North American Ducks Research Articles

Highly Pathogenic H5N1 Influenza A Virus (IAV) in Blue-Winged Teal in the Mississippi Flyway Is Following the Historic Seasonal Pattern of Low-Pathogenicity IAV in Ducks.

Highly pathogenic H5N1 (HP H5N1) influenza A virus (IAV) has been detected annually in North American ducks since its introduction during 2021, but it is unknown if this virus will follow the same seasonal and geographic patterns that have been observed with low-pathogenicity (LP) IAV in this reservoir. We monitored blue-winged teal in the Mississippi flyway prior to the detection of HP H5N1 and during two post-introduction migration cycles from spring 2022 to spring 2024, testing birds for infection and antibodies to IAV nucleoprotein (NP), hemagglutinin subtype H5, and neuraminidase subtype N1. Antigens representing clade 2.3.4.4b HP H5 and LP North American H5 were used for hemagglutination inhibition (HI) and virus neutralization (VN) tests for H5 antibodies. Virologic results were consistent with historic seasonal and geographic patterns reported for LP IAV with peak infections occurring in pre-migration staging areas in Minnesota during fall 2022. However, the high prevalence of the H5 subtype was exceptional compared to historic prevalence estimates at this same site and for the Mississippi flyway. HP H5N1 was detected on wintering areas in Louisiana and Texas during the fall of that same year and this was followed by an increase in estimated antibody prevalence to NP, H5, and N1 with no HP H5N1 detections during the wintering or spring migration periods of 2022/2023. HP H5N1 was not detected in Minnesota during fall 2023 but was detected from a single bird in Louisiana. However, a similar increase in antibody prevalence was observed during the winter and spring period of 2023 and 2024. Over the two migration cycles, there was a temporal shift in observed prevalence and relative titers against the H5 antigens with a higher proportion of ducks testing positive to the 2.3.4.4b H5 antigen and higher relative titer to that antigen compared to the representative LP North American H5 antigen. The seasonal and geographic patterns observed appear to be driven by population immunity during the migration cycle. Results support an initial high infection rate of HP H5N1 in blue-winged teal in the Mississippi flyway followed by a high prevalence of antibodies to NP, H5, and N1. Although prevalence was much reduced in the second migration cycle following introduction, it is not known if this pattern will persist in the longer term or affect historic patterns of subtype diversity in this reservoir.

Wildlife professionals' and graduate students' perceptions regarding scientific publishing

AbstractWithin academia and research, publishing peer‐reviewed articles is expected for dissemination of knowledge and is used as a measure of professional performance. However, few papers have been published on professionals' and graduate students' perceptions of student publication performance, how professionals encourage student publishing, and what types of publication barriers exist for graduate students to publish. In 2019, we emailed a survey to professional and student attendees of the 2013 and 2016 North American Duck symposia as a representative cadre of waterfowl and wildlife biologists. We surveyed 469 professionals and 98 students who attended the symposia. Response rates were 42% and 45% for professionals and students, respectively, and deemed reliable as response rates approached 50%. Fifty percent and 69% of professional respondents indicated they felt frustration motivating their Ph.D. and M.S. students to publish, respectively. Of strategies used to motivate graduate student publishing, 79% of students ranked providing congenial encouragement most effective, while 60% of professionals ranked playing a major role in drafting and editing as most effective. Both professionals and students considered lack of time during and outside work hours as barriers to publishing graduate work. Professionals and students agreed that asking students to sign a contract at time of their initial matriculation may be an effective strategy to publish. Information from this study adds to knowledge on graduate student publishing practices and can be used to improve methods to increase graduate student publication rates in wildlife science and conservation. Similar surveys can be conducted face‐to‐face or remotely at international wildlife and other ecological conferences to broaden application of our results. We encourage students to generate publications from their research by writing separate thesis/dissertation chapters to expedite the publication process.

