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At least 1,585 records · Page 88Linked to original sources
USGS2005

Parasitism, productivity, and population growth: response of Least Bell's Vireos (Vireo bellii pusillus) and Southwestern Willow Flycatchers (Empidonax traillii extimus) to cowbird (Molothrus spp.) control

Cowbird (Molothrus spp.) control is a major focus of recovery-oriented management of two endangered riparian bird species,the Least Bell's Vireo (Vireo bellii pusillus) and Southwestern Willow Flycatcher (Empidonax traillii extimus). During the past 20 years, annual trapping of cowbirds at Least Bell's Vireo and Southwestern Willow Flycatcher breeding sites has eliminated or reduced parasitism in comparison with pretrapping rates and, thereby, significantly increased seasonal productivity of nesting pairs. Enhanced productivity, in turn, has resulted in an 8-fold increase in numbers of Least Bell's Vireos; Southwestern Willow Flycatcher abundance, however, has changed little, and at some sites has declined despite cowbird control. Although generally successful by these short-term measures of host population response, cowbird control poses potential negative consequences for long-term recovery of endangered species. As currently employed, cowbird control lacks predetermined biological criteria to trigger an end to the control, making these species' dependence on human intervention open-ended. Prolonged reliance on cowbird control to manage endangered species can shift attention from identifying and managing other factors that limit populations--in particular, habitat availability. On the basis of our analysis of these long-term programs, we suggest that cowbird control be reserved for short-term crisis management and be replaced, when appropriate, by practices emphasizing restoration and maintenance of natural processes on which species depend. /// El manejo orientado hacia la recuperación de dos especies de aves ribereñas Vireo belli pusillus y Empidonax trailli extimus se ha focalizado principalmente en el control de los Molothrus spp parásitos. Durante los pasados 20 años, la captura anual de los Molothrus en las áreas de nidificación de Vireo belli pusillus y Empidonax trailli extimus ha eliminado o reducido el parasitismo en comparación con las tasas previas a la captura y, en consecuencia, ha incrementado significativamente la productividad estacional de las parejas reproductivas. Ese mejora en productividad, a su vez, ha resultado en que el número de Vireo belli pusillus se incrementara 8 veces. La abundancia de Empidonax trailli extimus en cambio, ha variado poco, e incluso en algunos sitios, se ha reducido a pesar del control de los Molothrus. Aunque aparentemente el control de Molothrus fue exitoso por los resultados obtenidos a corto plazo, el control de los Molothrus posee consecuencias potencialmente negativas para la recuperación a largo plazo de las especies en peligro. De la forma en que es actualmente aplicado, el control de los Molothrus carece de criterios biológicos predeterminados que permitan dejar de aplicarlo. Esto implica que las especies que se quiera proteger dependan eternamente de la intervención humana. El hecho de que que el manejo de las especies en peligro se base en la dependencia prolongada en el control de los Molothrus podría distraer la atención sobre la identificación y el manejo de otros factores que limitan dichas poblaciones-en particular, la disponibilidad de hábitat. Basándonos en nuestro análisis de estos programas a largo plazo, sugerimos que el control de Molothrus quede reservado para las crisis de manejo de corto plazo. Cuando fuera apropiado, es de esperar que dicho manejo sea reemplazado por prácticas enfatizadas hacia la restauración y el mantenimiento de los procesos naturales de los cuales esas especies en realidad dependen.

Ornithological Monographs↗
USGS2010

Modeling seasonal dynamics of small fish cohorts in fluctuating freshwater marsh landscapes

Small-bodied fishes constitute an important assemblage in many wetlands. In wetlands that dry periodically except for small permanent waterbodies, these fishes are quick to respond to change and can undergo large fluctuations in numbers and biomasses. An important aspect of landscapes that are mixtures of marsh and permanent waterbodies is that high rates of biomass production occur in the marshes during flooding phases, while the permanent waterbodies serve as refuges for many biotic components during the dry phases. The temporal and spatial dynamics of the small fishes are ecologically important, as these fishes provide a crucial food base for higher trophic levels, such as wading birds. We develop a simple model that is analytically tractable, describing the main processes of the spatio-temporal dynamics of a population of small-bodied fish in a seasonal wetland environment, consisting of marsh and permanent waterbodies. The population expands into newly flooded areas during the wet season and contracts during declining water levels in the dry season. If the marsh dries completely during these times (a drydown), the fish need refuge in permanent waterbodies. At least three new and general conclusions arise from the model: (1) there is an optimal rate at which fish should expand into a newly flooding area to maximize population production; (2) there is also a fluctuation amplitude of water level that maximizes fish production, and (3) there is an upper limit on the number of fish that can reach a permanent waterbody during a drydown, no matter how large the marsh surface area is that drains into the waterbody. Because water levels can be manipulated in many wetlands, it is useful to have an understanding of the role of these fluctuations.

