Anas Rubripes Research Articles

Food Resources for Wintering and Spring Staging American Black Ducks

Abstract Habitat restoration and enhancement objectives for wintering waterfowl are typically derived by a bioenergetics modeling approach. This approach has been developed as a planning tool to identify the amount of foraging habitat required to meet North American Waterfowl Management Plan population objectives. Our objective was to provide the energetic supply component of the bioenergetics model at an important wintering area for American black ducks Anas rubripes in the Atlantic Flyway, the Eastern Shore of Virginia. We estimated food availability among four main wetland cover types used by overwintering American black ducks: brackish water, freshwater, mudflat, and salt marsh. Mudflat (221 ± 50 kg/ha) and salt marsh (728 ± 175 kg/ha) had the highest amounts of available invertebrate food density, and freshwater (42 ± 9 kg/ha) had the highest amounts of available seed biomass. Our results suggest that seed density found in freshwater wetlands on the Eastern Shore of Virginia is considerably lower than densities found in inland freshwater cover types used by dabbling ducks. We also found that levels of invertebrate density found in Virginia mudflat and salt marsh are considerably lower than levels on Long Island, New York, and in southern New Jersey. Lower levels of food density compared with both more inland and northern wintering areas suggest that American black ducks wintering in Virginia are more likely to be limited by forage availability than American black ducks and other dabbling ducks wintering both inland and in the northern portion of the wintering range.

Clade 2.3.4.4 H5 North American Highly Pathogenic Avian Influenza Viruses Infect, but Do Not Cause Clinical Signs in, American Black Ducks (Anas rubripes).

Highly pathogenic avian influenza virus (HPAIV) from the goose/Guangdong/1996 clade 2.3.4.4 H5 lineage spread from Asia into North America in 2014, most likely by wild bird migrations. Although several variants of the virus were detected, H5N8 and H5N2 were the most widespread in North American wild birds and domestic poultry. In early 2015, the H5N2 virus spread through commercial poultry in the Midwest, and >50 million chickens and turkeys died or had to be culled. Related H5 HPAIVs are still endemic in much of the Eastern Hemisphere. The wild bird species that were involved with dissemination of the virus in North America are not known. Dabbling ducks, especially mallards (Anas platyrhynchos), typically have the highest detection rates for avian influenza viruses. To better characterize the wild avian species that could spread the virus, American black ducks (Anas rubripes), which are closely related to mallards, were challenged with the North American H5N2 and H5N8 index HPAIV isolates: A/Northern Pintail/WA/40964/2014 H5N2 and A/Gyrfalcon/WA/41088/2014 H5N8. Although the American black ducks could be infected with low doses of both isolates (≤102 50% egg infective doses), ducks shed the H5N2 longer than the H5N8 (10 vs. 7 days) and the titers of virus shed were higher. Although there were too few ducks available on which to draw definitive conclusions, this suggests that American black ducks could serve as a more efficient reservoir for the H5N2 virus than the H5N8 virus.

The American Black Duck: Three Decades of Science-Based Adaptive Management

The American black duck (Anas rubripes) population declined by 50% between 1955 and 1985, prompting more than three decades of intensive scientific research and strategic management. Analyses of band recovery data suggest that the historical declines may have been caused in part by harvest, but even with restrictive hunting regulations implemented in the mid 1980s, populations have not recovered. Increasing competition and hybridization with mallards (Anas platyrhynchos), coupled with habitat loss and fragmentation on northern breeding grounds are hypothesized to have contributed to a lower continental black duck population. Simultaneously, there is a concern that declines in the quantity and quality of wintering habitat—coastal salt marshes of the eastern United States—may have deleterious cross-seasonal effects on black duck demographics. Black ducks have a long legacy of intensive research and management, and ongoing threats to their populations make this a well-rooted and timely case study in science-based conservation.

