The role of olfaction in birds is poorly understood, in part because of our limited knowledge of signal-transmission mechanisms. Here we evaluate the hypothesis that the secretions of the uropygial gland serve as olfactory signals in birds by testing the prediction that size of the olfactory bulb, a proxy for olfactory ability, covaries positively with the size of the uropygial gland's tuft, a circlet of feathers whose size varies extraordinarily from species to species. The function of this tuft has remained a mystery, but mechanical or protective roles are unlikely on the basis that these feathers are downy and always saturated with gland secretion. These observations instead suggest that the tuft may be involved in trapping the compounds produced by the gland's secretions to facilitate conspecifics' perception of odor. We therefore predicted that the uropygial gland's tuft should be more developed in birds with better capacity to smell. Using a dataset of 29 species of birds of 20 families of nonpasserines, we show that the sizes of the tuft (relative to the uropygial gland) and olfactory bulb (relative to the cerebral hemisphere and body mass) are positively correlated after the confounding effects of colonial breeding and phylogeny are controlled for. This suggests that the tuft may have evolved because of the adaptive benefits of enhancing the transmission of body odors. Additionally, colonial species have tufts larger than those of solitary species, as expected because sociality increases encounter rates and the prevalence of odor-producing bacteria.

Wind turbines provide a source of renewable energy to meet increasing human demand and offset the costs of fossil fuel usage and nuclear power generation. Birds are killed and displaced at wind facilities, so increased understanding of the drivers of mortality and displacement will assist planners considering the future placement and use of wind facilities. Our objectives were to assess the effect on birds of a wind facility in southeastern Wisconsin by (1) recording the species composition of recovered bird carcasses, (2) estimating mortality rates, and (3) identifying variables correlated with fatalities. We found 20 bird carcasses during scheduled searches. On this basis, we estimated that over two springs and two autumns of study from 2008 to 2010, 607 birds (0.026 per turbine per day, 0.017 per megawatt per day) were killed over 277 days of searching at this facility containing 86 turbines. Nocturnally migrating passerines accounted for 50% of the birds found killed. We found a significant negative relationship between bird fatalities and northward movement of birds through the wind facility. Despite the close proximity of Horicon Marsh National Wildlife Refuge, a wetland of international importance, we found no relationship between distance to Horicon Marsh and bird fatalities. Our study provides a timely assessment of fatal bird collisions with turbines at a wind facility in agricultural lands, uniquely located near a large wetland at which migrating birds stage.

California oak savanna is a habitat of sparse tree canopy that extends from northern Baja California to southern British Columbia and is under threat from land-use pressures such as conversion to agriculture, overgrazing, urban development, and fire suppression. Bird-conservation plans have been drafted for the region's oak woodlands. Yet it is unclear whether birds use California oak savanna at different frequencies than they do neighboring oak habitats. In the foothills of the central and northern Sierra Nevada, California, we explored patterns of avian community structure and habitat occupancy in four habitats: blue oak (Quercus douglasii) savanna with a well-developed grass and forb layer, blue oak savanna with a well-developed shrub layer, and two habitats with a denser canopy, blue oak woodland, and montane hardwood. Additionally, we assessed the effect of habitat characteristics on avian community structure and occupancy. Avian communities were uniquely grouped among the four habitats. Five species of management and conservation concern—the Western Kingbird (Tyrannus verticalis), Western Bluebird (Sialia mexicana), Lark Sparrow (Chondestes grammacus), Western Meadowlark (Sturnella neglecta), and Bullock's Oriole (Icterus bullockii)—were predicted to occupy oak savanna habitats at frequencies higher than in oak woodland or montane hardwood. Shrub cover was the most influential habitat characteristic shaping the avian community and was negatively associated with occupancy of the five savanna-affiliated birds. The distinctive structure and occupancy patterns observed for species of concern in California oak savanna suggest that birds perceive this as unique habitat, highlighting the need for its conservation.