Multi‐level habitat selection of boreal breeding mallards

AbstractCanada's western boreal forest is a vital breeding habitat for North American duck populations. This region has experienced considerable demand for its valuable natural resources (e.g., oil and gas, forestry) resulting in extensive industrial development (e.g., infrastructure), which is predicted to continue. The potential effects of industrial development on breeding ducks in the western boreal forest, however, remains largely unexplored. We used backpack harness global positioning system (GPS) transmitters to document habitat selection by breeding female mallards (Anas platyrhynchos) across a gradient of industrial development in the western boreal forest of Alberta, Canada. We modeled breeding home range (second order) selection and habitat selection within the home range (third order) using resource selection functions, and spatially predicted our models across the landscape to highlight important breeding habitat. Contrary to our predictions, breeding female mallards did not avoid all industrial development at the second and third orders. Females established home ranges (second order) with greater proportions of marsh, graminoid fen, and well pads, and lower proportions of forest. Within their home range (third order), females selected shrub swamp, graminoid fen, marsh, well pads, and borrow pits, and avoided open water, swamp, treed peatland, forest, forest harvest areas, and industrials (e.g., buildings). Females also selected habitat that was close to pipelines and roads. Our results suggest that the magnitude and direction of breeding season habitat selection by female mallards varies depending on the scale and landscape features, but current levels of industrial development within our study area still allowed for the establishment of breeding season home ranges. Our spatially predicted maps contribute to the increasing body of work surrounding boreal waterfowl ecology by highlighting important breeding habitat for mallards in Canada's western boreal forest.

Long-term trends and drought: Spatiotemporal variation in juvenile sex ratios of North American ducks.

Sex ratios affect population dynamics and individual fitness, and changing sex ratios can be indicative of shifts in sex‐specific survival at different life stages. While climate and landscape changes alter sex ratios of wild bird populations, long‐term, landscape scale assessments of sex ratios are rare. Further, little work has been done to understand changes in sex ratios in avian communities. In this manuscript, we analyze long‐term (1961–2015) data on five species of ducks across five broad climatic regions of the United States to estimate the effects of drought and long‐term trends on the proportion of juvenile females captured at banding. As waterfowl have a 1:1 sex ratio at hatch, we interpret changes in sex ratios of captured juveniles as changes in sex‐specific survival rates during early life. Seven of 12 species‐region pairs exhibited evidence for long‐term trends in the proportion of juvenile females at banding. The proportion of juvenile females at banding increased for duck populations in the western United States and typically declined for duck populations in the eastern United States. We only observed evidence for an effect of drought in two of the 12 species‐region pairs, where the proportion of females declined during drought. As changes to North American landscapes and climate continue and intensify, we expect continued changes in sex‐specific juvenile survival rates. More broadly, we encourage further research examining the mechanisms underlying long‐term trends in juvenile sex ratios in avian communities.

Reassessing perennial cover as a driver of duck nest survival in the Prairie Pothole Region

AbstractConservation plans designed to sustain North American duck populations prominently feature a key hypothesis stating that the amount of the landscape in perennial cover surrounding upland duck nests positively influences nest survival rates. Recent conflicting research testing this hypothesis creates ambiguity regarding which management actions to pursue and where to prioritize conservation delivery. We compared existing models and new formulations of existing models explaining spatiotemporal variation in nest survival using independent data documenting the fate of >20,000 duck nests within the Drift Prairie, Missouri Coteau, and Prairie Coteau physiographic regions of the United States Prairie Pothole Region during 2002–2018. Our results suggest an inconsistent relationship between perennial cover and survival of upland duck nests, which depended upon physiographic region and current and time‐lagged landscape and environmental conditions. The magnitude and direction of how perennial cover correlated with daily nest survival depended on its dominance as a landcover type. A positive relationship existed when perennial cover was a minor component of landcover in all physiographic regions (<30% of a 10.4‐km2 area) and, in the Drift Prairie and Prairie Coteau, when perennial cover was the dominant landcover type (>60%). A constant or negative relationship was predicted at locations of about 30–60% perennial cover. Additionally, environmental conditions (i.e., density of wetlands and estimated gross primary productivity in the previous year) moderated or enhanced the effect of perennial cover on nest survival, depending on physiographic region. Our finding of inconsistency in the relationship between perennial cover and nest survival contradicts the conservation premise that nest survival universally increases linearly when uplands are converted to perennial cover. Promoting policies and management actions designed to increase perennial cover can be expected to be situationally but not consistently associated with higher survival of upland duck nests.

Investigating the Diversity and Host Range of Novel Parvoviruses from North American Ducks Using Epidemiology, Phylogenetics, Genome Structure, and Codon Usage Analysis.

