Adults of several species of western North American passerines are known to migrate to the Mexican monsoon region to undergo molt from July to October before continuing migration to their wintering grounds in the neotropics, but little is known about the biology and habitat requirements of these birds on their molting grounds. Therefore we established 13 banding stations during the monsoon seasons of 2007 and 2008 in southeastern Arizona, central Sonora, and central Sinaloa. We studied the spatial and temporal occurrence of 10 previously known and 9 new species of molt migrants on the molting grounds. In many of these species most or all individuals appeared to undertake molt migration but in others it appeared to be limited to a small proportion of the population, underscoring that molt migration must be defined at the level of the individual rather than of the population. Our results suggest that during the drier 2007 monsoon season molt migrants sought out riparian habitats, whereas in the wetter 2008 season, when the flush of vegetation was greater, they were more widely distributed in drier habitats. Site fidelity to molting grounds was virtually zero, significantly less than site fidelity to banding stations on breeding and winter grounds. Our results suggest that molt migration to the Mexican monsoon region is a stochastic or plastic process, substantially influenced by individual choices related to variation in weather and the preceding breeding season. Our study also emphasizes the need to conserve a mosaic of habitats in the monsoon region appropriate for molting birds.

The global redistribution of pathogens, such as highly pathogenic avian influenza, has renewed interest in the connectivity of continental populations of birds. Populations of the Northern Pintail (Anas acuta) wintering in Japan and California are considered separate from a management perspective. We used data from band recoveries and population genetics to assess the degree of biological independence of these wintering populations. Distributions of recoveries in Russia of Northern Pintails originally banded during winter in North America overlapped with distributions of Northern Pintails banded during winter in Japan. Thus these allopatric wintering populations are partially sympatric during the breeding season. The primary areas of overlap were along the Chukotka and Kamchatka peninsulas in Russia. Furthermore, band recoveries demonstrated dispersal of individuals between wintering populations both from North America to Japan and vice versa. Genetic analyses of samples from both wintering populations showed little evidence of population differentiation. The combination of banding and genetic markers demonstrates that these two continental populations are linked by low levels of dispersal as well as likely interbreeding in eastern Russia. Although the levels of dispersal are inconsequential for population dynamics, the combination of dispersal and interbreeding represents a viable pathway for exchange of genes, diseases, and/or parasites.

Little is known about the movements and habitat selection of California Black Rails (Laterallus jamaicensis coturniculus) in coastal California. We captured 130 Black Rails, of which we radio-marked 48, in tidal marshes in San Francisco Bay during 2005 and 2006. Our objective was to examine their home ranges, movements, and habitat selection to improve the species' conservation. The mean fixed-kernel home range was 0.59 ha, the mean core area was 0.14 ha. Home ranges and core areas did not differ by year or site. Males had significantly larger home ranges and core areas than did females. All sites combined, Black Rails used areas with ≥94% total vegetative cover, with perennial pickleweed (Sarcocornia pacifica) the dominant plant. The rails' habitat selection varied by year and site but not by sex. A multivariate analysis of variance indicated that Black Rails selected areas with pickleweed taller and denser than average, greater cover and height of alkali bulrush (Bolboschoenus maritimus) and common saltgrass (Distichlis spicata), more stems between 20 and 30 cm above the ground, maximum vegetation height, and shorter distance to refugia. On average, Black Rails moved 27.6 ±1.8 (SE) m daily and 38.4 ± 5.5 m during extreme high tides. Understanding the California Black Rail's movements, home range, and habitat use is critical for management to benefit the species.

The globally threatened Sarus Crane (Grus antigone) has low annual productivity and occurs mostly in landscapes dominated by agriculture; it is therefore vulnerable to extinction caused by human-related disturbance and mortality. The Sarus Crane's increased use of rice paddies as breeding habitat has fueled concerns that the species is being forced to use suboptimal habitats. To assess the issue, I studied nest-site selection and quantified nest and brood survival of Sarus Cranes in Uttar Pradesh, northern India, during 2000 and 2001 and evaluated differences between natural wetlands and rice paddies. The cranes preferred wetlands as nesting habitat at the levels of both the landscape and individual territory. The success (daily survival rate) of nests closer to roads was lower, suggesting that human-related mortality played a role. The effect of habitat on nest success was equivocal, suggesting that rice fields per se are not suboptimal as nesting sites. This result is unique to this area, suggesting that favorable attitudes of farmers still allow Sarus Cranes to nest in rice paddies. Broods hatching later and those in territories with fewer wetlands had a lower probability of survival. Vegetation changes and disturbance during crop harvesting likely decreased brood survival. Maintaining a patchwork of shallow wetlands in rice-dominated landscapes and ensuring that farmers retain a positive attitude toward the species are crucial for survival of Sarus Crane nests and broods.