Body condition of American Black Ducks (Anas rubripes) wintering in Atlantic Canada using carcass composition and a scaled mass index

Body condition is commonly used in ecology to assess the physiological health of an organism or population and can be used to predict individual survival or breeding success. Waterfowl have been the focus of much research on body condition, and we studied body condition via carcass composition and using a scaled mass index (SMI) in American Black Ducks (Anas rubripes Brewster, 1902) wintering in coastal, agricultural, and urban areas of Atlantic Canada. Carcass composition varied between sexes and body mass decreased through winter as fat reserves depleted. Carcass composition was compared with American Black Ducks wintering in the United States, and American Black Ducks wintering in Atlantic Canada were structurally smaller yet proportionally fatter than those wintering in the United States, likely as a mechanism to survive Atlantic Canada’s harsher winters. SMI did not differ between coastal, agricultural, or urban American Black Ducks, indicating that despite known differences in the diets of the Black Ducks from these three areas, they can maintain similar body conditions capable of surviving the winter. We show that the SMI is a nondestructive alternative to study body condition in waterfowl. Our research highlights the adaptability and hardiness of American Black Ducks at the northern limit of their winter range.

Monitoring responses to variation in food supply for a migratory waterfowl: American Black Duck (Anas rubripes) in winter.

Wintering Black Ducks (Anas rubripes) concentrate in wetlands along the Atlantic coast where natural and anthropogenic disturbances have increased over the last 50 years, a period in which the population of Black Ducks has declined. We studied the sensitivity of Black Ducks to perturbations in food supply that often result from disturbances by storms, predators, and people. In the paper, we characterize the responses of captive Black Ducks to shifts in food quality and availability during winter and apply those measures to a comparison of wild birds. Captive ducks that were fed intermittently (3 consecutive days/week) compensated for fasted days to achieve similar body mass and body fat to control birds that were fed every day on both animal- and plant-based diets. However, birds that were fed intermittently expended 15% more energy each day than controls when both groups were fed (536 vs. 464kJ/kg0.75), which indicates that variable food supply increases the costs of maintenance and thus reduces the number of birds that can be supported on the same resource of food without interruptions to foraging. Egg production was not affected by diet quality provided in spring or by the frequency of feeding during the preceding winter months. Black Ducks lost body fat through winter in captivity and in the wild. Fat stores of birds in New Jersey were greater than those of birds in Maine (13.3 vs. 8.3% of body mass) in January, which reflected the high energy demands of cold temperatures in Maine. Values for ∂15N were greater in Maine than in New Jersey for both red blood cells and plasma, which indicated a consistent diet of marine invertebrates in Maine. Greater isotopic variation in red blood cells indicated that diets were more diverse in New Jersey than in Maine for both ∂15N (9.7 ± 1.1 vs. 11.2 ± 0.4‰) and for ∂13C (- 15.1 ± 2.2 vs. - 13.8 ± 1.4‰). Plasma ∂13C was enriched over red blood cells in wild birds especially those with low fat stores, which suggested birds with low energy stores were shifting diets. Black Ducks can compensate for disturbances in feeding by increasing intakes if they have access to high quality wetlands where they are able to find abundant food. High energy demands at cold temperatures may constrain fat stores and thus the tolerance of feeding disturbances especially at the northern limits of the winter range. We hypothesize that decreasing variation in diet may indicate an increase in vulnerability to disturbance in winter when body fat is low. Recent efforts to assess and improve habitat quality of Black Ducks could be enhanced by monitoring the body composition and diet of birds to assess their vulnerability to disturbances in food supply and energy demands.

