Health effects following chronic dosing with tungsten-iron and tungsten-polymer shot in adult game-farm mallards

Permanent approval of shot composed of tungsten-iron and tungsten-polymer for waterfowl hunting by the U.S. Fish and Wildlife Service was pending the results of the present study that examined the health and reproductive effects of the two shot types on mallards (Anas platyrhynchos) over a 150-day period. We collected data pertaining to the effects of tungsten-iron and tungsten-polymer shot on mortality, body weight, organ weight, tissue pathology, and shot erosion. Thirty-two bird groups (sexes equal) of adult mallards were dosed orally with eight #4 steel shot (control), eight #4 tungsten-iron shot, or eight #4 tungsten-polymer shot on days 0, 30, 60, 90, and 120 of a 150-day trial (26 January 1998 to 25 June 1998). An additional 12 mallards (sexes equal) were dosed orally with eight #4 lead shot (positive control) on day 0 of the study. All lead-dosed ducks died by day 25, whereas no ducks died in the other treatment groups. Significant liver hemosiderosis was present in all control and tungsten-iron-dosed males, in five of eight control and three of eight tungsten-iron-dosed females, and in one tungsten-polymer-dosed male examined. The rate of shot erosion was highest for tungsten-polymer shot (99%), followed by tungsten-iron (72%), and steel (55%) shot. Tungsten-iron or tungsten-polymer shot repeatedly administered to adult mallards did not have deleterious health effects during the 150-day trial based on mortality, body weights, organ weights, and histology of the liver and kidneys.     

Hematological effects and metal residue concentrations following chronic dosing with tungsten-iron and tungsten-polymer shot in adult game-farm mallards

The U.S. Fish and Wildlife Service required a chronic dosing study that assessed the health and reproductive effects of tungsten-iron and tungsten-polymer shot in adult game-farm mallards (Anas platyrhynchos) prior to granting permanent approval of the shot for waterfowl hunting. Herein, we present the effects of tungsten-iron and tungsten-polymer shot on various hematologic parameters and metal residue concentrations in the femur, liver, kidneys, and gonads. Thirty-two-bird groups (sexes equal) of adult mallards were dosed orally with eight #4 steel shot (control), eight #4 tungsten-iron shot, or eight #4 tungsten-polymer shot on days 0, 30, 60, 90, and 120 of a 150 day trial (26 January 1998 to 25 June 1998). An additional 12 mallards (sexes equal) received eight #4 lead shot (positive control) on day 0 of the study. Lead-dosed mallards had significantly decreased hematocrit, hemoglobin concentration, and whole-blood delta aminolevulinic acid dehydratase activity on day 7, as well as significant changes in a number of plasma chemistry parameters compared to ducks in the control, tungsten-iron, or tungsten-polymer groups. Mallards dosed with tungsten-iron or tungsten-polymer shot had occasional significant differences in hematocrit and plasma chemistry values when compared to control mallards over the 150 day period, but these changes were not considered to be indicative of deleterious effects. Low concentrations of tungsten were detected in gonad and kidney samples from males and females and in liver samples from females dosed with tungsten-polymer shot. Tungsten was also detected in femur samples from tungsten-polymer-dosed mallards. Higher concentrations of tungsten were detected in femur, liver, kidney, and gonad samples from tungsten-iron-dosed ducks. Tungsten-iron or tungsten-polymer shot repeatedly administered to adult mallards did not cause adverse hematological effects during the 150 day trial. Concentrations of tungsten in the femur, liver, kidneys, and gonads were generally higher in tungsten-iron-dosed ducks when compared to tungsten-polymer-dosed ducks.     

Acute toxicity of lead, steel, and an iron-tungsten-nickel shot to mallard ducks (Anas platyrhynchos)

Twenty mallards (Anas platyrhynchos) of both sexes were dosed by oral gavage with Heavi-Shot (H-S; Environ-Metal, Inc., Sweet Home, Oregon, USA) pellets, 20 with steel shot, and 10 with lead (Pb) pellets, all of equal size. All pellets were fired from a shotgun into an absorbent material, retrieved, and weighed prior to introduction into the ducks. Birds were fed whole kernel corn and grit and observed for signs of toxicity for 30 days following dosing. Hevi-Shot pellets lost an average of 6.2% of their mass and steel shot pellets lost 57% of their mass in the birds' gizzards. Almost all (90%) of the Pb shot dosed birds died before the end of the study, while no mortality was observed in the steel or H-S dosed groups. Even though total food consumption differed between the H-S and steel shot groups, mean bird weight change was not different. There were no significant morphologic or histopathologic abnormalities of the liver and kidney in the H-S and steel shot groups. Results indicated that mallards dosed orally with eight No. 4 H-S pellets were not adversely affected over a 30-day period, and that H-S provides another environmentally safe nontoxic shot for use in waterfowl hunting.     