Parvoviruses are small single-stranded DNA viruses that can infect both vertebrates and invertebrates. We report here the full characterization of novel viruses we identified in ducks, including two viral species within the subfamily Hamaparvovirinae (duck-associated chapparvovirus, DAC) and a novel species within the subfamily Densovirinae (duck-associated ambidensovirus, DAAD). Overall, 5.7% and 21.1% of the 123 screened ducks (American black ducks, mallards, northern pintail) were positive for DAC and DAAD, respectively, and both viruses were more frequently detected in autumn than in winter. Genome organization and predicted transcription profiles of DAC and DAAD were similar to viruses of the genera Chaphamaparvovirus and Protoambidensovirus, respectively. Their association to these genera was also demonstrated by subfamily-wide phylogenetic and distance analyses of non-structural protein NS1 sequences. While DACs were included in a highly supported clade of avian viruses, no definitive conclusions could be drawn about the host type of DAAD because it was phylogenetically close to viruses found in vertebrates and invertebrates and analyses of codon usage bias and nucleotide frequencies of viruses within the family Parvoviridae showed no clear host-based viral segregation. This study highlights the high parvoviral diversity in the avian reservoir with many avian-associated parvoviruses likely yet to be discovered.

"North American Ducks, Geese & Swans Identification Guide" by Frank S. Todd, 2018. [book review

"North American Ducks, Geese & Swans Identification Guide" by Frank S. Todd, 2018. [book review

Strengthening links between waterfowl research and management

ABSTRACTWaterfowl monitoring, research, regulation, and adaptive planning are leading the way in supporting science‐informed wildlife management. However, increasing societal demands on natural resources have created a greater need for adaptable and successful linkages between waterfowl science and management. We presented a special session at the 2016 North American Duck Symposium, Annapolis, Maryland, USA on the successes and challenges of linking research and management in waterfowl conservation, and we summarize those thoughts in this commentary. North American waterfowl management includes a diversity of actions including management of harvest and habitat. Decisions for waterfowl management are structured using decision analysis by incorporating stakeholder values into formal objectives, identifying research relevant to objectives, integrating scientific knowledge, and choosing an optimal strategy with respect to objectives. Recently, the consideration of the value of information has been proposed as a means to evaluate the utility of research designed to meet objectives. Despite these advances, the ability to conduct waterfowl research with direct management application may be increasingly difficult in research institutions for several reasons including reduced funding for applied research and the lower perceived value of applied versus theoretical research by some university academics. In addition, coordination between researchers and managers may be logistically constrained, and communication may be ineffective between the 2 groups. Strengthening these links would help develop stronger and more coordinated approaches for the conservation of waterfowl and the wetlands upon which they depend. © 2017 The Wildlife Society.

Targeting waterfowl habitat restoration in the Prairie Pothole Region: A spatial analysis of marginal benefits and costs

Federal, state, and local government conservation program operators and policy-makers may wish to maximize benefits relative to costs, but have few economic measures to draw upon. The objective of this research is to estimate the costs of restoring and preserving prairie pothole wetlands and the value of their duck hunting benefits. The Prairie Pothole Region (PPR) lies in the north-central plains of the United States and produces 50% to 80% of all North American ducks. The ducks fly all major US flyways; hence they benefit hunters across the country. Historical data are used to estimate a wetland cost function that generates county-level estimates of the cost of restoring and preserving wetlands. Data availability limited the number of model variables so that coefficients are likely to be bias. Nonetheless, the model demonstrates predictive capability with an adjusted R2 of 0.74. To estimate benefits, we first developed geographic information system (GIS) models—one for each of five species—that predict the numbers of ducks produced by a new wetland by 1 km2 (0.4 mi2) grid point. Findings of previous research provide a quantitative link between changes in duck numbers and changes in hunting quality. We then use benefits transfer to value changes in hunting quality. Results reveal benefit-cost ratios ranging from near-zero to over nine.

Environmental variability and population dynamics: do European and North American ducks play by the same rules?

Density dependence, population regulation, and variability in population size are fundamental population processes, the manifestation and interrelationships of which are affected by environmental variability. However, there are surprisingly few empirical studies that distinguish the effect of environmental variability from the effects of population processes. We took advantage of a unique system, in which populations of the same duck species or close ecological counterparts live in highly variable (north American prairies) and in stable (north European lakes) environments, to distinguish the relative contributions of environmental variability (measured as between‐year fluctuations in wetland numbers) and intraspecific interactions (density dependence) in driving population dynamics. We tested whether populations living in stable environments (in northern Europe) were more strongly governed by density dependence than populations living in variable environments (in North America). We also addressed whether relative population dynamical responses to environmental variability versus density corresponded to differences in life history strategies between dabbling (relatively “fast species” and governed by environmental variability) and diving (relatively “slow species” and governed by density) ducks. As expected, the variance component of population fluctuations caused by changes in breeding environments was greater in North America than in Europe. Contrary to expectations, however, populations in more stable environments were not less variable nor clearly more strongly density dependent than populations in highly variable environments. Also, contrary to expectations, populations of diving ducks were neither more stable nor stronger density dependent than populations of dabbling ducks, and the effect of environmental variability on population dynamics was greater in diving than in dabbling ducks. In general, irrespective of continent and species life history, environmental variability contributed more to variation in species abundances than did density. Our findings underscore the need for more studies on populations of the same species in different environments to verify the generality of current explanations about population dynamics and its association with species life history.