In species of birds with biparental care, each sex may have its own energy requirements and/or schedule for feeding, possibly leading the sexes to differ in foraging strategy. In estuaries, shorebirds such as the Kentish Plover (Charadrius alexandrinus alexandrinus) may forage on intertidal mudflats and in adjacent supratidal habitats during winter as well as during the breeding season. In this study, we analyzed the diet, use of foraging habitat, food-intake rate (biomass ingested per unit time), and time allocated to foraging by male and female Kentish Plovers at both seasons in an estuary near Cádiz, Spain, where intertidal mudflats and adjacent salt works are the main habitats for foraging. The plovers' main prey was the rag worm (Nereis diversicolor), an intertidal polychaete that supplied more than 80% of the biomass consumed at each season. During the breeding season, both sexes increased their intake rate and decreased their daylight foraging time. By increasing the diurnal intake rate during the breeding season, the birds minimized their time spent foraging on the intertidal mudflats, allowing them to maximize the time for activities associated with breeding in the adjacent salt works. Therefore, the plovers solved the conflict between foraging on the mudflats and breeding in the salt works by shortening the foraging time on the mudflats, minimizing time away from the nesting areas. The sexes differed in the daylight time allocated to foraging, with females spending 2 hr less on foraging and concentrating their feeding activity into the central hours of low tide.

Recent studies reveal that hummingbirds' songs are more variable and complex than previously realized. The Wedge-tailed Sabrewing (Campylopterus curvipennis) forms leks composed of small adjacent territories defended individually from intruders. When females visit these territories, males emit long songs composed of structurally complex syllables. Their elaborate acoustic signals are intriguing because most species of lekking hummingbirds studied in detail have relatively simple and stereotyped songs. Here we report song variation among territorial males at one lek over 4 years. Despite variation in syllable composition, cluster analyses classified songs of territorial males into three groups (“song neighborhoods”), and the pattern of clustering was consistent over time. Song neighborhoods were also clustered spatially. The syllabic composition of songs from members of a given neighborhood was relatively constant from year to year, suggesting site and territory fidelity, and introductory syllables were emitted consistently as the signature of a group. Our findings demonstrate the existence (and persistence) of song neighborhoods in a lekking hummingbird. This spatial structure might result from reduced competition among males to occupy a territory for singing, kinship between members of a neighborhood, and/or as a consequence of sexual selection. Therefore, documentation of song neighborhoods should be considered more often in future studies of song variation in hummingbirds that breed in leks.

Despite widespread efforts to avert wildfire by reducing the density of flammable vegetation, little is known about the effects of this practice on the reproductive biology of forest birds. We examined nest-site selection and nest survival of the Black-chinned Hummingbird (Archilochus alexandri) in New Mexico riparian forests treated or not for fuel reduction. In untreated plots the hummingbirds frequently nested in exotic trees such as saltcedar (Tamarix spp.) and Russian olive (Eleagnus angustifolia). Following fuel reduction, they increased use of cottonwood (Populus deltoides ssp. wislizenii) as a nest substrate and nested at greater heights. Though fuel reduction influenced habitat and nest-site selection, it did not immediately affect nest survival. A logistic exposure model containing effects of year and interaction of nest height and substrate explained nest survival better than did other models. Estimates of daily nest-survival rates from this model varied by year from 0.970 (95% CI: 0.949– 0.982) to 0.992 (95% CI: 0.983–0.996), corresponding with period survival rates of 31% (95% CI: 13.7%–50.1%) to 73% (95% CI: 52.1%–85.9%). In addition, in all substrates except saltcedar, nest survival decreased with nest height. Our relatively high nest-survival estimates suggest that the riparian forest along the Middle Rio Grande provides high-quality nesting habitat for this species. Fuel reduction, however, reduces nest-site availability and can lower nest survival by removing potential nest sites in the forest understory, forcing hummingbirds to nest at greater heights where predation risk is higher.