Comparative toxicity of lead shot in black ducks (Anas rubripes) and mallards (Anas platyrhynchos)

In winter, pen-reared and wild black ducks (Anas rubripes), and game farm and wild mallards (Anas platyrhynchos), maintained on pelleted feed, were sham-dosed or given one number 4 lead shot. After 14 days, dosed birds were redosed with two or four additional lead shot. This dosing regimen also was repeated in summer using pen-reared black ducks and game farm mallards. Based upon mortality, overt intoxication, weight change, delta-aminolevulinic acid dehydratase activity and protoporphyrin concentration, black ducks and mallards were found to be equally tolerant to lead shot. However, captive wild ducks were more sensitive than their domesticated counterparts, as evidenced by greater mortality and weight loss following lead shot administration. This difference may be related to stress associated with captivity and unnatural diet.     

Influence of diet on the hematology and serum biochemistry of zinc-intoxicated mallards

Changes in hematological and serum biochemistry parameters in female zinc (Zn)-dosed farm-raised mallards (Anas platyrhynchos) fed four different diets were examined. Sixty ducks received an average dose of 0.97 g of Zn in the form of eight, 3.30-mm diameter shot pellets containing 98% Zn and 2% tin, and another 60 ducks were sham-dosed as controls. Fifteen ducks from each of the two dosing groups were assigned to one of four dietary treatments: corn only, corn with soil, commercial duck ration only, or commercial duck ration with soil. Shot-pellet dissolution rates ranged from 7 mg/Zn/day to 27 mg/Zn/day. Regardless of diet, the Zn dose resulted in mortality; incoordination; paralysis and anorexia; decreased body, liver, pancreas, gonad, and gizzard weight; increased kidney weight; and macroscopic lesions. Zn-dosed ducks had a lower mean erythrocyte packed cell volume (PCV), higher mean reticulocyte count, and a greater number of individuals with immature and/or abnormal erythrocytes, than did control mallards. Mean total leucocyte counts were higher in Zn-dosed ducks than in controls. Zn-dosed ducks that had soil available had higher leucocyte counts than those without soil. Zn-dosed ducks were characterized by a marked heterophilia and relative lymphopenia. In Zn-dosed ducks, the mean lymphocyte count was highest in those provided a commercial duck ration, and lowest in those fed corn. In control ducks, the mean lymphocyte count was highest in ducks fed corn, and lowest in those provided soil along with a commercial duck ration. Zn-dosed mallards had higher serum aspartate aminotransferase and amylase levels, and lower alkaline phosphatase activities than control ducks. Serum phosphorus and uric acid concentrations were higher, and calcium, glucose, and total protein levels lower, in Zn-dosed ducks than in control ducks. Diet did affect serum calcium, phosphorus, total protein, and uric acid concentrations. Differences in erythrocyte and leucocyte parameters, serum enzyme activities, and metabolite concentrations were associated with dose and diet effects. Diets high in protein and other organic matter and calcium and phosphorus did not prevent or substantially alleviate Zn toxicosis in farm-raised mallard ducks.     

克隆鸭乙型肝炎病毒DNA双体体内转染的研究

用一种含头尾相连DHBVDNA双体的质粒体内转染2日龄芙蓉鸭,大多数鸭（86％）产生了短暂病毒血症。血清DHBs／preSAg和DHBVDNA于转染后第9天出现,第12～14天达峰值,第28天时多数转阴;少数鸭的病毒血症可持续50天以上。转染鸭肝组织中也检测到复制中间型DHBVDNA的存在。用转染鸭病毒血症期的血清作磷钨酸负染电镜观察,找到了完整的DHBV病毒颗粒,并且用此血清腹腔注射1日龄鸭,60％的鸭被感染成功,证明体内转染后有生物活性的DHBV病毒颗粒的产生。该研究方法的建立．对于研究DHBV变异株．DHBV基因结构与功能的关系等,均有一定理论意义及应用价值。     

Inhibition of adult social behaviour in ducks induced by juvenile administration of gonadal hormones