Integrated population models facilitate ecological understanding and improved management decisions

ABSTRACTIntegrated population models (IPMs) represent a formal statistical methodology for combining multiple data sets such as population counts, band recoveries, and fecundity estimates into a single unified analysis with dual objectives: better estimating population size, trajectory, and vital rates; and formally describing the ecological processes that generated these patterns. Although IPMs have been used in population ecology and fisheries management, their use in wildlife management has been limited. Data sets available for North American waterfowl are unprecedented in terms of time span (>60 years) and geographic coverage, and are especially well‐suited for development of IPMs that could improve the understanding of population ecology and help guide future harvest and habitat management decisions. In this overview, we illustrate 3 potential benefits of IPMs: integration of multiple data sources (i.e., population counts, mark‐recapture data, and fecundity estimates), increased precision of parameter estimates, and ability to estimate missing demographic parameters by reanalyzing results from a historical study of canvasbacks (Aythya valisineria). Drawing from our own published and unpublished work, we demonstrate how IPMs could be used to identify the critical vital rates that have had the greatest influence on population change in lesser scaup (Aythya affinis), evaluate potential mechanisms of harvest compensation for American black ducks (Anas rubripes), or prioritize the most appropriate places to conduct habitat management to benefit northern pintails (Anas acuta). Integrated population models provide a powerful platform for evaluating alternative hypotheses about population regulation and they have potential to advance the understanding of wildlife ecology and help managers make ecologically based decisions. © 2017 The Wildlife Society.

Mercury in wintering American black ducks (Anas rubripes) downstream from a point-source on the lower Penobscot River, Maine, USA

Waterfowl wintering along the lower Penobscot River, Maine continue to be exposed to elevated Hg concentrations from the HoltraChem chlor-alkali plant that operated along the river between 1967 and 2000. In American black ducks (Anas rubripes) total Hg in duck breast muscle increased with residence time on contaminated marshes, reaching means of 0.82±0.21μg/g ww (wet weight) by the end of the fall hunting season, and prompting Maine to issue a human consumption advisory on duck breast muscle. Methyl Hg comprised over 99% of the total Hg in breast muscle. The ratio of Hg concentrations in blood and muscle were strongly correlated and approached 1:1 after extended residence times. Primary feather (P1) total Hg concentrations averaged 2.2±1.3μg/g fw (fresh weight), verifying low Hg exposure during feather growth on distant breeding grounds the preceding summer. Mercury concentrations in black ducks, following winter residence along the lower Penobscot exceeded levels associated with reproductive toxicity. Carry-over of Hg to summer breeding grounds may limit the subsequent reproductive success of black ducks.

Increases in the Number of American Black Ducks Wintering in Nova Scotia, 1970–2015

AbstractDespite extensive conservation and management efforts, American black duck (Anas rubripes) populations remain below desired population levels. Researchers have noted declines at a number of wintering areas, but indications are that wintering populations in the northern part of the range are increasing. Intermittently between 1970 and 1989, and annually since 1992, wildlife biologists have flown aerial surveys of the entire coast of Nova Scotia, Canada, to count wintering waterfowl. This survey counted the total number of ducks seen in predefined lengths of coastline. We analyzed those survey data using generalized linear mixed models, over the entire time period (1970–2015) and in the recent time period (1992–2015, when surveys were done every year), across five general regions of Nova Scotia. We fit models using Bayesian methods with uninformative priors; models with negative binomial response distributions performed well. Due to the large variation in mean numbers of black ducks across the predefined lengths of coastline, we treated these blocks of coastline as a fixed effect, while annual trend (slope) was modeled as a random effect to account for variation in annual trend across blocks of coastline. Results were similar between the entire time series (1970–2015) or the most recent and more complete time series (1992–2015). In general, numbers of wintering black ducks in Nova Scotia increased (1.0–16.0%/y) and increases were significant (Bayesian credible intervals did not bound 0) in four of five regions. Increasing numbers in Nova Scotia are consistent with other observations of increasing wintering numbers at the northern range of American black ducks and may indicate a shift in the wintering range that has been ongoing for decades. Future habitat management actions will benefit from consideration of regional trends and the distributional change of black duck.