In many passerines, rates of extra-pair paternity are high, but relatively few studies have examined the behaviors females use to obtain extra-pair copulations. We studied extra-pair behavior in females of the Acadian Flycatcher (Empidonax virescens), a species in which 58% of females produce extra-pair young and males often sire offspring on distant territories. We used radiotelemetry to document the extent and frequency of females' forays away from their territories and used behavioral observations and playback experiments to test if females' vocalizations advertise their fertility. No radio-tagged females left their territories (n = 12 females, 105 hr of tracking) even during the hour before dawn. Paternity analyses revealed that at least six of these females produced extrapair young. Focal observations revealed that fertile females spent significantly more time calling “chiff,” gave more calls per hour, and called in longer bouts than did incubating females. Extent of females' chiffing during their fertile period was not related to subsequent extra-pair paternity in their nest (n = 12). We tested for a territory-defense function of chiffing by playing chiff calls to females at both the fertile and incubation stages. Females responded to playbacks on their territory by approaching or flying over the speaker, did not increase their chiffing rate significantly, but did give other high-intensity calls. Our results suggest that extra-pair fertilizations of Acadian Flycatchers occur primarily on a female's own territory and that extra-pair males could use females' vocalizations to locate fertile females.

Over 3 years we observed the breeding behavior of four groups of the Sichuan Jay (Perisoreus internigrans), a poorly known Chinese endemic restricted to fragments of high-altitude spruce—fir forest on the east side of the Qinghai—Tibetan Plateau. The first seven known nests of this species were well hidden in tall conifers at 3350–3700 m above sea level, and clutches were initiated in cold, snowy conditions in late March and early April. Only the presumed breeding males fed incubating females, but all members of a group (mean size 3.8) regularly fed nestlings and fledglings. We contrast this pattern of alloparenting with that seen in the two other species of Perisoreus: alloparenting is restricted to the fledgling period in the Gray Jay (P. canadensis) and entirely absent in the Siberian Jay (P. infaustus). The differences may be explained by differences in the defensive capabilities of the three species and/ or by differences in the abundance and capabilities of their potential nest predators.

Hatching asynchrony results in age and size hierarchies within broods, and the subsequent asymmetric competition among siblings has important consequences for nestlings' fitness. In this study, we compare the growth of Barn Swallow (Hirundo rustica) nestlings in relation to their order of hatching. The aim was to test the prediction that early-hatched nestlings develop differently from late-hatched nestlings, which should be under greater pressure to trade investment in growth in favor of traits important to simultaneous fledging. Early-hatched nestlings were always larger than late-hatched nestlings, but when the age difference was taken into account, the two classes of nestling gained mass and head-bill length in similar ways, including having similar asymptotes, as predicted by nonlinear curve models. For wing length, however, late-hatched nestlings reached the inflection point of growth sooner than early-hatched nestlings, and although scaled rates of wing growth were similar, early-hatched nestlings had significantly longer wings, both before fledging, when the oldest nestling was 14 days old, and as suggested by the asymptotic, age-independent values derived from the nonlinear curve models. This finding suggests that nestlings hatched later preferentially develop body mass and the skeleton at the expense of wing feathers. As swallows rely on their wings for foraging and avoiding predators, this pattern of resource allocation is likely to have negative consequences for the late-hatched nestlings.

In the tropics, passerines are expected to have survival rates higher than those in temperate zones, though their post-fledging survival has rarely been quantified. Furthermore, because of increased care by adults, cooperative breeding species should have even higher rates of juvenile survival, but few data are available on such species also. Over three breeding seasons we examined factors relating to post-fledging survival of the cooperatively breeding Puff-throated Bulbul (Alophoixus pallidus) in an evergreen forest in northeastern Thailand. These factors included the presence of helpers, fledgling age, sex, body mass prior to fledging, date of fledging within a year, and variation by year. The probability of surviving the 8-week period of dependency was 0.61 ± 0.09 (SE), higher than most reports from the temperate zone. Survival probability was strongly associated with age though not with the presence of helpers. Weekly survival was lowest during the first week post-fledging (0.73 ± 0.08), while average weekly survival during the 7 subsequent weeks was constant at 0.97 ± 0.01. The other factors tested appeared to have little or no effect on survival. Understanding factors influencing post-fledgling survivorship provides insights to this limiting life-history stage, which is typically characterized by high mortality and affects population dynamics. The survivorship of fledglings we report adds to the body of evidence that the post-fledgling survivorship of tropical passerines is generally higher than that of temperate-zone species. However, fledglings from cooperative breeders may not experience significantly higher survivorship than those from noncooperatively breeding birds in the same system.