Pleistocene refugia likely contributed to the modern biodiversity of northern areas. Using the mitochondrial DNA cytochrome-b gene, we compared 11 forest-dwelling bird species from Haida Gwaii (formerly the Queen Charlotte Islands) with populations from Alaska, Washington, and other locations in the United States. If Haida Gwaii was an unglaciated refugium, its modern populations should feature a high number of endemic lineages and divergence times that predate the end of the last glacial maximum, ca. 13,000–19,000 years before present (ybp). Furthermore, the genetic diversity of these populations should be higher than that in areas colonized after the glacial retreat. Four of the species examined from Haida Gwaii showed old divergences and a high percentage of endemic lineages: the Northern Saw-whet Owl (Aegolius acadicus), Hairy Woodpecker (Picoides villosus), Steller's Jay (Cyanocitta stelleri), and Pine Grosbeak (Pinícola enucleator); all four have endemic subspecies on these islands. The Pacific Wren (Troglodytes pacificus) and Song Sparrow (Melospiza melodia) showed genetic trends associated with populations in refugia, including high genetic diversity on Haida Gwaii. Estimated divergence dates of these six species were fairly uniform (∼20,000–30,000 ybp), being greatest for the Hairy Woodpecker (>70,000 ybp) and Pine Grosbeak (>120,000 ybp). There was an association between apparent occurrence in a refugium and a sedentary lifehistory strategy and a trend for endemic subspecies (4 of 6) also to show this association. Our findings suggest that the Haida Gwaii area hosted a forested refugium during the cycles of climatic change in the late Pleistocene.

In migratory birds, population-genetic structure is generally low, but philopatric species can have fine-scale patterns of differentiation. We investigated the population-genetic structure of the Canada Goose (Branta canadensis) across multiple spatial scales to determine whether genetic data support current delineations of subspecies and populations for management. We collected samples from two subspecies (B. c. interior and B. c.maxima) and four management populations across Ontario and Quebec. Using 7 microsatellites and 442 base pairs of mtDNA we found that genetic structure varied with scale. F_ST revealed low levels of genetic differentiation between subspecies and management populations, and individual-based clustering revealed no genetic differentiation. However, fine-scale spatial autocorrelation revealed significant levels of relatedness at distances <85 km. The lack of clear genetic structure may reflect recent human management. That our fine-scale analysis revealed significant genetic relationships suggests that genetic structure may increase and in time reflect that revealed by banding data. As our markers were unable to accurately distinguish between subspecies they will be of little use in estimating subspecific contribution to harvested stock. Alternative molecular markers under selective pressure may be more informative in assess targets for harvest.

Along the Pacific coast of North America, White-winged Scoters (Melanitta fusca) winter in sites that vary widely in habitat conditions. This habitat variation likely alters the relative costs and benefits of using specific sites in terms of factors such as foraging conditions, degree of predation danger, and thermoregulatory costs. To assess how White-winged Scoters adjust their physiology and energy management in response to variation in habitat conditions, we contrasted overwinter dynamics in several physiological indicators across five sites in British Columbia and Washington. We tested the relative support for various hypotheses that considered exposure to wind and waves, water depth, predation danger, diet composition, and collection period as possible underlying causes of variation in physiological indicators. Total body mass and lipid mass were highest at an exposed offshore site, which may reflect an adaptive response to buffer against unpredictable foraging conditions and increased thermoregulatory costs. At nearshore sites where exposure was lower, scoters maintained lower lipid masses throughout the winter. Total lipid mass declined through the winter in all sites, a result consistent with findings for many waterfowl species. However, levels of plasma metabolites (triglycerides, β-hydroxybutyrate) varied little by site, suggesting that, irrespective of body composition, birds at all sites maintained physiological homeostasis. Digestive morphology was strongly related to diet, with smaller gizzards and longer intestines associated with a greater fraction of soft-bodied foods in the diet. Our results suggest that the physiology and energy management of wintering White-winged Scoters is related to local habitat conditions.

