstrate a link between melanin-based speculum patterning and CORT at time of trait development, although speculum patterning was both affected by immune challenge treatment and correlated with adult immune function. These results support the “quality hypothesis,” which states that variation in CORT could be positively correlated with traits reflecting individual quality under certain conditions, and also suggest that the speculum in Mallards may be able to serve as an honest signal.

We used observational and experimental approaches to assess the possible functional significance of the often extensive “pavements” of livestock dung constructed by female Black Larks (Melanocorypha yeltoniensis) around their nests. These pavements are conspicuous to human observers, suggesting that they may also attract predators. The size of the pavement was correlated with, but not limited by, the density of dung in the vicinity of the nest. The relationship between pavement size and local dung density did not differ significantly between habitats or years, suggesting that females might scale their pavements according to the perceived trampling risk. Even in heavily grazed areas nest trampling was rare, and nest survival rates were similar to those in areas with few grazing animals, suggesting that pavements may reduce trampling risk without incurring an additional predation risk or, alternatively, that trampling is currently not an important threat to lark nests. An experimental manipulation of grazing animals around artificial nests yielded equivocal support for a trampling-deterrent effect of dung pavements. Dung pavements might also provide thermal benefits; experiments on artificial nests suggested that dung pavements buffer nests against extremes of heat and cold, and there was equivocal support for a positive effect of pavement size on chick tarsus growth rates. These pavements may therefore be multifunctional, but identifying the adaptive drivers of the behavior requires further research.

Extent of parental care, such as food provisioning to offspring, can vary widely between sexes in socially monogamous birds. The determinants of the sex-specific provisioning behaviors remain unclear as they could be associated with individual condition, as well as mate contribution and quality. In a population of Mountain White-crowned Sparrows (Zonotrichia leucophrys oriantha), we tested the good parent hypothesis, the reproductive compensation hypothesis, and the differential allocation hypothesis among 29 nesting pairs during the breeding season. Using multivariable linear regression, we examined the relationship of offspring food provisioning rate, a measure of parental care, with each parent's own demographic (sex, age), morphological (tail length, wing chord, tarsus, mass, body fat, and cloacal protuberance), and physiological (hematocrit and infection status) characteristics, as well as crown-white, a reliable status signal in this species. In male birds, feeding rate was positively correlated with partner crown-white (0.31 [95% CI: 0.18, 0.44] times hour⁻¹ per 1% crown-white), and inversely associated with partner mass and body fat (mass: (−0.42 [95% CI: −0.80, −0.05] times hour⁻¹ per 1 g; body fat: −1.74 [95% CI: −2.58, −0.90] times hour⁻¹ per 1-unit increment in body-fat score). These findings suggest that mate quality is a predictor of food provisioning behavior in males but not females, thus providing some sex-specific support for the differential allocation hypothesis. On the other hand, we did not find conclusive evidence for the good parent or reproductive compensation hypotheses.

While foraging, animals often trade off between food and safety, reducing feeding in response to increased predation risk. This response, however, may not be a viable option for animals that are energetically compromised. Many single-species studies have shown that hungry animals select habitats within which foraging opportunities are greater even if predation pressures are higher, but it is unclear whether these patterns can be extrapolated to entire communities. Here, we examined the stopover habitat use of 28 frugivorous landbird species along the coast of Maine, USA, during an energetically demanding period of the annual cycle, fall migration. Across 6 stopover sites, we determined whether or not a tradeoff existed between using safe habitat patches (patches with high woody plant stem density) and patches with high food resources (patches with high fruit abundance). Controlling for raptor abundance at a site, landbird migrants were captured at higher rates in sites where no tradeoff existed, suggesting that birds avoided staying in sites where there was a predation risk–foraging tradeoff. At all sites, regardless of the presence or absence of a tradeoff, longer-distance migrants used patches with high food availability more frequently than shorter-distance migrants; patch use by shorter-distance migrants was explained by habitat cover alone. Our findings suggest that, for the Gulf of Maine, birds reduce predation risk at the scale of a stopover site, and differences in habitat selection at finer patch scales are mediated by migration strategy.