Habitat selection and activities of dabbling ducks during non‐breeding periods

ABSTRACTWestern Tennessee is an important region for waterfowl during non‐breeding periods, supporting >40% of the Mississippi Flyway population of American black ducks (Anas rubripes). Understanding habitat selection and activities of waterfowl during the non‐breeding period is important for directed habitat management on national wildlife refuges and in other wetlands important in meeting regional waterfowl conservation objectives. During November–February 2011–2013, we investigated diurnal habitat selection and activities of dabbling ducks (Anatini) among 5 common wetland types relative to emergent cover, water depth, and energetic carrying capacity (i.e., duck energy days [DEDs]) in western Tennessee, USA. We observed waterfowl daily and sampled food resources monthly at Tennessee and Cross Creeks National Wildlife Refuges. Mallard (Anas platyrhynchos), gadwall (A. strepera), northern pintail (A. acuta), and American green‐winged teal (A. carolinensis) selected moist‐soil wetlands over wooded, aquatic bed, and open water wetland types. Gadwall also selected deeper wetlands containing submersed aquatic vegetation. Foraging was the dominant activity of all dabbling ducks in mudflats and moist‐soil wetlands, and it was also dominant in wooded wetlands for green‐winged teal and gadwall. Deep, open water was avoided by dabbling ducks. Selection of wetland types was negatively correlated with increasing water depth and positively correlated with increasing emergent cover and DEDs. Shallowly flooded moist‐soil and wooded wetlands provide high‐energy foods and dense emergent cover, and are important to a diversity of dabbling ducks during winter. © 2017 The Wildlife Society.

Agricultural food resources and the foraging ecologies of American black ducks (Anas rubripes) and mallards (Anas platyrhynchos) at the northern limits of their winter ranges

Atlantic Canada is the northern limit of the American black duck (Anas rubripes) wintering range, and more recently, the mallard (A. platyrhynchos) wintering range. Atlantic Canada has experienced considerable urban and agricultural development over the last century, and wintering black ducks and mallards appear to be using these habitats, in addition to traditional coastal habitat to survive the winters. This paper combines digestive tract content and stable isotope analysis to provide a comprehensive analysis of winter black duck and mallard diet in Atlantic Canada, and compares mallard and black duck winter diet in agricultural and coastal areas. Coastal black ducks wintering in Atlantic Canada had a diet consisting mainly of marine invertebrates and wild plant matter, while coastal mallards appeared to rely less on invertebrates and more on plant matter through winter. As winter progressed, the agricultural black ducks relied more on the feed provided by the farmer at our study site, indicated by a significant decline in their δ15N ratios. Mallards at the agricultural site maintained a low and steady δ15N ratio through winter, indicating a less diverse winter diet than sympatrically-wintering black ducks. Urban black ducks had a diet consisting entirely of vegetation and anthropogenic food supplies, which appeared to meet their nutritional requirements for winter survival. Our research highlights the importance of agricultural and urban habitats for wintering black ducks and mallards in Atlantic Canada.

Carrying capacity of wintering American black ducks in forested wetlands

ABSTRACTWintering populations of American black ducks (Anas rubripes) have declined throughout their range since the 1950s. Conservation for wintering black ducks is based largely on bioenergetic models of carrying capacity, and managers seek to provide wetlands with sufficient food energy to support target populations. Although the significance of salt marsh to wintering black ducks along the Atlantic Coast is well documented, forested wetlands are also used by wintering black ducks, and yet their foraging value and energetic potential is unknown. Therefore, quantifying the energetic value of forested wetlands is important for accurately estimating the carrying capacity of black ducks. We collected soil core samples from forested wetlands on Prime Hook National Wildlife Refuge, Milton, Delaware in 2014 (n = 46) and Edwin B. Forsythe National Wildlife Refuge, Galloway, New Jersey, USA in 2015 (n = 41) to estimate food biomass and quantify the energetic capacity of forested wetlands representative of the mid‐Atlantic region. We estimated the mean energetic value of forested wetlands as 84,667 ± 21,822 kcal/ha. Considering all forested wetlands that are potentially available to black ducks along the mid‐Atlantic Coast, we estimated that forested wetlands within a 24.1‐km distance from the coast provide between 103,696,524–192,678,224 duck‐use days supporting 489,135–908,860 black ducks over the 212 days of winter. Forested wetlands may be an important foraging resource for wintering black ducks, and our estimates of energetic value will help to refine bioenergetic models for black ducks in the Atlantic Flyway. © 2017 The Wildlife Society.