We quantified the duration and phenology of remigial molt of Barrow's Goldeneyes (Bucephala islandica) in northwestern Alberta, Canada. We estimated that the remiges' average (± SE) growth rate was 3.94 ± 0.13 mm day^-1, slightly slower than that of most waterfowl. Barrow's Goldeneyes regained flight with the ninth primary 77% grown, a percentage similar to or greater than that of most waterfowl. By several metrics, remigial molt of Barrow's Goldeneye was longer than that of most waterfowl. We estimated that it took 6.5 ± 1.2 days for a new ninth primary to become visible once the old primary was dropped (pre-emergence interval). The periods in which males and females were flightless were 30 ± 0.4 and 28 ± 0.5 days, respectively, and 36.5 ± 0.5 and 34.5 ± 0.8 days, respectively, including the pre-emergence interval. Complete maturation of primaries after emergence took 39 ± 0.5 and 36 ± 0.7 days for males and females, respectively, and 45.5 ± 0.6 and 42.5 ± 0.9 days, respectively, including the pre-emergence interval. These results suggest a lack of strong selective pressure to reduce the duration of remigial molt in this species. Initiated over a range of nearly 2 months, remigial molt was asynchronous both between and within age and sex cohorts, suggesting a lack of strong temporal optima for remigial molt of Barrow's Goldeneyes at our study sites.

Monitoring of birds often requires the use of very-high-frequency radios or satellite telemetry to enhance detectability of individuals. An assumption implicit in such studies is that radio-marked individuals are representative of the population at whole, which requires that radios do not influence an individual's behavior or demographics. We present results from a capture—mark—recapture study of male Greater Sage-Grouse (Centrocercus urophasianus), some radio collared, others only banded, in an experimental framework to assess whether radio collars influenced the birds' behavior or survival. We generated encounter histories of 906 male Greater Sage-Grouse and used a robust-design framework in program Mark to estimate probabilities of annual survival, detection, and temporary emigration from the lek for the radio-collared and banded segments of the sample population. Results of models suggested that seasonal detection rates at leks the year after capture were 3–5 times higher for males only banded than for males equipped with radio collars. These results also suggested a possible negative influence of radio collars on males' annual survival and annual lek attendance. We suggest researchers should exercise caution when designing studies or analyzing data that rely on radio-collared male Greater Sage-Grouse.

To understand how animals cope with environmental variability it is necessary to identify the degree of flexibility in a species' diet and foraging mode and the consequences of this flexibility for reproduction. We examined rates of feeding and energy delivery to chicks by a long-lived pelagic seabird, the Short-tailed Shearwater (Puffinus tenuirostris). Individual adults alternated between foraging trips of short and long duration in a dual foraging strategy, but the allocation of time on those trips varied significantly from year to year. In two years when sea-surface temperatures of feeding grounds exploited during short trips were cooler (2005, 2006) adults initially fed their chick more often, then feeding decreased through the chick-rearing period. In the following year of warmer sea-surface temperature (2007), the number of feedings per day was initially low but increased through chick rearing. Despite varied feeding patterns, breeding success was consistently high, yet in 2006 the chicks' poor condition indicates the capacity for buffering chicks from these effects was lower than in other years. The relative contribution of short and long trips to the amount of energy delivered to chicks also varied by year. During local food shortages, shearwaters appeared to deliver more oil from long trips and increased the frequency of short trips. Yet in 2006, low-calorie prey from short trips coincided with low volume of stomach oil from long trips, resulting in chicks' poorer condition. Oil volume and increased short-trip foraging provide potential mechanisms of flexibility enabling adults to buffer prey delivery to chicks during food shortages.

Interest in monitoring the population viability of the Brown Pelican (Pelecanus occidentalis) has recently risen in the context of the species' 2009 delisting as endangered, rapid degradation of nesting habitat, and recent oil spills. To assess the Brown Pelican's patterns of movement (across natal colony, nonnatal colony, and noncolony islands), age and sex structure, and survival probabilities, we banded 1177 chicks in Louisiana from 2007 to 2009. In band-resighting surveys within the Isles Dernieres archipelago from 2008 to 2010, we detected 92 of our banded birds. Neither age nor sex appeared to influence where we observed pelicans resting on beaches across the islands, and we found the highest proportions of pelicans at their natal island. Yet few observations of banded birds suggest either movement outside our study area or mortality. Conditions at colonies and proximity to other sites of loafing or colonies may in part explain the disparity in proportions of resightings of individuals banded on different islands. Finally, the apparent probability of survival of one-year-old pelicans was lower than that of two- and three-year olds. Insights into these trends in movement and survival of young Brown Pelicans can improve future management of colony sites.