Landscape Ecology↗
USGS2015

Comparative demographics of a Hawaiian forest bird community

Estimates of demographic parameters such as survival and reproductive success are critical for guiding management efforts focused on species of conservation concern. Unfortunately, reliable demographic parameters are difficult to obtain for any species, but especially for rare or endangered species. Here we derived estimates of adult survival and recruitment in a community of Hawaiian forest birds, including eight native species (of which three are endangered) and two introduced species at Hakalau Forest National Wildlife Refuge, Hawaiʻi. Integrated population models (IPM) were used to link mark–recapture data (1994–1999) with long-term population surveys (1987–2008). To our knowledge, this is the first time that IPM have been used to characterize demographic parameters of a whole avian community, and provides important insights into the life history strategies of the community. The demographic data were used to test two hypotheses: 1) arthropod specialists, such as the ‘Akiapōlā‘au Hemignathus munroi , are ‘slower’ species characterized by a greater relative contribution of adult survival to population growth, i.e. lower fecundity and increased adult survival; and 2) a species’ susceptibility to environmental change, as reflected by its conservation status, can be predicted by its life history traits. We found that all species were characterized by a similar population growth rate around one, independently of conservation status, origin (native vs non-native), feeding guild, or life history strategy (as measured by ‘slowness’), which suggested that the community had reached an equilibrium. However, such stable dynamics were achieved differently across feeding guilds, as demonstrated by a significant increase of adult survival and a significant decrease of recruitment along a gradient of increased insectivory, in support of hypothesis 1. Supporting our second hypothesis, we found that slower species were more vulnerable species at the global scale than faster ones. The possible causes and conservation implications of these patterns are discussed.

Hawaii↗
USGS1995

Use of implanted satellite transmitters to locate Spectacled Eiders at-sea

Population estimates of Spectacled Eiders ( Somateria fischeri ) on the Yukon-Kuskokwim Delta (YKD), Alaska, suggest that by 1992 the number of birds on this major nesting area had declined to 1,721 pairs, 4% of that estimated in the 1970s (Stehn st al 1993). Consequently, Spectacled Eiders were listed as threatened under the Endangered Species Act. As nesting habitats for this species are believed to have changed little over the past 100 years, hypotheses concerning the cause of this decline include factors away from nesting areas. The non-nesting distribution of this eider is unknown, but birds are believed to molt and winter in the Bering and Chukchi seas (Dau and Kistchinski 1977). Systematic aerial surveys to locate areas where birds concentrate are expensive because of the vast area to be surveyed and dangerous because of restricted daylight and extreme weather conditions. Surveys from ships along the ice margin in the Bering Sea failed to locate concentrations of birds (Irving et al 1968, Everett et al 1989). We initiated a study to determine if at-sea areas used by Spectacled Eiders could be identified using satellite telemetry.

Alaska↗
USGS1986

Demography, environmental uncertainty, and the evolution of mate desertion in the snail kite