Identification of Type A Influenza Viruses from Wild Birds on the Delmarva Peninsula, 2007-10.

Wild waterfowl and shorebirds in the Delaware-Maryland-Virginia (Delmarva) Peninsula region within the Atlantic Flyway were sampled as part of the Early Detection of Highly Pathogenic H5N1 Avian Influenza (AI) in Wild Migratory Birds program. The U.S. Department of Agriculture (USDA) and state wildlife agencies submitted 7858 samples for AI virus (AIV) testing by real-time reverse transcription PCR (rRT-PCR) to the University of Delaware Poultry Health System from April 2007 to March 2011. Virus isolation attempts were performed on samples with matrix gene cycle threshold (Ct) values ≤33.9. Using rRT-PCR, AIV was detected in 14% (1091/7857) of the samples. In species with sample sizes >100, American black duck ( Anas rubripes ; 28%), ruddy turnstone ( Arenaria interpres ; 27%), American green-winged teal ( Anas crecca ; 21%), semipalmated sandpiper ( Calidris pusilla ; 27%), greater snow goose ( Chen caerulescens atlanticus; 12%), mallard ( Anas platyrhynchos ; 10%), and northern pintail ( Anas acuta ; 14%) showed the highest rates of AIV detection. Forty-two AIVs were recovered from eight species: American black duck, mallard, ruddy turnstone, American green-winged teal, greater snow goose, Canada goose ( Branta canadensis ), ring-necked duck ( Aythya collaris ), and mallard × American black duck ( Anas platyrhynchos × Anas rubripes ). Recovered H5 (n = 2) and H7 (n = 2) viruses were found to be low pathogenicity by the USDA National Veterinary Services Laboratory. Additional AIVs represented a diversity of subtype combinations: H1-H4, H6, and H10 and H11 and N subtypes N1-N9 and N6-N9. The rate of AIV recovery from swabbings was inversely related to Ct value, ranging from 50% for Ct values of 16.0-18.9 to 5.1% for Ct values of 31-33.9.

Disentangling density‐dependent dynamics using full annual cycle models and Bayesian model weight updating

Summary Density dependence regulates populations of many species across all taxonomic groups. Understanding density dependence is vital for predicting the effects of climate, habitat loss and/or management actions on wild populations. Migratory species likely experience seasonal changes in the relative influence of density dependence on population processes such as survival and recruitment throughout the annual cycle. These effects must be accounted for when characterizing migratory populations via population models. To evaluate effects of density on seasonal survival and recruitment of a migratory species, we used an existing full annual cycle model framework for American black ducks Anas rubripes, and tested different density effects (including no effects) on survival and recruitment. We then used a Bayesian model weight updating routine to determine which population model best fit observed breeding population survey data between 1990 and 2014. The models that best fit the survey data suggested that survival and recruitment were affected by density dependence and that density effects were stronger on adult survival during the breeding season than during the non‐breeding season. Analysis also suggests that regulation of survival and recruitment by density varied over time. Our results showed that different characterizations of density regulations changed every 8–12 years (three times in the 25‐year period) for our population. Synthesis and applications. Using a full annual cycle, modelling framework and model weighting routine will be helpful in evaluating density dependence for migratory species in both the short and long term. We used this method to disentangle the seasonal effects of density on the continental American black duck population which will allow managers to better evaluate the effects of habitat loss and potential habitat management actions throughout the annual cycle. The method here may allow researchers to hone in on the proper form and/or strength of density dependence for use in models for conservation recommendations.