We documented the diet of the Black Oystercatcher (Haematopus bachmani) on Haida Gwaii, British Columbia, Canada, (2004–2010) with prey remains from 99 territories in Gwaii Haanas National Park Reserve, National Marine Conservation Area Reserve and Haida Heritage Site. The ranking of its four main prey types did not vary annually. Numerically, the prey comprised 50–75% limpets, 14–34% mussels, 8–18% chitons, 1–2% Northern Abalone (Haliotis kamtschatkana), and <1% other species. In 2009, we estimated prey availability and prey preference by Ivlev's electivity index. Northern Abalone, limpets, and chitons were highly preferred; mussels, turban snails, and barnacles were taken in proportion to their occurrence or avoided. Black Oystercatchers preferred abalone of 50 mm, smaller than the mean size available, in contrast to the selection of larger-than-average prey, typical for other prey species. In 2010, 52% of nesting territories sampled contained remains of Northern Abalone, despite that species' small contribution to the diet. The Northern Abalone has never been reported as prey of the Black Oystercatcher despite its high vulnerability to predation at low tides. We speculate that the recent inclusion of the Northern Abalone in the Black Oystercatcher diet on Haida Gwaii may indicate a greater abundance of Northern Abalone than in other regions of its distribution. The frequency of the oystercatcher's feeding on abalone was unexpected because under Canada's Species at Risk Act, the Northern Abalone was listed as “endangered” in 2010, after the population continued to decline after legal protection from harvest in 1990.

Shorebirds are exceptionally diverse in their social mating systems, and genetic analyses of a few species have revealed further variation in genetic mating systems. The ecological and evolutionary causes of this variation are largely unknown. We studied the mating system of American Golden-Plovers (Pluvialis dominica) at Barrow, Alaska, between 2004 and 2009. In this arctic population, birds bred in socially monogamous pairs, as has been seen in other populations, yet rates of philopatry and mate fidelity were low. We used microsatellite DNA markers to analyze parentage and found that most offspring originated from monogamous matings, but 8% of 131 offspring and 16% of 37 nests were the result of nonmonogamous matings. One nest was ambiguous; it was either a full clutch sired through extra-pair copulation or represented an instance of social polygyny. Despite wide variation in synchrony of breeding and density of nesting both within and among breeding seasons, these ecological variables were unrelated to mating patterns. We speculate that the low rate of pair fidelity and the high rate of extrapair paternity are linked, because the lack of familiarity between partners reduces benefits of monogamy. Further study of breeding behavior and rates of philopatry are needed to understand why, among socially monogamous shorebirds, the American Golden-Plover has high rates of extra-pair paternity.

Although much of the Arctic Coastal Plain has remained undeveloped, oil and gas industries, new and expanding villages, as well as tourism are likely to increase in the near future. One potential effect of increased human development is increased anthropogenic waste and the need to dispose of this waste in landfills. We investigated potential indirect effects of the North Slope Borough landfill on breeding shorebirds by examining changes in environmental conditions (predator densities and timing of snow melt) and measures of shorebird reproduction (nest-initiation dates, nest density, nest survival, and return rates) in relation to construction and deposition of waste in the landfill. This study included one year of pre-construction data (2004), three years when landfill roads and fences were being constructed (2005–2007), and five years when waste was being deposited (2008–2012). We monitored 364 shorebird nests within a 36-ha plot (approximately half of which was inside the landfill and half outside). Construction of a fence around the landfill reduced snow levels inside the landfill, leading to earlier snow melt and likely to shorebirds initiating nests earlier. Densities of avian predators increased following waste deposition, but nest densities, nest survival, and return rates were generally greater inside the landfill than outside in all years after landfill construction. Our results indicate that fences placed around landfills and procedures to reduce attraction of predators to landfills can minimize indirect negative effects of landfill construction and operation and even favor species breeding in the area.

