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The strain of West Nile virus (WNV) currently epidemic in North America contains a genetic mutation elevating its virulence in birds, especially species in the family Corvidae. Although dead American Crows (Corvus brachyrhynchos) have been the hallmark of the epidemic, the overall impact of WNV on North America’s avifauna remains poorly understood and has not been addressed thoroughly in California. Here, we evaluate variation by species in the effect of WNV on California birds from 2004 to 2007 by using (1) seroprevalence in free-ranging birds, (2) percentage of carcasses of each species reported by the public that tested positive for WNV, (3) mortality determined from experimental infections, and (4) population declines detected by trend analysis of Breeding Bird Survey (BBS) data. Using Bayesian linear models, we extrapolate trends in BBS data from 1980–2003 (pre-WNV) to 2004–2007 (post-WNV). We attribute signifcant declines from expected abundance trends in areas supporting epiornitics to WNV transmission. We combine risk assessed from each of the four data sets to generate an overall score describing WNV risk by species. The susceptibility of California avifauna to WNV varies widely, with overall risk scores ranging from low for the refractory Rock Pigeon (Columba livia) through high for the susceptible American Crow. Other species at high risk include, in descending order, the House finch (Carpodacus mexicanus) , Black- crowned N i g h t - Heron ( Nycticorax nycticorax) , Western Scrub - Jay ( Aphelocoma californica), and Yellow-billed magpie (Pica nuttalli). Our analyses emphasize the importance of multiple data sources in assessing the effect of an invading pathogen.
Alteration of habitat has been highlighted as the most important factor causing declines in populations of grassland birds. In this study we characterized bird assemblages in cultivated and natural grasslands under four different agricultural schemes in Uruguay. We surveyed birds along transects to determine species richness, composition, and population density along this agricultural gradient. Although species richness was lower in the most natural grassland type (24 species), community composition and abundance patterns highlighted the importance of natural grasslands for the conservation of habitat specialists. An analysis of similarity showed that bird communities in the four grassland types were distinct (ANOSIM global R = 0.68). Several species using grassland facultatively were characteristic of cultivated habitats, whereas the opposite was true for grassland obligates and natural grasslands. An indicator-species analysis further supported the association of grassland-facultative and grassland-obligate birds with cultivated and natural grassland, respectively. Population-density patterns varied by species; some were more abundant in cultivated habitats, others in natural grasslands, but threatened species attained relatively high densities in natural grasslands only. Some threatened species, such as the Ochre-breasted Pipit (Anthus nattereri) and Pampas Meadowlark (Sturnella defilippii), were largely restricted to natural grasslands, so their long-term survival will depend on the conservation of suitable patches of these habitat types.
Woody vegetation has been linked to increased rates of Brown-headed Cowbird (Molothrus ater) parasitism for some grassland hosts. In northern North Dakota, however, studies reported that parasitism of grassland passerine nests was lower in landscapes with trees than in those without trees. We looked for evidence of this pattern elsewhere, using data from two studies conducted on the Sheyenne National Grassland in southeastern North Dakota. Specifically, we examined the probability of parasitism relative to percent tree cover within 2 km of a nest. We found a negative relationship for grassland passerine nests of all species tested. Our results support the suggestion that cowbirds are less likely to parasitize nests of grassland passerines where tree cover on the landscape is greater. This pattern could be explained by cowbirds switching to alternative hosts in woodlands, but this hypothesis needs further testing.
Counts of migrating raptors are done worldwide and are seen as a useful tool for hawk-population management. Rigorous monitoring of animal abundance, however, requires an evaluation of the detectability associated with the survey methods. Incorporating detectability into abundance estimates has never been done in the case of counts of migrating hawks. The goal of this study was to evaluate the extent to which variation in weather and raptor behavior can affect the detectability of migrating hawks and, consequently, the assessment of long-term population trends. We used a double-observer approach to evaluate detection probability at the Observatoire d'oiseaux de Tadoussac in Québec, Canada. We then did a population-viability analysis, adjusted for detectability, on historical raw counts to evaluate how detectability affects the assessment of extinction probability of diurnal raptors' populations. The detectability of migrating raptors ranged between 75% and 90% and was influenced mainly by weather and flight altitude. High-flying hawks were easier to detect against a cloudy sky than against a blindingly sunny sky. Optimal detection probability was associated with hawks flying at eye level. Probability of detection was higher for larger hawks and for hawks migrating in a cluster. No observer effects were found. No differences were found in the quasi-extinction probability of a species when detectability was accounted for, but this could be the result of the highly variable weather conditions experienced at our maritime site coupled with short time series. We suggest repeating our study at both maritime and continental monitoring sites if our results are to be generalized.