Addressing variability in estuarine food density for American black ducks

ABSTRACTHabitat limitation and availability of food energy may be the cause of decline in American black duck (Anas rubripes) populations. Estimating food availability is a critical step in developing winter carrying‐capacity estimates for black ducks. Recent research has estimated the biomass and energy supply of winter black duck foods in coastal marshes using a single‐core sampling method, but estimates had large variability. We tested whether taking bulk core samples (i.e., homogenizing multiple core samples and subsampling a single core from it) at the same location (vs. a single core sample) would alter the mean and variance in food estimates in 7 different landscape–vegetation types (hereafter, habitat types: mudflat, subtidal, low marsh, high marsh, quasi‐tidal pools, a tidal impoundment, and a freshwater impoundment) in coastal New Jersey, USA, during 2011. In all but one habitat type, there were no differences in the mean biomass or the mean energy density estimates for the single core samples and the bulk core samples, across all food types. Variance was reduced in subtidal and quasi‐tidal pool habitats, but variance increased in tidal impoundments. Assessing time and cost‐efficiency of bulk samples over single samples, cost per sample increased 4–12%/core. Because we observed little difference in food biomass, energy, and their respective variance estimates between single and bulk methods, while recording a slightly greater cost, we recommend researchers use the single‐core sampling method to save both time and money. Further reductions in variance will likely need to be achieved through increased sample sizes. © 2016 The Wildlife Society.

Updating movement estimates for American black ducks (Anas rubripes).

Understanding migratory connectivity for species of concern is of great importance if we are to implement management aimed at conserving them. New methods are improving our understanding of migration; however, banding (ringing) data is by far the most widely available and accessible movement data for researchers. Here, we use band recovery data for American black ducks (Anas rubripes) from 1951–2011 and analyze their movement among seven management regions using a hierarchical Bayesian framework. We showed that black ducks generally exhibit flyway fidelity, and that many black ducks, regardless of breeding region, stopover or overwinter on the Atlantic coast of the United States. We also show that a non-trivial portion of the continental black duck population either does not move at all or moves to the north during the fall migration (they typically move to the south). The results of this analysis will be used in a projection modeling context to evaluate how habitat or harvest management actions in one region would propagate throughout the continental population of black ducks. This analysis may provide a guide for future research and help inform management efforts for black ducks as well as other migratory species.

Winter survival of female American Black Ducks in Tennessee, USA

ABSTRACT Many migratory bird species, including the American Black Duck (Anas rubripes; hereafter black duck), face challenges to their survival during winter due to potentially limited resources and high energetic demands. These winter processes can be especially important for the population dynamics of migratory species. Despite stabilization of the overall black duck breeding population, historical declines continue for black ducks wintering in the Mississippi Flyway. It remains unclear if declining abundance in this region reflects high winter mortality. We radio-tagged 111 females in the Tennessee National Wildlife Refuge (TNWR), a major wintering area for mid-continent Mississippi Flyway black ducks, from December to February of 2010–2012 to estimate winter survival and investigate factors that may influence survival rates, including body mass, age, hunting period, and weather. Winter survival (0.83–0.85) was greater than or comparable with previous estimates for black duck populations in North Amer...

A FULL ANNUAL CYCLE MODELING FRAMEWORK FOR AMERICAN BLACK DUCKS

AbstractAmerican black ducks (Anas rubripes) are a harvested, international migratory waterfowl species in eastern North America. Despite an extended period of restrictive harvest regulations, the black duck population is still below the population goal identified in the North American Waterfowl Management Plan (NAWMP). It has been hypothesized that density‐dependent factors restrict population growth in the black duck population and that habitat management (increases, improvements, etc.) may be a key component of growing black duck populations and reaching the prescribed NAWMP population goal. Using banding data from 1951 to 2011 and breeding population survey data from 1990 to 2014, we developed a full annual cycle population model for the American black duck. This model uses the seven management units as set by the Black Duck Joint Venture, allows movement into and out of each unit during each season, and models survival and fecundity for each region separately. We compare model population trajectories with observed population data and abundance estimates from the breeding season counts to show the accuracy of this full annual cycle model. With this model, we then show how to simulate the effects of habitat management on the continental black duck population.