Although sex biases in survival and dispersal are thought to be linked to avian mating systems, little is known about these demographic patterns in less common mating strategies such as polygynandry. We investigated breeding-site fidelity, natal philopatry, and apparent survival of the polygynandrous Smith's Longspur (Calcarius pictus) over a 7-yr period at 2 areas in Alaska's Brooks Range. We used capture–recapture histories of 243 color-banded adults and 431 juveniles to estimate annual survival and determined dispersal patterns from 34 adults that were found breeding within the study areas over multiple years. Most adults (88%) returned to nest in the same breeding neighborhood as in previous years; mean dispersal distance was 300.9 ± 74.2 m and did not differ between sexes. Juveniles exhibited low natal philopatry; only 4% of banded hatch-year birds were resighted as adults during subsequent years. Those that did return dispersed, on average, 1,674.4 ± 465.8 m from their natal nests (n = 6). Model-averaged survival estimates indicated that annual survival of adult females (50–58%) was only slightly lower than that of males (60–63%); juvenile survival was 41% but was paired with a low (13%) encounter probability. We attribute the lack of sex bias in adult dispersal to this species' polygynandrous mating strategy. Within this system, there are multiple mates within a breeding neighborhood. We argue that natural selection may favor females that remain on the same, familiar breeding site, because they do not have to disperse to a new area to find a suitable mate. Dispersal among breeding populations most likely occurs by juveniles returning as adults. Our findings support hypotheses that suggest a relationship between dispersal and mating strategy and provide some of the first insight into the demographic patterns of a polygynandrous passerine.

Estimating population abundance for territorial species is challenging because individuals often differ in behavior (e.g., transience, multiterritoriality), and thus in detectability. How well prevailing methods detect and quantify individuals using multiple strategies is rarely addressed. In our effort to efficiently quantify avian abundance and transience among diverse nonbreeding habitats, we combined ‘unmarked' (distance sampling) with traditional ‘marked' (territory mapping) survey methods using a migratory passerine, the American Redstart (Setophaga ruticilla), wintering in 3 habitats in Jamaica from 2010 to 2012. We predicted that the ‘unmarked' survey method would detect not only all known (marked) territorial individuals, but also individuals that were transient or nonterritorial in the same habitats. Comparisons of abundance estimates generated by our best distance sampling (unmarked) model with territory mapping data identified high proportions of transient individuals (up to 50%) in 2 habitats, coastal dry scrub and mangrove, and virtually none in the third habitat, higher-elevation wet limestone forest. Documenting so many nonterritorial individuals, disproportionately weighted toward females and yearlings, in some habitat–year combinations prompts questions of what conditions favor transience and what role these individuals play in population processes. Our results illustrate how unmarked and marked survey methods can be used jointly to establish the number and identity of transients from neighboring areas. The unmarked survey method was sufficient for estimating population size among different habitats, but marked survey methods were necessary to identify and quantify transient individuals. Combined, these methods provide a powerful tool for assessing the range and variation of space-use strategies deployed by nonbreeding individuals.

Examples of phenotypic convergence in plumage coloration have been reported in a wide diversity of avian taxonomic groups, yet the underlying evolutionary mechanisms driving this phenomenon have received little scientific inquiry. We document a striking new case of plumage convergence in the Helmeted Woodpecker (Dryocopus galeatus) and explore the possibility of visual mimicry among Atlantic Forest woodpeckers. Our multilocus phylogenetic analyses unequivocally place D. galeatus within Celeus, indicating that the former has subsequently converged in appearance upon the distantly related and syntopic Dryocopus lineatus, to which it bears a remarkable resemblance in plumage coloration and pattern. Although details of the Helmeted Woodpecker's ecology and natural history are only now beginning to emerge, its smaller size and submissive behavior are consistent with predictions derived from evolutionary game-theory models and the hypothesis of interspecific social-dominance mimicry (ISDM). Moreover, estimates of avian visual acuity suggest that size-related mimetic deception is plausible at distances ecologically relevant to Celeus and Dryocopus foraging behavior. In light of our results, we recommend taxonomic transfer of D. galeatus to Celeus and emphasize the need for detailed behavioral studies that examine the social costs and benefits of plumage convergence to explicitly test for ISDM and other forms of mimicry in these Atlantic Forest woodpecker communities. Future field studies examining potential cases of competitive mimicry should also take into account the mimic's acoustic behavior, particularly in the presence of putative model species and other heterospecific competitors, as any discontinuity between morphological and behavioral mimicry would likely preclude the possibility of deception.

In a recent paper, we described a new bird species, Scytalopus gonzagai, and proposed the English name “Bahian Mouse-colored Tapaculo” for it (Maurício et al. 2014). However, this proposal conflicts with general principles of the formation of English names and, thus, we propose the name “Boa Nova Tapaculo” for S. gonzagai.