Spring migration strategies vary within and among species. Examination of this variability extends our understanding of life histories and has implications for conservation. I used satellite transmitters to determine migration strategies and evaluate factors influencing the timing of spring migration of Pacific Common Eiders (Somateria mollissima v-nigrum) that nest along the western Beaufort Sea coast. Adult females were marked at nesting colonies in the summers of 2000, 2001, and 2003, and were followed throughout spring migration the following year. Each year approximately equal proportions of eiders used three distinct migration strategies varying in duration, staging locations (waters near the Chukotka Peninsula, Russia, and the Chukchi and Beaufort seas, Alaska), and arrival dates at the nesting areas. It is unlikely that differences in the timing of movements to stopover sites in the Chukchi and Beaufort seas were a result of responses to changes in weather, particularly wind direction. Ice distribution and melt/movement patterns vary substantially among staging areas and thus may affect risk of starvation and reproductive potential. Long-term (decadal) changes in climate may favor birds using one strategy during “warmer” and another during “colder” years.
Little is known of the chronology of wing molt and the intensity of body molt among flightless Red-breasted Mergansers (Mergus serrator) in North America. We examined molt of 39 postbreeding males collected at Anticosti Island, Québec, Canada, from July to September 2005 and 2006. We estimated that birds were flightless for 30-33 days. Nearly 75% of males initiated the flightless period between 20 July and 3 August, and >90% were flightless from 8 to 25 August. Subadult males became flightless slightly earlier ( = 28 July; n = 7) than adult males ( = 1 August; n = 28). Most of the head and neck attained the cryptic alternate plumage before remiges were lost, consistent with observations at a nearby breeding site in eastern New Brunswick, where >75% of adult males were undergoing prealternate molt on the head region by late June. Molt intensities declined on the side, collar, and back and increased on the belly and tail throughout the period of flightlessness. Greater coverts generally molted in synchrony with new flight feathers, and most median and lesser coverts initiated replacement within 10 days of loss of remiges. This study provides evidence that the prealternate molt functions effectively to replace bright upperpart feathers with cryptic plumage prior to the flightless period. We suggest that the staggered pattern of molt among feather regions may minimize nutritional and energetic demands associated with feather replacement and changes in plumage insulation among flightless male Red-breasted Mergansers in the upper Gulf of St. Lawrence.
In colonial species, it is often assumed that locations in the center of the colony are of highest quality and provide highest breeding success. We tested this prediction, known as the “central-periphery model,” in a King Penguin colony in the subantarctic Crozet Archipelago. Breeding activity and survival of 150 penguins, ftted with transponder tags, were monitored over an entire breeding season. Among these 150 bird s, 50 bred on t he slope at the upper periphery of the colony, where the rates of predation and parasitism by ticks were high. fifty birds bred in the center of the colony, where rates of predation and tick parasitism were low, and 50 bred at the lower end of the colony, where the rate of tick parasitism was low but predation and fooding were important risks. We predicted that the center of the colony should provide the safest breeding place and consequently be characterized by the highest breeding success and be used by the highest-quality individuals. Yet we found that penguins breeding in the center of the colony had the same breeding success as those at both peripheral locations. In addition, penguins breeding on the upper slope had a higher survival rate than penguins breeding at the center or bottom of the slope and were likely of higher quality. Our study does not support the central-periphery model and emphasizes the complexity behind the relationships among breeding site, breeding success, and individual quality.
The Red Knot (Calidris canutus) is a long-distance migrant breeding on tundra in the high Arctic and wintering along temperate and tropical coasts. Preflight fueling rate is a major determinant of successful migration, yet individual fueling rates are impossible to determine because Red Knots cannot be recaptured easily. These problems can be overcome by estimating changes in body mass from plasma metabolites. Plasma metabolites are, however, sensitive to stress and time since last meal, limiting studies to situations where birds can be bled almost immediately after capture. Such sampling is almost impossible in the field, where Red Knots are often captured with mist nets in darkness. This study on captive Red Knots investigates whether plasma metabolites obtained from blood samples taken up to 3 hr after capture can be used to predict individual long-term (weekly) body-mass changes during the natural spring preflight fueling period. Triglyceride decreased and β-hydroxybutyrate increased with time since capture, and these changes varied with time since start of the spring fueling period. β-Hydroxybutyrate and uric acid were correlated with weekly body-mass change, but triglyceride was not. Triglyceride was correlated with overall body mass. Weekly body-mass change was best predicted with a model including all metabolites and body mass. Time of blood sampling (immediately or 3 hr after capture) did not affect the accuracy of the predictions. The predictions were not accurate enough to allow comparisons of individuals; they should be used only to compare groups.