This study examines the relationship of carotenoid-based plumage coloration to mating and reproductive success in a migratory songbird, the American Redstart (Setophaga ruticilla). Adult male redstarts have several highly variable orange carotenoid-based patches of plumage on their wings, flanks, and tail. We tested the prediction that males with larger and more intensely orange plumage patches have higher pairing success, as well as higher within-pair, extra-pair, and total reproductive success. We also quantified nestling-provisioning rates, to determine if carotenoid-based coloration was related to males' provisioning behavior. Males that paired successfully had significantly brighter orange flanks than unmated males. Paternity testing with microsatellites showed that 64% of broods (18 of 28) had extra-pair young and 44% of nestlings (36 of 81) were extra-pair. We found no relationship between carotenoid-based coloration and within- or extra-pair paternity. Contrary to our predictions, males with less saturated orange flanks sired more total young than more saturated males. Males with brighter orange flanks also provisioned nestlings at rates lower than did less bright males. These unexpected results highlight the need for further research on the mechanisms governing the production and maintenance of carotenoid-based signals in redstarts, as well as incorporating the ecology of migratory species into the study of carotenoid-based signals, which heretofore has been based primarily on nonmigratory species.

By affecting the probability of nest predation and brood parasitism, avian nest-site selection has significant implications for reproduction and fitness. Therefore, understanding factors associated with habitat use at the nest-site scale is imperative, especially for species of conservation concern. One such species, the Swainson's Warbler (Limnothlypis swainsonii), is a rare neotropical migrant that breeds mainly in bottomland hardwood forests throughout the southeastern United States, but relatively little is known about its nesting habitat. From 2004 through 2007, we studied nest-site selection of Swainson's Warblers at two study areas in eastern Arkansas. We found that, relative to paired random plots, nest sites were characterized by dense understory vegetation, high total canopy cover, abundant leaf litter, and high density of woody stems, especially of giant cane (Arundinaria gigantea), a once-abundant bamboo native to the southeastern U.S. Indeed, most nests (90%) were placed in giant cane either exclusively or in combination with other nest substrates. However, understory vegetation density and total canopy cover were the best predictors of nest sites. We suggest that management for Swainson's Warbler nesting habitat should focus on providing forests with uniformly dense understory vegetation and well-developed structure of the canopy and subcanopy. When possible, conservation efforts should focus on maintaining, enhancing, or restoring dense cane thickets.

A phenomenon currently not well understood is the reproduction of some species in clustered territories. We evaluated two ecological hypotheses (material-resources and predation hypotheses) that could explain cluster formation during reproduction of the Blue-black Grassquit (Volatinia jacarina), a socially monogamous bird with a high rate of extra-pair fertilization. Additionally, we considered the breeding synchrony of females, since synchronization may serve as a means of diluting predation. To this end, we compared the breeding synchrony of females in clusters to that of those in solitary territories. We found support for the material-resources hypothesis in terms of vegetation structure and food availability: territories were clustered in habitat with more food and a more complex vegetation structure. Breeding synchrony of females in clusters was higher than that of females in solitary territories, but probably not as a mechanism to dilute predation: pairs within clusters suffered lower reproductive success because of elevated predation. The advantages gained by reproducing in clusters in areas of more complex vegetation structure and more food may offset the disadvantages due to predation, especially over a longer temporal scale. Our results suggest that the aggregation of Blue-black Grassquit territories could be explained by habitat heterogeneity in terms of vegetation and food, but these results do not exclude other hypotheses related to sexual selection and/or competition leading to aggregation.

Young, immature-plumaged males of species in which plumage maturation is delayed are often unable to obtain mates or pair later in the season to lower-quality females. Their clutches are smaller and the quality of their eggs is lower, leading to fewer nestlings, whose quality and fledging success are reduced. The Saffron Finch (Sicalis flaveola) is an understudied and declining secondary-cavity nester with delayed plumage maturation; yearling males look like females. The aims of this study, of birds nesting in boxes, were to compare the morphology of immature- and mature-plumaged males of the Saffron Finch and their respective females and to compare the breeding success of females mated to each category of male. We expected that females mated to immature-plumaged males would follow the pattern described for other species, having lower reproductive investment and success. Immature- and mature-plumaged males were similar in morphology, but females paired to mature-plumaged males were bigger in bill height and weight. We found no differences, however, between females paired to each category of male in any biologically meaningful variable. Reduced competition among males for nest cavities and enough females willing to pair with immature-plumaged males might be partially responsible for these results.