The Snail Kite (Rostrhamus sociabilis), an endangered hawk, has a unique mating system in Florida (Beissinger and Snyder 1987): when food is abundant, males or females desert their mates at nearly equal frequency (ambisexual mate desertion) in the midst of a nesting cycle. I examined the demographic and environmental constraints selecting for a clutch size that permits one parent to desert, yet optimizes the number of offspring produced by each parent. Demographic studies, conducted from 1979—1983, indicated that kites have (1) a very high nest failure rate (° = 68%) due most often to unstable nest sites and predation, (2) a variable nesting season (5—10 mo/yr), (3) an early age of first reproduction for a bird this size (10 mo), (4) a high degree of iteroparity (double and potentially triple clutching within a season), and (5) unstable populations. Both nesting success and population size were directly related to Everglades water levels and resultant snail densities. Kites responded to large annual changes in food abundance, not by adjusting clutch size but by deserting their mates and presumably attempting to renest. Kite demographic traits appear to be adaptations to or results of an uncertain environment. Based on 67 yr of Everglades water levels, environmental predictability, measured by spectral analysis and Colwell's (1974) index, was low and influenced by water management regimes: (1) water levels were lowered, (2) annual variation in levels increased and annual cycles became stronger, (3) the period length of long—term drought—flood cycles shifted from 10 or more yr toward 5 yr, and (4) levels became a less predictive cue for favorable nesting conditions. A potential evolutionary pathway from biparental care to mate desertion in Snail Kites is proposed. It is unlikely that mate desertion evolved solely from a context of conflict between the sexes, because kite nesting success is so low that it is probably maladaptive for a parent to desert and jeopardize the survival of any of its first brood. Instead, mate desertion behavior probably evolved in response to a smaller average clutch size; this would allow females to be highly iteroparous and avoid the costs of overinvestment, and should be strongly favored in a highly uncertain environment. Analysis of clutches in museum collections suggests that an apparent decline in clutch size may have occurred in Florida during the past century. The potential for ambisexual mate desertion to occur in other vertebrates is during the past century. The potential for ambisexual mate desertion to occur in other vertebrates is examined.

Ecology↗
USGS2020

GIS-Modeling of island hopping through the Philippines demonstrates trade-offs migrant grey-faced buzzards during oceanic crossings

Migration can be costly with consequences that can influence population trajectories. These costs and consequences are especially heightened during over-water travels, which can be high-risk events for birds. We created spatial models to evaluate potential migratory responses of “oceanic”, island-hopping grey-faced buzzards that encounter variation in landscape parameters and weather as they move through and out of the Philippine archipelago. We constrained the modeled routes to enter the island chain at Basco and to use one of four potential exit points in the south of the country, either Balabac, Bongao, Balut Island, or Cape San Agustin. We used all possible combinations of our three external parameters (stopover sites, water crossings and wind direction) to model alternative migratory routes for each of the four exit points (n = 20 migratory routes). Modeled grey-faced buzzard routes were between 1,582 and 2,970 km. Routes overlapped over eastern and central Luzon, along a leading line created by the Sierra Madre Mountains. Routes also overlapped and suggested unavoidable over-water crossings between Mindoro and Palawan, Negros and Zamboanga del Norte, and Leyte and Surigao. Our models suggest that the optimal migratory strategy for these birds is to find the shortest route to an exit point with the greatest possible access to stopover habitats and fewest open-water crossings under wind resistance. Understanding how each of these external factors affected the geography and characteristics of the migratory routes helps us to understand the context for different migratory strategies of birds that face dangerous open-water crossings on migration.

Journal of Engineering, Environment and Agricultur↗
USGS2020

Learning from real-world experience to understand renewable energy impacts to wildlife

The project team sought to use real-world data to understand adverse effects to wildlife of renewable energy production that is critical to meeting California’s climate and clean energy goals. The project had three main components. First, a systematic literature review studied 20 peer-reviewed publications and 612 reports from other nonreviewed sources from 231 wind and solar facilities in North America. Within California, 50 percent of facilities collected pre- and post-construction data, 30 percent had experimental study designs, and fewer than 7 percent estimated detection probability during habitat use surveys. Mitigation at wind power plants focused on repowering to reduce risk to soaring birds and at solar facilities emphasized wildlife deterrence and compensatory mitigation. Second, the authors developed a best-practices approach to employ environmental isotopes (for example, hydrogen obtained from animal tissue) and rescaling functions (a statistical approach to modeling the relationship between variables) to assign individual birds or bats to their place of origin. The team applied this approach to feathers from 411 individuals of 12 species killed at wind facilities and 515 individuals of 19 species killed at solar facilities. From 24 percent to 100 percent (mean +/- SD = 49 percent +/- 33 percent) and 25 percent to 100 percent (73 percent +/- 25 percent) of birds grew feathers at a location outside the collection site at wind and solar facilities, respectively. Third, the authors constructed Bayesian integrated population models (probability models) for 29 focal species affected by wind or solar energy generation in California. Species predominantly local in origin generally had lower population growth rates than did species that were predominantly nonlocal in origin. These patterns illustrate the complex linkages between behavioral ecology, vulnerability to mortality, and population-level impacts to wildlife from fatalities at renewable energy facilities. This project benefits the renewable energy sector by providing a framework and specific tools for understanding environmental impacts of renewable energy generation.