Relatively little is known about birds during the postfledging period when flighted chicks have left the nest and must learn to forage independently. We examined postfledging movements, habitat selection, and colony attendance of Forster's Terns (Sterna forsteri) radio-marked just before they fledged in San Francisco Bay, California. The proportion of the day spent at their natal colony declined as juveniles aged, from 65% at the time of fledging to <5% within two weeks of fledging. Accordingly, the distance postfledging terns were located from their colony increased as they aged, from <500 m within the first week of fledging to >5000 m by their fifth week. Time of day also influenced colony attendance, with older terns spending more time at the colony during nighttime hours (20:00 to 05:00) than during the day (06:00 to 19:00), when they were presumably foraging. Home ranges and core-use areas averaged 12.14 km2 and 2.23 km2, respectively. At each of four spatial scales of analysis, postfledging terns selected salt pond habitats strongly. No other habitat types were selected at any scale, but terns consistently avoidedd uplands. Terns also avoided open bay habitats at the two largest spatial scales, tidal marsh habitats at the two smallest scales, and sloughs and managed marshes at several scales. Within salt ponds, terns were located closer to salt-pond levees (58 m) than was expected (107 m). Our results indicate that tern chicks disperse from their natal colony within a few weeks of fledging, with older chicks using their natal colony primarily for roosting during the night, and that postfledging terns are highly dependent on salt ponds.
The breeding schedules of birds may not change at a rate sufficient to keep up with the current pace of climate change, causing reduced reproductive success. This disruption of synchrony is called the “mismatch hypothesis.” We analyzed data on the breeding of Thick-billed Murres (Uria lomvia) at a colony in northern Hudson Bay, Canada, to examine the relative importance of matched and mismatched timing in determining the growth rates of nestlings. From 1988 to 2007 the date of break-up and 50% clearance of sea ice in surrounding waters advanced by 17 days, and the date on which the count of murres at the colony peaked, an index of food availability, advanced by the same amount. However, the median date of egg-laying advanced by only 5 days so that the number of days between the date of hatching and the date of peak attendance and 50% ice cover increased over the study period. Nestlings' growth was reduced in years when the counts of attending adults peaked early in the season and early relative to the date of hatching. These observations suggest that the timing of breeding is not advancing to keep pace with changes in the timing of events in the arctic marine environment, leading to greater difficulty in provisioning nestlings. We also demonstrate a relationship between the state of the North Atlantic Oscillation and both the date of peak colony attendance and the growth of nestlings. This relationship suggests that large-scale ocean-atmosphere interactions influence the availability of prey for murres, although the mechanism by which this occurs is not yet understood. Our results support the idea that mismatching of avian breeding cycles with peaks in food abundance is an important consequence of global climate change.
Seabirds maintain plasticity in their foraging behavior to cope with energy demands and foraging constraints that vary over the reproductive cycle, but behavioral studies comparing breeding and nonbreeding individuals are rare. here we characterize how Marbled Murrelets (Brachyramphus marmoratus) adjust their foraging effort in response to changes in reproductive demands in an upwelling system in central California. we radio-marked 32 murrelets of known reproductive status (9 breeders, 12 potential breeders, and 11 nonbreeders) and estimated both foraging ranges and diving rates during the breeding season. Murrelets spent more time diving during upwelling than oceanographic relaxation, increased their foraging ranges as the duration of relaxation grew longer, and reduced their foraging ranges after transitions to upwelling. When not incubating, mur relets moved in a circadian pat ter n, spending nighttime hours resting near flyways used to reach nesting habitat and foraging during the daytime an average of 5.7 km (SD 6.7 km) from night time locations. breeders foraged close to nesting habitat once they initiated nesting and nest attendance was at a maximum, and then resumed traveling longer distances following the completion of nesting. Nonbreeders had similar nighttime and daytime distributions and tended to be located farther from inland flyways. breeders increased the amount of time they spent diving by 71â€“73% when they had an active nest by increasing the number of dives rather than by increasing the frequency of anaerobiosis. Thus, to meet reproductive demands during nesting, murrelets adopted a combined strategy of reducing energy expended commuting to foraging sites and increasing aerobic dive rates.