This experiment was designed to study the long-term behavioural effects of gonadal hormone injections into domestic ducklings. Ten male and four female ducklings were injected daily from their 4th day onwards: the males with testosterone propionate and the females with oestradiol benzoate. Five males were injected for 40 days, the other ducklings (5♀ + 4♂) for 80 days. Behavioural effects of these treatments were assessed when the birds were six months old. Social displays were strongly inhibited in the injected males as compared with control birds but no general significant effect was observed for the aggressive and sexual behaviour. The inhibition of social displays is consistent with the view that these behaviour patterns are regulated by the pituitary-gonadal axis and the different levels at which the functional inhibition could have occured are discussed. Far fewer social displays were performed by the males presented with injected females which, on the other hand, showed less sexual behaviour than the control females. This supports the idea that the female plays an important part in the social display of ducks.     

A comparison of two methods to establish the prevalence of lead shot ingestion in mallards (Anas platyrhynchos) from The Netherlands

Two collection methods for screening the mallard (Anas platyrhynchos) population in the Netherlands for the ingestion of spent lead shot were compared. One method consisted of examination of gizzards from mallards shot by hunters (n = 2,859) and the other method consisted of examination of gizzards from mallards caught in duck traps (n = 865). The 95% confidence interval of lead shot ingestion in the mallard population estimated by the first method was 1.7 to 2.9% and by the second method 1.1 to 3.1%. These values were not significantly different. From the numbers of lead pellets embedded in the gizzard wall in hunter-killed and trapped mallards it was estimated that at least 22 to 68% of the trapped ducks had been hit by lead shot previously, but survived. Furthermore, this study shows that it is reasonable to assume that a substantial part of the pellets which are identified (in this study and other studies) as ingested, may well have been shot into the gizzard lumen at some time before the birds were actually killed. To avoid lead poisoning in mallards and in raptors depredating waterfowl hit by lead shot, a change to steel shot is advocated.     

The influence of seven environmental and physiological factors on duck plague virus shedding by carrier mallards

Duck plague (DP) carrier mallards (Anas platyrhynchos) were subjected to seven environmental and physiological conditions in an attempt to stimulate DP virus shedding. The conditions were: food quality, social interaction, reproductive state, time dependency of food and water, noise, exercise, and sex of bird. Cloacal and oral swabs were taken daily for 10 days and assayed for DP virus content. The stimulated carrier ducks shed DP virus intermittently in amounts up to 10(8) ffus/swab/day (the highest 10-fold dilution still showing specific fluorescence). Unstimulated DP carrier ducks shed only up to 10(3) ffus/swab/day. Reproductive state and exercise were the only two factors that acted in concert to stimulate the shedding of virus in oral secretions.     

A Century of Hybridization: Decreasing Genetic Distance Between American Black Ducks and Mallards

American black ducks (Anas rubripes) and mallards (A. platyrhynchos) are morphologically and behaviorally similar species that were primarily allopatric prior to European colonization of North America. Subsequent sympatry has resulted in hybridization, and recent molecular analyses of mallards and black ducks failed to identify two distinct taxa, either due to horizontal gene flow, homoplasy, or shared ancestry. We analyzed microsatellite markers in modern and museum specimens to determine if the inter-relatedness of mallards and black ducks was an ancestral or recent character. Gst, a measure of genetic differentiation, decreased from 0.146 for mallards and black ducks living before 1940, to 0.008 for birds taken in 1998. This is a significant reduction in genetic differentiation, and represents a breakdown in species integrity most likely due to hybridization. Using modern specimens, we observed that despite a lower incidence of sympatry, northern black ducks are now no more distinct from mallards than their southern conspecifics.     

Carbon and nitrogen isotopes trace nutrient exchange in an ant-plant mutualism

By combining and reanalysing data from two independent field experiments we explore whether food limitation at the brood stage affects habitat selection in nesting mallards (Anas platyrhynchos). In an introduction experiment we found that, independent of treatment, some study lakes remained empty of wild mallard pairs (”empty lakes”), whereas on other lakes introduced birds attracted wild mallards (”attractive lakes”). In the other experiment we used mallard ducklings to address brood-stage food limitation by studying mass change of ducklings. We found that ducklings foraging on lakes that did not attract wild mallard pairs in the introduction experiment gained much less mass than those foraging on attractive lakes. In most cases ducklings even lost mass in the empty-lake foraging trials, providing strong evidence for food limitation. Therefore, lakes that remained empty of wild mallard pairs in the introduction experiment proved to be inferior brood habitats, particularly in terms of food. Our results give insight into the mechanisms underlying the general habitat selection hypotheses, specifically the ideal preemptive and conspecific attraction rules. The results further support our earlier conclusion that mallards do not use the ideal preemptive rule when selecting nesting lakes. However, conspecific attraction may not be generally applicable either, because, independent of the presence of introduced conspecifics, wild mallards somehow anticipated the low quality of the empty lakes as brood-rearing habitats and made their habitat-selection decision accordingly. Received: 22 March 1999 / Accepted: 7 October 1999     