Prevalence and genetic diversity of haematozoa in South American waterfowl and evidence for intercontinental redistribution of parasites by migratory birds.

To understand the role of migratory birds in the movement and transmission of haematozoa within and between continental regions, we examined 804 blood samples collected from eleven endemic species of South American waterfowl in Peru and Argentina for infection by Haemoproteus, Plasmodium, and/or Leucocytozoon blood parasites. Infections were detected in 25 individuals of six species for an overall apparent prevalence rate of 3.1%. Analysis of haematozoa mitochondrial DNA revealed twelve distinct parasite haplotypes infecting South American waterfowl, four of which were identical to lineages previously observed infecting ducks and swans sampled in North America. Analysis of parasite mitochondrial DNA sequences revealed close phylogenetic relationships between lineages originating from waterfowl samples regardless of continental affiliation. In contrast, more distant phylogenetic relationships were observed between parasite lineages from waterfowl and passerines sampled in South America for Haemoproteus and Leucocytozoon, suggesting some level of host specificity for parasites of these genera. The detection of identical parasite lineages in endemic, South American waterfowl and North American ducks and swans, paired with the close phylogenetic relationships of haematozoa infecting waterfowl on both continents, provides evidence for parasite redistribution between these regions by migratory birds.

Isolation of Type A Influenza Viruses from Red-necked Grebes (Podiceps grisegena)

Six type-A low pathogenic influenza viruses from 14 Red-necked Grebes (Podiceps grisegena) from Agassiz National Wildlife Refuge were sequenced. The grebe viruses were closely related to North American duck viruses. The genetic and temporal subtype consistency between the duck and grebe isolates suggest spillover events, potentially enhanced by feather eating.

Hydroclimate Variability and Change in the Prairie Pothole Region, the “Duck Factory” of North America*

Abstract The Prairie Pothole Region (PPR) of the northern Great Plains is a vital ecosystem responsible each year for producing 50%–80% of new recruits to the North American duck population. Climate variability and change can impact the hydrology and ecology of the region with implications for waterfowl populations. The historical relationship between PPR wetlands, duck populations, and seasonal hydroclimate are explored. Model experiments from phase 5 of the Coupled Model Intercomparison Project are used to determine whether a recent wetting trend is due to natural variability or changing climate and how PPR hydroclimate will change into the future. Year-to-year variations in May duck populations, pond numbers, and the Palmer drought severity index are well correlated over past decades. Pond and duck numbers tend to increase in spring following La Niña events, but the correlation is not strong. Model simulations suggest that the strengthening of the precipitation gradient across the PPR over the past century is predominantly due to natural variability and therefore could reverse. Model projections of future climate indicate precipitation will increase across the PPR in all seasons except summer, but this gain for surface moisture is largely offset by increased evapotranspiration because of higher temperatures and increased atmospheric evaporative demand. In summer, the combined effects of warming and precipitation changes indicate seasonal surface drying in the future. The presented hydroclimate analyses produce potential inputs to ecological and hydrological simulations of PPR wetlands to inform risk analysis of how this North American waterfowl habitat will evolve in the future, providing guidance to land managers facing conservation decisions.

The effects of a large‐scale wind farm on breeding season survival of female mallards and blue‐winged teal in the Prairie Pothole Region