The Patagonian Forest Earthcreeper (Upucerthia saturatior) is a distinctive furnariid that inhabits the Patagonian forests of central-western Argentina and adjacent Chile within the Nothofagus Center of Endemism. After its description as a species in 1900, U. saturatior was quickly subsumed, without comment or study, as a subspecies of the Scale-throated Earthcreeper (U. dumetaria), a treatment followed by most subsequent authors. On the basis of an apparent geographical cline within U. dumetaria and a reanalysis of the morphology and plumage of reported intergrades between U. dumetaria and U. saturatior, there is no evidence of intergradation between these taxa. Upucerthia saturatior differs from U. dumetaria by its song (“p-p-tirik-tirik-tirik-tirik-tirik-tiruk” vs. “pli-pli-pli-pli-pli . . .”), which is also three times faster in dumetaria, call (“pep” vs. “keep”), morphology (smaller and darker with a short black bill vs. larger and paler with a long brown bill), distinctive tail pattern, breeding habitat (forest borders vs. shrubby steppe and open highland habitats), and migration patterns (trans-Andean vs. north-south). These differences exceed those between U. jelskii and U. albigula and are far greater than those between U. jelskii and U. validirostris; they overwhelmingly support ranking U. saturatior as a full species. The existence of a forest-dwelling species of Upucerthia parapatric to an open-country Upucerthia provides an opportunity for testing the role of habitat shift between dry exposed habitats and forest habitats (and vice versa) during speciation.
For sit-and-wait predators, a key factor influencing foraging decisions is the ability to detect and track prey, which is expected to vary with the physical and light properties of the environment. We assessed how changes in visual and physical prey availability altered perch selection and visual search activity (head-movement rate and bout length) in the Black Phoebe (Sayornis nigricans), a sit-and-wait flycatcher. We used an observational approach by studying individuals in foraging areas that varied in the ecological factors of interest across an urbanized landscape. Black Phoebes selected perches with high levels of tree and grass cover and low light intensity, which could increase access to prey and reduce predation risk. Visual searching for prey decreased as grass cover increased and tree cover decreased, likely because of less physical and visual obstruction. Visual searching increased with light intensity, probably as a result of the effects of glare, but chromatic contrast did not exert a significant influence. We suggest that for Black Phoebes in urbanized areas physical access to prey may be mediated through the availability of an open understory and visual access through illuminance rather than discrimination of prey against the background.
Flee-inducing alarm calls often communicate information about the type of predator and the danger associated with it. Less is known about how approach-inducing mob calls encode this information. We studied the mob calls of the American Crow to determine whether these calls convey information about the predator type or the level of danger by presenting a model owl (representing an avian predator) and raccoon (representing a mammalian predator). We found that crows emit the same types of vocalizations in response to both of these predator classes. Our results, however, suggest that calls with a longer duration, higher rate, and shorter interval between caws reflect a higher degree of danger. We also found significant differences in call structure from trial to trial, possibly reflecting variations in call structure among individuals or groups. The ability to encode specific information about urgency and individual or group identity while mobbing may be particularly important for efficient coordination of group activities in species—such as the American Crow—that live in stable social groups.
The Red Crossbill (Loxia curvirostra complex) endemic to the South Hills and Albion Mountains in southern Idaho has coevolved in a predator-prey arms race with the lodgepole pine (Pinus contorta latifolia). The resulting divergent selection has favored a sedentary, locally adapted crossbill population whose size and vocalizations differ from those of co-occurring Red Crossbills of other call types. It has also led to high levels of reproductive isolation between the “South Hills crossbill” and nomadic taxa with different vocalizations that move in and out of the area yearly. Genetic analyses of amplified fragment length polymorphisms (AFLP) indicate that about 5% of the loci in the South Hills Crossbill have diverged in spite of the potentially homogenizing influence of gene flow. Given these differences in genetics, morphology, and behavior, and the high level of reproductive isolation in sympatry with other call types (99% of South Hills Crossbills pair assortatively), we recommend that this crossbill be recognized as a distinct species.
We measured the reproductive rate of Red-shouldered Hawks (Buteo lineatus) nesting in suburban southwestern Ohio and in a rural forested region in Hocking Hills, southeastern Ohio, from 1997 to 2005. In the suburban region, the reproductive rate varied greatly from nest to nest, less so from year to year, indicating that some nest areas consistently produced more young than others. The most productive 25% of nest areas produced 44% of all the nestlings in the study, whereas the least productive 25% of nest areas produced only 7% of all nestlings. In the rural area, the reproductive rate varied significantly from year to year, less so from nest area to nest area. Overall reproductive rates in the two study areas differed only in 2000. We suggest that differences among the nest areas in reproductive rate likely indicate differences in habitat quality of individual nest areas, whereas differences among years likely reflect regional factors that influence nesting birds, such as weather.