Genetic structure of Eurasian and North American mallard ducks based on mtDNA data

To elucidate the origin and genetic structure of the domesticated duck in Eurasia and North America, we sequenced 114 duck D-loop sequences and retrieved 489 D-loop sequences from GenBank. In total, 603 ducks including 50 duck breeds/populations from eight countries (China, France, Russia, India, Kazakhstan, Mongolia, Thailand and USA) were used in this study. One hundred and thirty-four haplotypes and 81 variable sites were detected. H49 was the predominant haplotype, which was considered to be the same dominant haplotype found in the previous studies, and was found in 309 birds. The smallest values for both genetic differentiation index (F(ST), 0.04156) and the number of the net nucleotide substitutions between two populations (D(A), 0.00018) were observed between Eurasian domestic ducks and Eurasian mallards. No geography, breed or population clusters were observed in the Eurasian domestic ducks and mallards. Five haplotypes were shared by USA mallards and Eurasian domestic duck/Eurasian mallards. Only one haplotype (H49) was shared by Eurasian domestic ducks and China spot-billed ducks. By combining phylogenetic analyses, haplotype network profile, genetic distances and shared haplotypes, we can draw two major conclusions: (i) Eurasian and North American mallards show a clear geographic distribution pattern; (ii) Eurasian domestic ducks are derived from the Eurasian mallards, not from the spot-billed ducks.     

Mallard ducklings (Anas platyrhynchos) experimentally infected with Echinostoma trivolvis (Digenea)

Of 8, day-old mallard ducklings, each fed 50 encysted metacercariae of Echinostoma trivolvis, 4 (50%) were infected 15-31 days postinfection (PI) with a total of 10 (2.5%) worms. The worms were attached loosely to the mucosa of the lower ileum and rectum-cloaca, and some were ovigerous by day 15 PI. Ducklings, 4-14 days old when fed encysted metacercariae, became infected with E. trivolvis adults, but ducks 150 days old were refractory to infection. Compared to our previous studies on experimental infections of echinostomes, mallard ducklings were less susceptible than golden hamsters to E. trivolvis.     

Effects of ingested lead on antibody production in mallards (Anas platyrhynchos)

We examined the effects of lead ingestion on in vitro and in vivo indices of immune function in mallards (Anas platyrhynchos). Twenty-four mallard drakes were randomly divided into three groups (I, II, III), then assigned to treatment or control subgroups (n = 4). On day 0, all treatment birds were dosed orally with one number 4 lead shot (mean = 0.2 g). We challenged all individuals in each group with washed sheep red blood cells (SRBC) injected intraperitoneally on days 0 (Group I), 7 (Group II) or 14 (Group III), and collected blood for analyses 7 and 8 days after SRBC challenge. We measured and compared blood lead concentrations, in vitro lymphocyte transformation responses to phytohemagglutinin A and lipopolysaccharide, and hemagglutination titers to SRBC. Mean blood lead concentrations were elevated (P less than or equal to 0.04) in treatment birds at each sampling period. Large individual variability in lymphocyte stimulation responses precluded further analysis of those data. Hemagglutination titers to SRBC were lower (P less than 0.0001) in lead-poisoned ducks than in controls, suggesting that ingested lead may have immunosuppressive effects on mallards.     