California↗
USGS1998

Waterfowl in the prairie pothole region

The prairie pothole region of the northern Great Plains is one of the most important areas for duck reproduction in North America. The region produces, on average, 50% of the primary species of game ducks on the continent (Smith 1995), yet accounts for only 10% of the waterfowl breeding habitat in North America (Smith et al. 1964). Twelve of the 34 species of North American ducks are common breeders in the region. For seven species—mallard, gadwall, blue-winged teal, northern shoveler, northern pintail, redhead, and canvasback—the prairie pothole region accounts for more than 60% of the breeding population (Smith 1995). The region is also a major migration corridor during fall and spring for other ducks, geese, and other water birds.

Book chapter↗
USGS2005

Southern marl prairies conceptual ecological model

About 190,000 ha of higher-elevation marl prairies flank either side of Shark River Slough in the southern Everglades. Water levels typically drop below the ground surface each year in this landscape. Consequently, peat soil accretion is inhibited, and substrates consist either of calcitic marl produced by algal periphyton mats or exposed limestone bedrock. The southern marl prairies support complex mosaics of wet prairie, sawgrass sawgrass ( Cladium jamaicense ), tree islands, and tropical hammock communities and a high diversity of plant species. However, relatively short hydroperiods and annual dry downs provide stressful conditions for aquatic fauna, affecting survival in the dry season when surface water is absent. Here, we present a conceptual ecological model developed for this landscape through scientific consensus, use of empirical data, and modeling. The two major societal drivers affecting the southern marl prairies are water management practices and agricultural and urban development. These drivers lead to five groups of ecosystem stressors: loss of spatial extent and connectivity, shortened hydroperiod and increased drought severity, extended hydroperiod and drying pattern reversals, introduction and spread of non-native trees, and introduction and spread of non-native fishes. Major ecological attributes include periphyton mats, plant species diversity and community mosaic, Cape Sable seaside sparrow ( Ammodramus maritimus mirabilis ), marsh fishes and associated aquatic fauna prey base, American alligator ( Alligator mississippiensis ), and wading bird early dry season foraging. Water management and development are hypothesized to have a negative effect on the ecological attributes of the southern marl prairies in the following ways. Periphyton mats have decreased in cover in areas where hydroperiod has been significantly reduced and changed in community composition due to inverse responses to increased nutrient availability. Plant species diversity and community mosaics have changed due to shifting gradients to more terrestrial or more aquatic communities, displacement of native communities by non-natives, expansion of woody plants, high-intensity dry season fires, tree-island burnout, and reduced microtopography resulting from alligator population decline. Cape Sable seaside sparrow populations are threatened by nest destruction resulting from extended hydroperiods, drying pattern reversals, and high intensity dry season fires, as well as by the expansion of woody plants into graminoid wetland habitats. Populations of marsh fishes and associated aquatic fauna that constitute the aquatic prey base for higher vertebrates have decreased in density and changed in species composition and size structure due to loss of wetland spatial extent, shortened hydroperiod, increased drought severity, loss of aquatic drought refugia in solution holes and alligator holes, and spread of exotic fishes. American alligator populations have declined in the Rocky Glades, and alligator holes have filled with sediment, as a result of shortened hydroperiod and increased drought severity. Habitat options for wading birds to forage during the early dry season and during unusually wet years have been reduced due to loss of spatial extent and shortened hydroperiod.

Wetlands↗
USGS2020

Determinants and consequences of dispersal in vertebrates with complex life cycles: a review of pond-breeding amphibians