We studied the breeding ecology of the White-browed Tit-Warbler (Leptopoecile sophiae) in the alpine zone of southern Tibet, elevations 4110–4780 m. The earliest breeder among local passerines, the species initiated egg-laying in early April and ended by late July. We located its domed nests in 13 species of shrubs at an average height of 0.9 m (range 0.2–2.5 m) above the ground. Clutch size averaged 4.7 (range 4–6) eggs, declining through the season. Brood size was 4.3 (range 2–6) at hatching and 3.8 (range 1–5) at fledging. Incubation lasted 20.5 (range 16–23) days, and nestlings fledged at 17.5 (range 14–21) days of age, when they were 4% above the adult weight. Of the nests we observed 66% fledged at least one young. Most pairs were monogamous, and both sexes shared all nesting duties. We noted two females attending a single nest, with a brood of normal size, and egg dumping by an additional female. At the population level, the sex ratio of offspring, determined by sexual differences in plumage of nestlings older than 6 days, did not deviate from equality. Birds breeding late in the season, however, tended to raise more females.
The nomadic Sedge Wren (Cistothorus platensis) breeds primarily in mesic grasslands in north-central North America. Following breeding in these regions from late May to early July, however, the species then “appears” en masse in the tallgrass prairie region farther south (e.g., Missouri and Kansas) and to the east to breed again from mid-July to early August (Herkert et al. 2001). The provenance of birds appearing in late summer to breed in these areas remains unknown because of problems inherent in mark—recapture surveys. Recent studies have shown how endogenous markers may be used to infer origins of individual birds. We analyzed levels of the stable hydrogen isotope 2H (δD) from liver, muscle, and claws of Sedge Wrens from known northern breeding locations first to establish the relationships between δD in the wrens' tissue and mean δD in precipitation during the growing season (δDp ). From these relationships we derived expected values (mean and 95% CI) for three sites in Kansas and Missouri where late breeders colonized. The observed values of δD in these late breeders were primarily within the range expected for those locations, but more individuals than expected had δD values higher than expected. In addition, in birds apparently originating from north or south of Kansas and Missouri, the values of δD in claws were positively correlated with those in other tissues, in contrast to those with the “local” signal. This supports the idea that the isotopic outliers at these sites were more recent arrivals. For small-bodied birds like the Sedge Wren, however, the isotopic approach based on soft tissues is limited to a very narrow temporal window of inference because of rapid elemental turnover. This greatly restricts the use of this technique in inferring origins of small nomadic species.
Infestations of bird blow flies (Protocalliphora spp. and Trypocalliphora braueri) have various negative effects on the condition of nestling birds. In the absence of other stressors such as inclement weather, however, infestation alone rarely reduces fledging success. Previous studies have documented effects of blow flies on nestling condition and fledging success. Without information regarding fledgling survival, the full effect of blow-fly infestation remains unclear. To fully investigate the effect of blow-fly infestation on reproductive success of the Ovenbird (Seiurus aurocapilla), we monitored infested and non-infested nests and monitored fledglings from each by using radio telemetry. Blow flies did not affect birds during the nestling period, as brood size, mean nestling mass, fledging success, and time to fledging in infested and non-infested nests were no different. Fledgling survival and minimum distance traveled the first day after fledging, however, were significantly lower for infected fledglings than for those that were not infected. We conclude that the stress of the early fledgling period combined with recent or concurrent blow-fly infection increases mortality in young Oven-birds. Our results demonstrate the importance of including the post-fledging period in investigations of the effects of ectoparasitic infestations on birds.
We studied a unique nest-dismantling behavior after fledging by Hair-crested Drongos (Dicrurus hottentottus) at the Dongzhai National Nature Reserve, Henan Province, China, during the summer of 2007. Of the 13 nests that fledged young, 12 were dismantled by the adults that built them after the young fledged. Some individuals initiated dismantling behavior on the same day the young birds left the nest and completed dismantling within a few days; others waited for a few days and took longer to finish. The mean rates of visitation and nest-dismantling behavior were 0.10 ± 0.04 and 0.06 ± 0.03 times hr-1 (± SE), respectively, once nest dismantling was initiated. We propose that the nest dismantling by the Hair-crested Drongo may be an adaptive behavior to increase fitness by reducing risk of future predation and competition for nest sites in the following breeding season.