ABSTRACTThe wetlands and grasslands of the Prairie Pothole Region (PPR) make it the most productive breeding habitat for North American ducks. The growth rate of mallard (Anas platyrhynchos) populations is sensitive to changes in survival of adult females during the breeding season. Much of the PPR is suitable for large‐scale wind‐energy development and collisions of breeding females with wind turbines may be a novel source of mortality in this area. We assessed the effects of wind energy on breeding female mallard and blue‐winged teal (A. discors) survival by monitoring 77 radio‐marked mallards and 88 blue‐winged teal during the 2009 and 2010 breeding seasons at the Tatanka Wind Farm (TWF) near Kulm, North Dakota. During the same period, we monitored 70 female mallards and 75 blue‐winged teal at an adjacent reference site without wind turbines (REF). We used an information‐theoretic approach to investigate relationships between female survival and site (TWF vs. REF), year (2009 vs. 2010), and date. Collision mortalities were rare. Only 1 radio‐marked female mallard and no blue‐winged teal collided with wind turbines. Most mortalities were caused by predators (78.3%; 36/46), irrespective of species and site. For mallards, the best‐approximating model indicated that breeding season survival was 1) lowest when a high proportion of radio‐marked females were incubating, and 2) dependent on year and site such that expected survival () in 2009 was higher at TWF ( = 0.90, 85% CI = 0.79–0.98) than at REF ( = 0.83, 85% CI = 0.68–0.95), but expected survival in 2010 was lower at TWF ( = 0.62, 85% CI = 0.46–0.79) than at REF ( = 0.84, 85% CI = 0.72–0.94). For blue‐winged teal, the constant model was the best‐approximating model and indicated that expected female survival was 0.75 (85% CI = 0.69–0.82). The most competitive model for blue‐winged teal that included the effect of wind turbines indicated that expected survival at TWF ( = 0.71, 85% CI = 0.62–0.79) was lower than survival at REF ( = 0.81, 85% CI = 0.73–0.89). The limited number of collisions observed for female mallards and blue‐winged teal nesting at TWF suggests that wind turbines had no direct effect on female survival. Thus, conservation strategies that include protection of wetland and grassland habitat in wind‐developed landscapes will most likely not cause a direct reduction in survival of breeding females due to collisions with wind turbines. © 2013 The Wildlife Society.

Harvest mortality in North American mallards: A reply to Sedinger and Herzog

AbstractSedinger and Herzog (2012) evaluated the evidence for the impact harvest mortality may have on North American duck populations. While doing that, they questioned our review (Pöysä et al. 2004) and conclusion that harvest mortality in North American mallards (Anas platyrhynchos) may have shifted from compensatory to additive over the period from the 1960s to the 1980s. In this reply, we correct Sedinger and Herzog's misrepresentations of our 2004 paper and argue that our interpretations of the results published at that time have not been invalidated. © 2013 The Wildlife Society.

The importance of prior choice in model selection: a density dependence example

Summary It is important to discern the magnitude of density dependence a species exhibits, as well as the time lag over which it operates. Knowledge of a species' likely response to natural as well as synthetic shocks will assist in effective species management. Statistically this is a challenging problem which does not usually admit closed‐form mathematical analysis. Consequently, many people have used Bayesian methods to fit state space models of density dependence to many different species, of which we take eleven species of North American duck as our motivating examples. A Bayesian analysis requires a choice of model and parameter prior. The latter is difficult to do without inducing bias in model selection, and we attempt to address this problem. Our priors will be obtained by considering which parameter values are representative of features we expect to see in the data, and which would produce unnatural behaviour. To fit the models, we use a novel sequential Monte Carlo method (particle learning) not previously applied to ecological data sets. We show that existing analyses on the duck data may have been susceptible to a common problem in Bayesian model selection (Lindley's paradox), and suggest methods for prior selection which mitigate this issue. We also discover that although it is possible to detect the existence of density dependence, it is unrealistic to expect to determine the time lag over which it operates without a great deal of data, even if said data are simulated from the model. We demonstrate that prior choices motivated by the above considerations can lead to substantially increased predictive accuracy over surprisingly long time scales whether model selection is of primary concern or not. We conclude from our analysis of real‐world data that there is little evidence of density dependence in many duck species, suggesting that such effects, if present, are likely to be small in magnitude.

Integrated modeling of communities: parasitism, competition, and demographic synchrony in sympatric ducks

Functionally similar species often co-occur within an ecosystem, and they can compete for or facilitate each other's access to resources. The coupled dynamics of such species play an important role in shaping biodiversity and an ecosystem's resilience to perturbations. Here we study two congeneric North American ducks: Redhead Aythya americana and Canvasback A. vaselineria. Both are largely sympatric during the breeding season, and in addition to competition, facultative parasitic egg-laying can lead to interspecific density dependence. Using multi-population integrated models, we combined capture-recovery data, population surveys, and age ratio data in order to simultaneously estimate the mechanistic drivers of fecundity, survival, and population dynamics for both species. Canvasback numbers positively affected Redhead fecundity, whereas Redhead numbers negatively affected Canvasback fecundity, as expected due to parasitism. This interaction was modulated by wetland habitat availability in a way that matched the observation that Redhead hens parasitize Canvasback nests under all conditions but exhibit typical nesting behavior more frequently during years with numerous ponds. Once these effects of density and habitat were statistically controlled for, we found high levels of interspecific synchrony in both fecundity and survival (respectively, 75% and 49% of remaining variation). Thus, both neutral and non-neutral mechanisms affected the dynamics of these functionally similar species. In this and other systems, our method can be used to test hypotheses about species coexistence and to gain insights into the demographic drivers of community dynamics.