Dispersal is a central process in ecology and evolution. It strongly influences the dynamics of spatially structured populations, by affecting population growth rate and local colonization-extinction processes. Dispersal can also influence evolutionary processes because it determines rates and patterns of gene flow in spatially structured populations and is closely linked to local adaptation. For these reasons, dispersal has received considerable attention from ecologists and evolutionary biologists. However, although it has been studied extensively in taxa such as birds and mammals, much less is known about dispersal in vertebrates with complex life cycles such as pond-breeding amphibians. Over the past two decades, researchers have taken an interest in amphibian dispersal and initiated both fundamental and applied studies, using a broad range of experimental and observational approaches. This body of research reveals complex dispersal patterns, causations and syndromes, with dramatic consequences for the demography and genetics of amphibian populations. In this review, our goals are to (1) redefine and clarify the concept of amphibian dispersal, (2) review current knowledge about the effects of individual (i.e., condition-dependent dispersal) and environmental (i.e., context-dependent dispersal) factors during the three stages of dispersal (i.e., emigration, immigration, transience), (3) identify the demographic and genetic consequences of dispersal in spatially structured amphibian populations, and (4) propose new research avenues to extend our understanding of amphibian dispersal. In particular, we emphasize the need to (1) quantify dispersal rate and distance rigorously using suitable model systems, (2) investigate the genetic basis and dispersal evolution patterns, and (3) examine dispersal-related eco-evolutionary dynamics. These proposed research avenues tap from the recent advances in quantitative and molecular methods and have the potential to improve our understanding of dispersal in organisms with complex life cycles.

The Quarterly Review of Biology↗
USGS2017

A genetic signature of the evolution of loss of flight in the Galapagos cormorant

INTRODUCTION Changes in the size and proportion of limbs and other structures have played a key role in the evolution of species. One common class of limb modification is recurrent wing reduction and loss of flight in birds. Indeed, Darwin used the occurrence of flightless birds as an argument in favor of his theory of natural selection. Loss of flight has evolved repeatedly and is found among 26 families of birds in 17 different orders. Despite the frequency of these modifications, we have a limited understanding of their underpinnings at the genetic and molecular levels. RATIONALE To better understand the evolution of changes in limb size, we studied a classic case of recent loss of flight in the Galapagos cormorant ( Phalacrocorax harrisi ). Cormorants are large water birds that live in coastal areas or near lakes, and P. harrisi is the only flightless cormorant among approximately 40 extant species. The entire population is distributed along the coastlines of Isabela and Fernandina islands in the Galapagos archipelago. P. harrisi has a pair of short wings, which are smaller than those of any other cormorant. The extreme reduction of the wings and pectoral skeleton observed in P. harrisi is an attractive model for studying the evolution of loss of flight because it occurred very recently; phylogenetic evidence suggests that P. harrisi diverged from its flighted relatives within the past 2 million years. We developed a comparative and predictive genomics approach that uses the genome sequences of P. harrisi and its flighted relatives to find candidate genetic variants that likely contributed to the evolution of loss of flight. RESULTS We sequenced and de novo assembled the whole genomes of P. harrisi and three closely related flighted cormorant species. We identified thousands of coding variants exclusive to P. harrisi and classified them according to their probability of altering protein function based on conservation. Variants most likely to alter protein function were significantly enriched in genes mutated in human skeletal ciliopathies, including Ofd1 , Evc , Wdr34 , and Ift122 . We carried out experiments in Caenorhabditis elegans to confirm that a missense variant present in the Galapagos cormorant IFT122 protein is sufficient to affect ciliary function. The primary cilium is essential for Hedgehog (Hh) signaling in vertebrates, and individuals affected by ciliopathies have small limbs and ribcages, mirroring the phenotype of P. harrisi . We also identified a 4–amino acid deletion in the regulatory domain of Cux1 , a highly conserved transcription factor that has been experimentally shown to regulate limb growth in chicken. The four missing amino acids are perfectly conserved in all birds and mammals sequenced to date. We tested the consequences of this deletion in a chondrogenic cell line and showed that it impairs the ability of CUX1 to transcriptionally up-regulate cilia-related genes (some of which contain function-altering variants in P. harrisi ) and to promote chondrogenic differentiation. Finally, we show that positive selection may have played a role in the fixation of the variants associated with loss of flight in P. harrisi . CONCLUSION Our results indicate that the combined effect of variants in genes necessary for the correct transcriptional regulation and function of the primary cilium likely contributed to the evolution of highly reduced wings and other skeletal adaptations associated with loss of flight in P. harrisi . Our approach may be generally useful for identification of variants underlying evolutionary novelty from genomes of closely related species.