A meta‐analysis of American black duck winter habitat use along the Atlantic Coast

ABSTRACTAmerican black duck (Anas rubripes) populations declined by more than 50% between the 1950s and 1990s, and the species serves as a flagship for conserving salt marsh habitats along the Atlantic Coast. Black ducks have generally been well studied throughout the annual cycle, but surprisingly, we lack a synthetic, quantitative understanding of their space use during the winter. This limits our ability to prioritize habitat acquisition and restoration efforts. We used >17,000 telemetry locations from 235 black ducks ranging from Connecticut to Virginia to study home range composition and space use during winter in relation to habitat quality, urbanization, and severe weather. Despite substantial environmental variation, home range sizes were similar among regions and years. Smaller home and core ranges contained a greater proportion of salt marsh habitat, and ducks experiencing more 4‐day freeze events had larger home and core ranges. Ducks exposed to prolonged periods of cold weather had smaller core ranges when those areas comprised more energy‐rich freshwater habitats. When we examined individual telemetry locations, we found that ducks used irregularly inundated high marsh more at night, presumably for foraging, and urban habitats more during the day and evening crepuscular periods. We found that black ducks used regularly inundated low marsh less on days where the temperature never rose above freezing, and instead used subtidal areas and forested wetlands more. Finally, we found ducks were marginally more likely to use freshwater habitats during high tides. Our study confirms that black ducks depend on salt marsh for wintering habitat, and points to an unexpectedly important role for forested wetlands during periods of cold weather. We found no evidence that black ducks avoided urban areas or roads, which supports the inclusion of all available habitats in carrying capacity modeling. We emphasize that new hypothesis‐driven, local telemetry studies are needed to further elucidate the relationships between black duck movements and environmental variation, especially cold weather. Further, given that most remaining coastal wetlands are currently protected via state and federal lands, we suggest black duck habitat management should strive to acquire and restore brackish and forested wetlands in close proximity to coastal marshes. Published 2015. This article is a U.S. Government work and is in the public domain in the USA.

Waterfowl Surveys in Canaan Valley: 1979–1993

Abstract From 1979 through 1993, we surveyed the waterfowl of Canaan Valley by using vantage-point, helicopter, and canoe surveys. We conducted monthly vantage-point surveys from April through November at four sites, we completed monthly canoe surveys during May–July along a 10-mi (16-km) segment of the Blackwater River, and we conducted helicopter surveys covering all of the Valley's drainages in May, August, and November. The most abundant species of waterfowl were the Branta canadensis (Canada Goose), Anas platyrhynchos (Mallard), Aix sponsa (Wood Duck), and Anas rubripes (American Black Duck). Thirteen other duck species were sighted, most of which were recorded during the November helicopter surveys. Of the several species of wading birds observed, the most abundant were Ardea herodias (Great Blue Heron) and Butorides virescens (Green Heron).

Implications of uncertainty in true metabolizable energy estimates for estimating wintering waterfowl carrying capacities

Carrying capacity models for wintering waterfowl require estimates of energy availability based on food densities and true metabolizable energy (TME) of various food types. However, because TME values vary widely between studies, estimates of carrying capacity may be less precise than previously acknowledged. We explored how variation in TME values affected estimates of landscape-level energy availability for American black ducks (Anas rubripes), using 4 distinct approaches for assigning TME values to waterfowl food items collected over the winter period in 2011–2012 and 2012–2013: a “best practices” approach, which typically used average TMEs across species, a minimum and maximum reported values approaches, and a coarse-scale “order-average” approach. We found that all 4 approaches yielded significantly different estimates of energy availability across all saltmarsh habitat types. Additionally, we evaluated the potential management implications of variation in TME values by comparing energy supply on 1,223 ha of marsh in Prime Hook National Wildlife Refuge (DE, USA) using all 4 approaches for assigning TME values. We estimated carrying capacity and modeled depletion of energy on this refuge over a hypothetical wintering period. We found that even relatively small variations in TME values produced highly variable estimates of carrying capacity for the refuge. Thus, we recommend that researchers consider the inherent uncertainty in TME values of waterfowl foods, and explicitly include this variation in carrying capacity models. © 2015 The Wildlife Society.