The chronology of birds' breeding has traditionally been thought to depend strongly on latitude, although it can be rather uneven among populations in close proximity. I estimated the extent of the breeding season in a population of the Red-necked Nightjar (Caprimulgus ruficollis) over three years in southwestern Spain. Data from brood patch development and appearance of recently fledged young provided evidence for a breeding season unusually extended (110 days) for a long-distance migrant. Flexible timing in reproduction appeared to affect individual's departure in migration. Although most adult nightjars gradually left the study area beginning in mid August, some late breeders left the area about a month later than the earliest migrants. Decreasing competition for food and free nesting territories, together with a food supply and predation pressure constant through the season, resemble conditions in the tropics and enable the nightjars to breed over an extended period despite inhabiting a temperate region. The nightjars' phenology was further expanded by a protracted period (≥35 days) of parental care, which also led to later molt prior to autumn migration. However, birds partially compensated for time costs to adjust to the annual cycle by initiating a simultaneous shedding of flight feathers immediately after hatching. In accordance with current studies showing how breeding events carry over into the annual routines of birds, results from this nightjar population come on top of recent hypotheses regarding new ways in which breeding, molt and migration could interact and condition each other.

Seasonal variation in food supply may not adequately explain avian breeding phenology in tropical areas with relatively stable climates. In northeastern Thailand the Red-headed Trogon (Harpactes erythrocephalus) and Orange-breasted Trogon (H. oreskios) share similar nest sites, diets, foraging habitats, and foraging techniques but differ in timing of reproduction even though their food supply of insects varies in a similar seasonal pattern, peaking in June for both species. For the Red-headed Trogon, egg laying lasts 5–6 months and peaks in May; nestling provisioning coincides with peak food availability. For the Orange-breasted Trogon, by contrast, egg laying lasts 2–3 months and peaks in February. This nesting period is shorter and earlier than that of most other species of birds at our study site, and nestling provisioning preceded the period of peak food by 4 months. Timing of breeding by the Orange-breasted Trogon may represent a compromise between breeding at the optimal time based on food resources and avoiding competition for nest sites from the larger Red-headed Trogon.

Cooperative breeding and complex social systems are known in woodpeckers, but most available information concerns species of the North Temperate Zone. The social behavior and breeding ecology of tropical woodpeckers are poorly documented. Here we present data on the breeding behavior and ecology of a tropical woodpecker, the Campo Flicker (Colaptes campestris campestris). We confirm for the first time, through behavioral observations, facultative cooperative breeding in this species. Both males and females were observed visiting the nest during the incubation and nestling period. In cooperative groups some helpers were identified as young from the previous year, but others seem to be unrelated individuals that joined the groups. In addition, our behavioral observations strongly suggest both polygyny (a male breeding with more than one female simultaneously) and joint nesting (multiple females contributing eggs to a single nest) in some but not all cooperative groups. These observations reveal a complex social system in this tropical species, with high potential for reproductive conflicts both within and among groups, due to the presence of new potential breeders or through advantages emerging from the effects of group size.

Understanding the evolution of cooperative breeding systems remains a central issue in evolutionary biology. In this context, molecular tools have contributed greatly toward understanding kinship patterns within groups and resolving long-standing issues regarding mating systems and parentage. We used such molecular tools to examine patterns of kinship in the Campo Flicker (Colaptes campestris campestris), a facultative cooperative breeder, to answer questions concerning its mating system and patterns of reproductive skew. Genetic analysis revealed that the species is predominantly monogamous in both cooperative groups and socially monogamous pairs, but in several cooperative groups auxiliary females contributed eggs to the nest. In contrast, within groups direct reproduction by auxiliary males was rare. We observed no cases of extra-group paternity but detected cases of quasi-parasitism in socially monogamous pairs. Levels of relatedness within groups were high; auxiliaries of both sexes were often offspring or siblings of the primary breeding pair but were sometimes unrelated to the breeding pair. The structural genetic complexity of groups and high intragroup relatedness generate a social system in which cooperation among group members coexists with competition for opportunities to breed.