A Comparison of Hybridization between Mottled Ducks (<Emphasis Type="Italic">Anas fulvigula</Emphasis>)and Mallards (<Emphasis Type="Italic">A. platyrhynchos</Emphasis>) in Florida and South Carolina using Microsatellite DNA Analysis

Interspecific hybridization has been implicated in population declines for some waterfowl species within the mallard complex, and hybridization with mallards (Anas platyrhynchos) is currently considered the largest threat to mottled ducks (A. fulvigula), one North American member of that complex. We assessed genetic variation among 225 mottled ducks and mallards using five microsatellite loci, and detected significant overall differences between these species within two geographic areas. We characterized hybridization in Florida, where mottled ducks are endemic and mallards are beginning to appear on the breeding grounds, and in South Carolina, where mottled ducks were introduced outside their native range. We used Bayesian genetic mixture analysis in an attempt to distinguish between these closely related species. In Florida, we detected two distinct genetic groups, and 10.9% of our samples from Florida mottled ducks were inferred to have been hybrids. In contrast only 3.4% of Florida mallards were inferred to have been hybrids, suggesting asymmetric hybridization. Populations from different geographic areas within Florida exhibited hybridization rates ranging from 0% to 24%. These data indicate a genetic component would be appropriate in actively managing interspecific hybridization in Florida mottled ducks. In contrast, South Carolina mottled ducks and mallards cannot be differentiated.     

Mottled ducks,feral mallards,and their hybrids in Florida

The nonmigratory and endemic Florida mottled duck (Anas fulvigula fulvigula) is facing conservation threats from the combined effects of urbanization and introgressive hybridization with feral mallards (Anas platyrhynchos) and mallard x mottled duck hybrids. In the past, the status of the Florida mottled duck population was assessed during annual aerial surveys and most brown ducks (mottled ducks, mallards, and hybrids of them) detected during the survey would have been mottled ducks. But the release of domesticated mallards for aesthetic purposes has led to increases in the prevalence of mallards-hybrids (mallards or mallard x mottled duck hybrids) throughout peninsular Florida, USA, and because it is impossible to differentiate among mottled ducks, female mallards, and hybrids during aerial surveys, helicopter surveys were halted in 2009 until state researchers could conduct a range-wide study to determine what proportion of brown ducks are mottled ducks versus mallards-hybrids. We used plumage keys and high-resolution photography to categorize brown ducks from 557 wetland grid points as either mottled ducks or mallards-hybrids. Of the 5,179 brown ducks categorized, 40.1% were mottled ducks and 59.9% were mallards-hybrids. We used logistic regression analysis to model the interactive effect of a site's latitude and level of urbanization (urban gradient value within a 2-km buffer) to generate a predictive raster surface (1-km resolution) of the study area with values corresponding to the probability that a brown duck observed within a cell is a pure mottled duck. Predicted values will be used as correction factors when estimating final mottled duck population abundance from brown-duck survey data. Additionally, the predictive raster surface will be used to identify wetlands where mottled ducks remain predominant so that these sites can be targeted for preservation. Overall, mallards-hybrids outnumbered mottled ducks throughout most of peninsular Florida, especially in more urbanized regions, and their current prevalence rate presents a serious conservation threat, via hybridization, to extant mottled duck populations.     

Influenza Virus in a Natural Host,the Mallard: Experimental Infection Data

Wild waterfowl, particularly dabbling ducks such as mallards (Anas platyrhynchos), are considered the main reservoir of low-pathogenic avian influenza viruses (LPAIVs). They carry viruses that may evolve and become highly pathogenic for poultry or zoonotic. Understanding the ecology of LPAIVs in these natural hosts is therefore essential. We assessed the clinical response, viral shedding and antibody production of juvenile mallards after intra-esophageal inoculation of two LPAIV subtypes previously isolated from wild congeners. Six ducks, equipped with data loggers that continually monitored body temperature, heart rate and activity, were successively inoculated with an H7N7 LPAI isolate (day 0), the same H7N7 isolate again (day 21) and an H5N2 LPAI isolate (day 35). After the first H7N7 inoculation, the ducks remained alert with no modification of heart rate or activity. However, body temperature transiently increased in four individuals, suggesting that LPAIV strains may have minor clinical effects on their natural hosts. The excretion patterns observed after both re-inoculations differed strongly from those observed after the primary H7N7 inoculation, suggesting that not only homosubtypic but also heterosubtypic immunity exist. Our study suggests that LPAI infection has minor clinically measurable effects on mallards and that mallard ducks are able to mount immunological responses protective against heterologous infections. Because the transmission dynamics of LPAIVs in wild populations is greatly influenced by individual susceptibility and herd immunity, these findings are of high importance. Our study also shows the relevance of using telemetry to monitor disease in animals.     