Science↗
USGS1998

Estimation of temporal variability of survival in animal populations

Temporal variation of demographic characteristics for animal populations is of interest to both ecologists and biological modelers. The standard deviation of a series of estimated parameter values (e.g., estimated population size) or some function thereof (e.g., log of the estimated parameters) is commonly used as a measure of temporal variability. These measures of temporal variation overestimate the true temporal variation by not accounting for sampling variability inherent to the estimation of unknown population parameters. Using a variance-components approach to partitioning the total variability of an estimated parameter, we demonstrate the ease with which sampling variation can be removed from the observed total variation of parameter estimates. Estimates of temporal variability of survival are given after removal of sampling variation for three bird species: the federally listed Roseate Tern ( Sterna dougallii ), Black-capped Chickadees ( Parus atricapillus ), and Mallard ducks ( Anas platyrhynchos ). Sampling variation accounted for the majority of the total variation in the survival estimates for nearly all of the populations studied. Substantial differences in observed significance levels were observed when testing for demographic differences in temporal variation using temporal variance estimates adjusted and unadjusted for sampling variance.

Ecology↗
USGS1993

Sex ratios of canvasbacks wintering in Louisiana

Disparate sex ratios in waterfowl are considered indicative of differential survival between the sexes and may limit reproductive potential. Because other studies have shown canvasbacks (Aythya valisineria) have highly skewed sex ratios, I examined aerial photographs from surveys flown monthly during winters 1987-88 and 1988-89 to determine sex ratios of canvasbacks in Louisiana where the wintering population is increasing. Estimated statewide sex ratios (proportion male) were balanced during the initial arrival period in November (1987: 0.505, 1988: 0.524), dominated by males during December-February (range = 0.600- 0.655), and then dominated by females as birds departed in March (1989: 0.419). Sex ratios varied geograph- ically with males being more prevalent in flocks at inland sites than in coastal areas. Canvasback sex ratios did not vary by flock size nor between major- and minor-use areas. Canvasback sex ratios in Louisiana appeared to be representative of the population wintering in the lower Mississippi Valley and Gulf Coast region and were less male dominated than the sex ratios projected for the population wintering in the Atlantic Flyway. Overall sex ratio characteristics imply that female canvasbacks in Louisiana may have higher annual survival or recruitment rates relative to males than occur in other wintering populations. I suggest that this may partially explain the observed increases in canvasbacks wintering in Louisiana.

Louisiana↗
USGS2022

In hot water? Patterns of macroinvertebrate abundance in Arctic thaw ponds and relationships with environmental variables

Ongoing environmental change across the Arctic is affecting many freshwater ecosystems, including small thaw ponds that support macroinvertebrates, thus potentially affecting important forage for fish and bird species. To accurately predict how fish and wildlife that depend on these macroinvertebrates will be affected by ecosystem change at high latitudes, understanding proximate factors that influence macroinvertebrate abundance is critical. To better understand factors that affect spatial and seasonal (i.e. phenology) patterns in abundance, we collected macroinvertebrates throughout the growing season of a single year from 33 thaw ponds on the Arctic Coastal Plain in northern Alaska. We used hierarchical N -mixture models to provide detection-corrected estimates of abundance (of the population exposed to sampling) in relation to pond type and seasonal patterns in environmental variables (i.e., cumulative water temperature, nutrient levels) for five taxonomic groups representing key food items for birds and fish—Anostraca (Arthropoda: Branchiopoda), Chironomidae (Insecta: Diptera), Cladocera (Arthropoda: Branchiopoda), Limnephilidae (Insecta: Trichoptera), and Physidae (Mollusca: Gastropoda). For three of five taxa (Anostraca, Cladocera, Limnephilidae), abundance varied across pond types and was lower in pond types where water temperatures increased more rapidly. Further, seasonal temperature profiles in ponds affected phenology, suggesting that seasonal patterns in abundance were influenced by changes in water temperature. These findings suggest that increases in water temperature in northern areas could alter macroinvertebrate phenology, possibly with consequences for upper level predators if availability of macroinvertebrate prey is reduced or shifted seasonally. Our results will facilitate improved predictions of how changing abiotic conditions could affect inland waters in northern areas, a critical need for conservation of Arctic wildlife and ecosystems.

Alaska↗
USGS1999

Bar-tailed Godwits Limosa lapponica in Alaska: A population estimate from the staging grounds

Bar-tailed Godwits Limosa lapponica were surveyed on their staging grounds in Alaska during September 1995 and 1997. The single high count of 94,000 birds closely matched that of counts from New Zealand and south-eastern Australia, the known non-breeding area for most of the baueri subspecies. Numbers recorded on the southern Yukon-Kuskokwim River Delta and at Egegik Bay, a small estuary along the Alaska Peninsula, qualify both areas as Hemispheric Reserves under the Westem Hemisphere Shorebird Reserve Network, as sites within the East Asian-Australian Shorebird Reserve Network, and as Ramsar sites. The breeding origins, destinations, and taxonomic affinities of Bar-tailed Godwits staging on the coast of south-west Alaska need further assessment.