Oaks (Quercus spp.) rely on vertebrates for seed dispersal. However, the extent to which dispersal is facilitated depends on the vertebrate species involved, its preferences for acorns of different species, and the relative availability of acorns. We examined foraging preferences by free-ranging individuals of four species of birds for acorns of white (Q. alba), black (Q. velutina), and pin (Q. palustris) oak. We photographed selection of acorns at feeding platforms and used discrete-choice models to estimate preferences. Blue Jays (Cyanocitta cristata) had strong and consistent preferences in the order pin oak > black oak > white oak, whereas Red-bellied Woodpeckers (Melanerpes carolinus) exhibited the same but weaker preferences. On the basis of acorn's chemical and physical attributes, these birds appear to focus on maximizing the net value of cached food. Tufted Titmice (Baeolophus bicolor) and White-breasted Nuthatches (Sitta carolinensis) preferred acorns of both pin and white oak over those of black oak. These smaller birds may be limited in their ability to crack harder shells. Our analysis also suggests that competition for limited food resources can lead to instances in which the dominant food item selected is not the most preferred. Ecological studies of oak dispersal and recruitment should consider the role of food preferences by avian assemblages in conjunction with spatial and temporal variation in acorn availability.

The song of the Canyon Wren (Catherpes mexicanus) is widely recognized but poorly studied. Here we present the first comprehensive description of Canyon Wren song, including data from a focused study in northern Colorado and data from songs recorded across the species' geographic range. Both male and female Canyon Wrens produce a stereotyped contact call and sex-specific song. Females sing infrequently, using a single song type. Males typically sing five song types, all consisting of a descending cascade of notes to which they variably append up to seven broadband notes. Songs are delivered with eventual variety in bouts that include an average of 4.6 repetitions of one song type, with temporal breaks between bouts. In the population we studied most song types were widely shared and males' repertoires overlapped by 94%. Some song types are apparently geographically restricted, while others are sung across large areas of the species' range. Overall, our results suggest that the Canyon Wren's song repertoires are relatively small compared with those of other wrens and that overlap of individuals' repertoires is high. Local and rangewide stability of certain song types may reflect high fidelity of copying during learning, this stability may be favored by ecological and life-history traits including low density of territories, lack of migration, and long-term monogamy.

Vegetation characteristics affect avian reproductive success for a variety of reasons, including predators, nesting sites, song perches, and food availability. We investigated the relationship between habitat composition and prey availability and the effect these variables have on reproductive success in the Golden-cheeked Warbler (Setophaga chrysoparia). Our objectives were to determine differences in pairing and fledging success of Golden-cheeked Warbler territories in two distinctive vegetation types and to explore the relationship between reproductive success and tree species composition, arthropod density, and foraging effort. We sampled in 2009 and 2010 at Fort Hood, north-central Texas, within 347 territories of two vegetation types: post oak (Quercus stellata) habitat and Texas oak (Q. buckleyi) habitat. Pairing and fledging success of territories was significantly higher in Texas oak habitat. The birds' rate of movement was considerably higher in post oak habitat, indicating a difference in prey-encounter rate. Golden-cheeked Warblers clearly switched substrates on which they foraged from oaks in April to juniper in May. Arthropod sampling revealed a correlation between preferred foraging substrates and arthropod density. Our study of foraging indicates that the interplay between tree species and the arthropod communities they support is a crucial element driving the Golden-cheeked Warbler's reproductive success.

Breeding birds must choose habitat at a multiple scales, from landscapes to nest sites. It is important to understand how birds make these decisions and the relationships between these choices and breeding success. Over 3 years, we studied nest-site selection of the Ringed Warbling-Finch (Poospiza torquata) in the semiarid Chaco of Argentina at the scales of the landscape, nest patch, and nest site. At the landscape scale, Ringed Warbling-Finches established nesting territories in shrub-grassland exclusively. Within shrub-grassland, nest patches and nest sites selected differed in few attributes of vegetation cover from patches and sites that were available, and those differences varied from year to year. Only in the year when brood survival and breeding success were lowest did features of sites of successful and depredated nests differ clearly. The Ringed Warbling-Finch selects features of nesting habitat at the landscape, nest-patch, and nest-site scales. Implications of choices at the scales of nest patch and nest site appear to vary by year, but the presence of shrubs, particularly Geoffroea decorticans and Ziziphus mistol, was most frequently important. The species appears to be plastic in its selection of nest patches and nest sites, perhaps because these choices do not lead to consistent selective advantages.