At the northern periphery of its range Brewer's Sparrow (Spizella breweri breweri) is in decline and breeds in small clusters within larger areas of suitable habitat. Clustered breeding that is unrelated to the distribution of resources may be explained by social attraction (conspecific cueing). We used a song-playback experiment to test the conspecific-cueing hypothesis in this species. The experiment was conducted during the spring settlement period in habitat that appeared physically suitable for breeding but had not been occupied during die previous two breeding seasons. Treatments were split between two periods that reflected peak settlement of experienced and first-time breeders. In both periods, more Brewer's Sparrows visited and established territories in treatment plots than in untreated control plots. There were not, however, more treatment than control plots containing breeding pairs. This difference could mean that males attracted to playbacks are of lower quality than males in established breeding clusters and thus less attractive to females, that females settle only in groups of males larger than some threshold, or that females' site fidelity is higher than that of males. These results lend support to the conspecific-cueing hypothesis in this species, indicating that social attraction may play a role in Brewer's Sparrow's habitat selection. They also suggest that traditional habitat models, which consider only resource distributions and not social factors, may be inadequate tools for the conservation of this and other species.

Fall arrival is an important period in the life history of migratory birds because processes during this period determine where birds spend the winter, which is linked to subsequent survival and condition at the time of spring migration. Henslow's Sparrows (Ammodramus henslowii) winter in savannas of longleaf pine (Pinus palustris), but their secretive behavior limits our knowledge of their winter ecology, including behavioral processes that lead to their documented preference of an ephemeral habitat—recently burned savannas. We expected that upon arrival Henslow's Sparrows actively seek out recently burned savannas and that their over-winter location is dependent on intraspecific interactions during the arrival period. We conducted a 2-year mark—recapture study in southeastern Louisiana longleaf pine savannas to examine these predictions. Bird densities were highest in savannas burned during the previous growing season. There was a large turnover of transient individuals in October and early November, but the proportion of dispersing birds was not related to the number of years since fire or to the bird's age or sex. Early-season movements did not result in skewed age or sex distributions associated with years since fire, suggesting the lack of a class-mediated competitive hierarchy. Birds were then site faithful from late November through spring departure, which began in March and lasted through mid April, with males departing first.

The epaulets of male Red-winged Blackbirds (Agelaius phoeniceus) function in both intersexual and intrasexual contexts, but there is little evidence that they covary with reproductive success in this well-studied species. We used path analysis of male/territory traits, mating success, and reproductive success of unmanipulated males to estimate current directional selection. Territory size had a positive effect on number of within-pair mates. Number of extra-pair mates had a positive effect on number of extra-pair fledglings, and number of within-pair and extra-pair fledglings had positive effects on total number of fledglings. We also reddened epaulets of free-living territorial males to determine whether manipulated and control males differ in territorial behavior, mating success, or reproductive success. Compared with control males, males with reddened epaulets incurred elevated rates of trespassing and territorial challenges, lost their territories more often, and were unable to produce extra-pair offspring. Despite these differences, however, the realized reproductive success of experimental and control males did not differ significantly, perhaps because males with reddened epaulets devoted more time to anti-predator vigilance and were more aggressive toward a simulated predator than were control males. An apparent lack of current sexual selection on epaulet color may be a form of counter-balancing sexual selection in which male aggression against redder epaulets opposes female preference for redder epaulets, or it might be the result of males compensating for lost extra-pair fertilizations by increasing their parental care.

Proximately, clutch size is determined by the termination of the sequential pattern of egg formation and laying. Egg laying is difficult to study, and documentation of detailed patterns is scarce. We used archived video recordings of the Eastern Bluebird (Sialia sialis) to contrast the times of day of laying with variability in egg-laying intervals. Ultimate eggs were laid over a significantly longer interval than previous eggs. The extended interval over which the ultimate egg is laid could be a symptom of physiological constraints in the formation of the ultimate egg, extra resources provided to compensate the ultimate egg, and/or a late egg may become the ultimate egg because being late prevents further ovulation.

Riparian forests are thought to be important habitat for fledglings in a diversity of temperate-forest passerines, yet few studies have examined this hypothesis. The main objective of this study, conducted in temperate deciduous forests in Michigan's Lower Peninsula, was to compare bird use of riparian and upland forests during and after breeding. Using mist-nets, we quantified populations in accordance with the Monitoring Avian Productivity and Survivorship (MAPS) protocol. Nets were set up in both riparian and upland forests in a paired sampling design. After fledging, we captured juveniles, including those of the Ovenbird (Seiurus aurocapilla), at a rate higher in riparian than in upland forests. The trend for adult Ovenbirds was similar. For riparian breeders, we found no difference in capture rates over time between upland and riparian forests; these species appeared to stay in riparian forests after fledging. Our results suggest that riparian forests are important habitat for passerines during the period following fledging; they need to be considered accordingly during conservation planning.