Embryonic Pituitary Adrenal Axis, Behavior Development and Domestication in Birds

ACTH and corticosterone exert opposite effects on the approachand imprinting behavior of newly hatched ducklings. Wild mallardand domesticated Pekin ducklings differ in the early posthatchperiod in both plasma corticosterone levels and approach/avoidancebehaviors. Injection of Pekin duckling embryos with pituitary—adrenocorticalhormones alters both later adrenal function and certain aspectsof posthatch behavior. These birds have behavioral and hormonalcharacteristics which resemble those of wild mallards. The hypothesisthat behavioral differences in wild and domesticated ducklingsresult from a higher level of pituitary adrenal function inthe wild embryo is explored. Although adrenocortical functionchanges during domestication in many species, evidence thatthe hormonal changes mediate the concomitant changes in approachand avoidance behavior remains inconclusive. Factors which causeadrenal function and early behaviors to differ in wild and domesticatedgenotypes must be sought in the gene action during embryonicdevelopment. Since imprinting behavior is modulated by pituitary—adrenalhormones, any factor which affects post—hatch adrenalfunction may potentially affect imprinting. Later behavior developmentin the adult is strongly dependent on neonatal experiences;and, therefore, hormonal modulation of early imprinting behaviormay constitute an important determinant of adult social behavior.     

Within-season sequential density dependence regulates breeding success in mallards Anas platyrhynchos

Density dependence in vital rates is a key issue in population ecology but remains largely unexplored experimentally. We studied breeding success, lake use, and prey availability in wild mallards Anas platyrhynchos on small nemoral lakes in a replicated, two-year cross-over experiment in which pair density was increased. The number of wild mallards that settled on lakes prior to introductions of extra pairs did not differ between control and introduction years. Introductions led to a lake-level reduction in the number of broods observed. However, the number of stage 2+ (almost fledged) ducklings did not differ between treatments, nor did lake utilization by nonbreeding adults, broods and ducklings. Prey resource availability differed greatly among lakes, but it did not correlate with breeding success. Partialling out the possible effect of food competition from wild adult nonbreeding mallards did not change this conclusion. Our study demonstrates sequential density dependence in breeding success; introductions caused a decrease in brood number, but despite fewer broods a similar number of nearly fledged ducklings were produced. We suggest that predation and/or lake change of broods soon after hatching created these patterns. We conclude that using a single and late measure of breeding success such as fledged birds can mask regulatory processes. Implications of density dependence and its relation to individual reproductive success are understood better if breeding success is decomposed into nest success, duckling survival and fledgling survival.     

Ecological studies on Cyathocotyle bushiensis (Digenea) and Sphaeridiotrema globulus (Digenea), possible pathogens of dabbling ducks in southern Québec

Over the past 20 yr, recurrent late-summer mortality of dabbling ducks in southern Québec has been attributed to the digenean parasite Cyathocotyle bushiensis. This study attempted to determine whether this parasite was still implicated in the epizootics, and examined aspects of the ecology of the parasite in the definitive host. Comparison of prevalence and mean intensity of infection of all species of Digenea between salvaged carcasses and hunter-shot ducks revealed that C. bushiensis and a second digenean Sphaeridiotrema globulus were both significantly more prevalent in salvaged ducks. Mean intensity of infection was consistently higher for S. globulus in the salvaged carcasses than in hunter-shot ducks. These data provide strong circumstantial support for the hypothesis that C. bushiensis continues to contribute to the annual mortality, but also strongly suggest that S. globulus may be involved. Comparison of the relative numbers of each species of duck between the salvaged carcass sample and the hunter-shot sample revealed that blue-winged teal (Anas discors) occurred significantly more frequently in the salvaged carcass sample whereas wood ducks (Aix sponsa) were common in the hunter-shot sample but were never found in the salvaged carcass sample. A comparison of susceptibility to experimental infection with C. bushiensis revealed that mallards (Anas platyrhynchos), black ducks (Anas rubripes), blue-winged teal, pintail (Anas acuta) and lesser scaup (Aythya affinis) had similar susceptibilities but that wood ducks were significantly less susceptible to infection. Experimental infections also showed that age of mallards significantly influenced the number of parasites obtained 7 days postinfection; ducklings were most heavily infected. Significant differences were detected in the gross pathology caused by C. bushiensis among duck species. Data obtained from sentinel ducks placed in the field for 24 hr revealed that transmission of both C. bushiensis and S. globulus occurs at least between mid-May and mid-September and that large, potentially lethal, infections can be acquired in as little as 24 hr. Finally evidence is presented to suggest that ducks acquire resistance to challenge infections of C. bushiensis.