Although plumage coloration has been widely used as a model in studies of the evolution of signaling, the role of plumage in intrasexual communication has been less studied, especially in cases of the structural plumage color. The aim of our study was to use choice trials and plumage manipulation to test whether or not the structural blue color of the male Blue Grosbeak (Passerina caerulea) works as a signal of social status. Additionally, we tested whether or not females exploit signals used in aggression between males. If they do, it suggests that plumage coloration is also used in mate choice. In 2007, we captured males and females from two different populations. We performed two choice experiments by manipulating plumage color: (1) a status-signaling experiment, in which a male had to choose one of two other males, and (2) a mate-choice experiment, in which a female had to choose one of two males. Males clearly foraged near less blue birds during their first encounter and on the first day. However, this preference was not evident on subsequent days. Females showed no preference for any color of male. Our data show that plumage color works as a “first impression” signal, which suggests that plumage coloration in the Blue Grosbeak represents a signal of dominance status between unfamiliar individuals and thus supports the status-signaling hypothesis. Our results support previous evidence for a lack of selection of males by females, which for this species thus rebuts the armament-ornament model of sexual selection.

Hosts of brood parasites may have not developed antiparasitic defenses either because host and parasite are recently sympatric or because costs of potential defenses outweigh their benefits. We studied antiparasitic defenses of the Brown-and-yellow Marshbird (Pseudoleistes virescens) against the Shiny Cowbird (Molothrus bonariensis), evaluating recognition and aggression toward female and male Shiny Cowbirds, estimating rates of rejection of cowbird eggs of different colors, and testing the effect of the size of parasite eggs on host rejection. We also observed and video-recorded host nests to estimate hosts' nest attentiveness, frequency of cowbird visits, and interactions between marshbirds and cowbirds. When marshbirds were faced with dummy models, they attacked first and more intensively those of cowbirds (both sexes) than those of a control species. Frequency of egg ejection increased as differences between cowbird and spotted marshbird eggs increased (immaculate > intermediate > spotted), and spotted eggs were ejected more frequently when laid before than during or after the hosts' laying. Marshbirds ejected artificially added immaculate eggs independently of their size. Cowbirds visited marshbird nests only at the egg stage. Hosts' nest attention was low during egg laying and increased during incubation and after hatching, but aggressiveness against cowbirds was always high. Nest defense was inefficient, as losses due to egg pecking by cowbirds were high. Ejection of cowbird eggs avoided the cost of lower survival of marshbird nestlings in highly parasitized nests. As this defense is cost-free, this small benefit would be sufficient to select for the evolutionary maintenance of egg ejection.

We tested whether the expression of melanin-based plumage traits reflects their bearer's levels of circulating antioxidants by repeatedly injected molting juvenile male House Sparrows (Passer domesticus) with a low or high dose of a substance (DL-buthionine-S,R-sulfoximine, hereafter BSO) that reduces the levels of cellular glutathione, a key antioxidant that inhibits melanin production. We predicted that birds with progressively lower glutathione levels would produce a progressively larger bib, a melanin-based trait present in males and related to their competitive ability. These injections were combined with dietary calcium supplements because we had found in a previous study that dietary calcium had a negative effect upon bib size in this species, perhaps because calcium level and glutathione level covary. We found that bib size was negatively related to dietary calcium but was unaffected by BSO injections. We also found no effect of BSO injections on the reflectance of bib feathers, although there was an interactive effect with dietary calcium, and no effect upon the size or reflectance of structurally based plumage traits. Bib size was not related to body condition at the time the bib was first being produced, nor was it related to the levels of circulating glutathione or total antioxidants. Evidence suggested that BSO injections reduced glutathione but that the effects were short-lived, possibly explaining why we found no effect on bib size. In sum, we found no evidence that bib size in House Sparrows is related to glutathione levels but confirmed that it is negatively related to dietary calcium.