Alaska↗
USGS2009

Estimating transition probabilities in unmarked populations: Entropy revisited

The probability of surviving and moving between 'states' is of great interest to biologists. Robust estimation of these transitions using multiple observations of individually identifiable marked individuals has received considerable attention in recent years. However, in some situations, individuals are not identifiable (or have a very low recapture rate), although all individuals in a sample can be assigned to a particular state (e.g. breeding or non-breeding) without error. In such cases, only aggregate data (number of individuals in a given state at each occasion) are available. If the underlying matrix of transition probabilities does not vary through time and aggregate data are available for several time periods, then it is possible to estimate these parameters using least-squares methods. Even when such data are available, this assumption of stationarity will usually be deemed overly restrictive and, frequently, data will only be available for two time periods. In these cases, the problem reduces to estimating the most likely matrix (or matrices) leading to the observed frequency distribution of individuals in each state. An entropy maximization approach has been previously suggested. In this paper, we show that the entropy approach rests on a particular limiting assumption, and does not provide estimates of latent population parameters (the transition probabilities), but rather predictions of realized rates.

Bird Study↗
USGS2006

Effects of methyl mercury on reproduction in American kestrels

Methyl mercury (MeHg) readily passes through biological membranes, accumulates in individuals, and biomagnifies in higher order predators. It is acutely toxic to some birds at 5-15 parts per million (ppm) wet weight in the diet, and it can damage the central nervous system, impair reproduction, and retard growth and development. The effects of MeHg on reproduction in wild raptors are poorly known, and experiments with captive raptors have not included measures of reproductive response. In this study, breeding pairs of captive American kestrels (Falco sparverius) were exposed to environmentally realistic concentrations of MeHg in the diet and their subsequent reproduction was measured. Egg production, incubation performance, and the number of eggs hatched markedly decreased between 3.2 and 4.6 ppm MeHg dry weight in the diet. The percent of eggs hatched declined between 0.75 and 2 ppm MeHg dry weight in the diet and further declined to almost total hatching failure between 3.2 and 4.6 ppm. The number of fledglings and the percent of nestlings fledged were greatly reduced at 0.75 ppm MeHg dry weight in the diet and began a final sharp decline between 2 and 3.2 ppm. Dietary concentrations of > 4.6 ppm MeHg dry weight were associated with total fledging failure. Mercury concentrations in a set of 19 `second-laid? eggs collected from all groups were related to dietary concentrations of MeHg and the reproductive responses of kestrels in each group. Observed percentage declines in fledgling production caused by diets containing >2 ppm MeHg dry weight would result in the production of insufficient numbers of young kestrels for maintenance of wild populations. Concentrations of total Hg in eggs from the highest diet group (5.9 ppm dry weight) were higher than egg concentrations reported for either wild birds or for captive birds fed dry commercial food containing 5 ppm MeHg wet weight.

Book chapter↗
USGS2004

Occupancy as a surrogate for abundance estimation

In many monitoring programmes it may be prohibitively expensive to estimate the actual abundance of a bird species in a defined area, particularly at large spatial scales, or where birds occur at very low densities. Often it may be appropriate to consider the proportion of area occupied by the species as an alternative state variable. However, as with abundance estimation, issues of detectability must be taken into account in order to make accurate inferences: the non?detection of the species does not imply the species is genuinely absent. Here we review some recent modelling developments that permit unbiased estimation of the proportion of area occupied, colonization and local extinction probabilities. These methods allow for unequal sampling effort and enable covariate information on sampling locations to be incorporated. We also describe how these models could be extended to incorporate information from marked individuals, which would enable finer questions of population dynamics (such as turnover rate of nest sites by specific breeding pairs) to be addressed. We believe these models may be applicable to a wide range of bird species and may be useful for investigating various questions of ecological interest. For example, with respect to habitat quality, we might predict that a species is more likely to have higher local extinction probabilities, or higher turnover rates of specific breeding pairs, in poor quality habitats.

Animal Biodiversity and Conservation↗