Harvest and dynamics of duck populations

AbstractThe role of harvest in the dynamics of waterfowl populations continues to be debated among scientists and managers. Our perception is that interested members of the public and some managers believe that harvest influences North American duck populations based on calls for more conservative harvest regulations. A recent review of harvest and population dynamics of North American mallard (Anas platyrhynchos) populations (Pöysä et al. 2004) reached similar conclusions. Because of the importance of this issue, we reviewed the evidence for an impact of harvest on duck populations. Our understanding of the effects of harvest is limited because harvest effects are typically confounded with those of population density; regulations are typically most liberal when populations are greatest. This problem also exists in the current Adaptive Harvest Management Program (Conn and Kendall 2004). Consequently, even where harvest appears additive to other mortality, this may be an artifact of ignoring effects of population density. Overall, we found no compelling evidence for strong additive effects of harvest on survival in duck populations that could not be explained by other factors. © 2012 The Wildlife Society.

Demographic response to perturbations: the role of compensatory density dependence in a North American duck under variable harvest regulations and changing habitat

1. Most wild animal populations are subjected to many perturbations, including environmental forcing and anthropogenic mortality. How population size varies in response to these perturbations largely depends on life-history strategy and density regulation. 2. Using the mid-continent population of redhead Aythya americana (a North American diving duck), we investigated the population response to two major perturbations, changes in breeding habitat availability (number of ponds in the study landscape) and changes in harvest regulations directed at managing mortality patterns (bag limit). We used three types of data collected at the continental scale (capture-recovery, population surveys and age- and sex ratios in the harvest) and combined them into integrated population models to assess the interaction between density dependence and the effect of perturbations. 3. We observed a two-way interaction between the effects on fecundity of pond number and population density. Hatch-year female survival was also density dependent. Matrix modelling showed that population booms could occur after especially wet years. However, the effect of moderate variation in pond number was generally offset by density dependence the following year. 4. Mortality patterns were insensitive to changes in harvest regulations and, in males at least, insensitive to density dependence as well. We discuss potential mechanisms for compensation of hunting mortality as well as possible confounding factors. 5. Our results illustrate the interplay of density dependence and environmental variation both shaping population dynamics in a harvested species, which could be generalized to help guide the dual management of habitat and harvest regulations.

Susceptibility of avian influenza viruses of the N6 subtype to the neuraminidase inhibitor oseltamivir

Avian influenza viruses are a source of genetic material that can be transmitted to humans through direct introduction or reassortment. Although there is a wealth of information concerning global monitoring for antiviral resistance among human viruses of the N1 and N2 neuraminidase (NA) subtypes, information concerning avian viruses of these and other NA subtypes is limited. We undertook a surveillance study to investigate the antiviral susceptibility of avian influenza N6 NA viruses, the predominant subtype among wild waterfowl. We evaluated 73 viruses from North American ducks and shorebirds for susceptibility to the NA inhibitor oseltamivir in a fluorescence-based NA enzyme inhibition assay. Most (90%) had mean IC50 values ranging from <0.01 to 5.0nM; 10% were from 5.1 to 50.0nM; and none were >50.0nM. Susceptibility to oseltamivir remained stable among all isolates collected over approximately three decades (P⩽0.74). Two isolates with I222V NA substitution had moderately reduced susceptibility to oseltamivir in vitro (IC50, 30.0 and 40.0nM). One field sample was a mixed population containing an avian paramyxovirus (APMV) and H4N6 influenza virus, as revealed by electron microscopy and hemagglutination inhibition assays with a panel of anti-APMV antisera. This highlights the importance of awareness and careful examination of non-influenza pathogens in field samples from avian sources. This study showed that oseltamivir-resistant N6 NA avian influenza viruses are rare, and must be tested both phenotypically and genotypically to confirm resistance.