Waterfowl Wintering Research Articles

Multi-stage sampling for large scale natural resources surveys: A case study of rice and waterfowl

Large-scale sample surveys to estimate abundance and distribution of organisms and their habitats are increasingly important in ecological studies. Multi-stage sampling (MSS) is especially suited to large-scale surveys because of the natural clustering of resources. To illustrate an application, we: (1) designed a stratified MSS to estimate late autumn abundance (kg/ha) of rice seeds in harvested fields as food for waterfowl wintering in the Mississippi Alluvial Valley (MAV); (2) investigated options for improving the MSS design; and (3) compared statistical and cost efficiency of MSS to simulated simple random sampling (SRS). During 2000–2002, we sampled 25–35 landowners per year, 1 or 2 fields per landowner per year, and measured seed mass in 10 soil cores collected within each field. Analysis of variance components and costs for each stage of the survey design indicated that collecting 10 soil cores per field was near the optimum of 11–15, whereas sampling >1 field per landowner provided few benefits because data from fields within landowners were highly correlated. Coefficients of variation (CV) of annual estimates of rice abundance ranged from 0.23 to 0.31 and were limited by variation among landowners and the number of landowners sampled. Design effects representing the statistical efficiency of MSS relative to SRS ranged from 3.2 to 9.0, and simulations indicated SRS would cost, on average, 1.4 times more than MSS because clustering of sample units in MSS decreased travel costs. We recommend MSS as a potential sampling strategy for large-scale natural resource surveys and specifically for future surveys of the availability of rice as food for waterfowl in the MAV and similar areas.

Shorebird Breeding Biology in Wetlands of the Playa Lakes, Texas, USA

Wetlands in the Playa Lakes Region of Texas are important habitats for North American wintering waterfowl and migrant shorebirds. However, shorebird breeding biology has been overlooked in characterizing the region’s ecological importance. In 1998 and 1999, American Avocet (Recurvirostra americana), Black-necked Stilt (Himantopus mexicanus), Killdeer (Charadrius vociferous), and Snowy Plover (C. alexandrinus) breeding ecology were studied in playas, saline lakes, and riparian wetlands in the Playa Lakes Region of Texas. Chronology of nest initiation, clutch sizes, and hatching success for 298 Snowy Plover, 111 American Avocet, 43 Killdeer, and 26 Black-necked Stilt clutches were measured. All four species nested in saline lakes, American Avocet and Killdeer also nested in playas, and Snowy Plover nested on riparian wetlands. American Avocet had higher hatching success in 1999 (52%) than 1998 (8%), because of more suitable hydrological conditions and lower predation. Hatching success was higher in 1998 than 1999 for Killdeer (1998, 63%; 1999, 21%) and Snowy Plover (1998, 47%; 1999, 33%) due to failures caused by flooding and hail in 1999. In other regions, clutch predation limits shorebird productivity, but hatching success in the Playa Lakes Region appears to be limited by unpredictable precipitation patterns and wetland hydroperiod. As such, breeding shorebird conservation and management should focus upon maintaining wetland hydrological integrity.

Effects of human disturbances on the behavior of wintering ducks

Human activity causes wintering waterfowl to expend energy to avoid humans at a time in their annual cycle when energy conservation is important to survival, migration, and breeding reserves. Understanding the effects of recreational activities on waterfowl is important to managing natural resource areas where migratory birds depend on wetland habitat for resting and feeding. We investigated responses of 7 species of dabbling ducks to 5 different experimental human activities, (a pedestrian, a bicyclist, a truck traveling at 2 different speeds, and an electric passenger tram). Responses of ducks depended on type of disturbance, species, and distance from disturbances. Most birds responded to the treatments. People walking and biking disturbed ducks more than vehicles did. Northern pintail (Anas acuta) was the species least sensitive to disturbance, whereas American wigeon (A. americana), green-winged teal (A. crecca), and gadwall (A. strepera) were most sensitive. Ducks were more likely to fly when closer to sources of disturbance. These results will be helpful to managers making decisions about public use that strive to minimize disturbance of dabbling ducks.

Long-Term Trends and Changes in Numbers and Distribution of some Wintering Waterfowl Species along the Swedish Baltic Coast

Regular counts of wintering waterfowl have been undertaken as a part of the International Waterfowl Census (IWC) and the National Swedish Environmental Monitoring Program since 1964. After the first years, a network of annually surveyed sites was established for the calculation of annual indices. Country-wide surveys (also including aerial counts) were undertaken 1971–1974, 1987–1989, 1992–1993 (partial) and 2004. During the survey period, increasing trends were found in Mallard (Anas platyrhynchos), Tufted Duck (Aythya fuligula), Pochard (Aythya ferina), Goldeneye (Bucephala clangula), Red-breasted Merganser (Mergus serrator), Mute Swan (Cygnus olor) and Cormorant (Phalacrocorax carbo). Some of these trends reflect genuine population changes; others are related to changing winter conditions, e.g. the establishment of a new wintering tradition for Wigeon (Anas penelope). Tufted Duck and Goldeneye decreased in the south and increased in the northern part of the coast. Mallard, on the other hand, increased in some coastal sites but not in others, but this species is also common inland.

Circulation of influenza viruses in wild waterfowl wintering in Italy during the 1993-99 period: evidence of virus shedding and seroconversion in wild ducks.

The mechanisms of perpetuation of influenza A viruses in aquatic birds, their main reservoir in nature, have not yet been completely clarified. One hypothesis is that they continue to circulate in waterfowl throughout the year, even though virus isolations during the winter months are rare. We analyzed influenza virus circulation in wild ducks in Italy during six winter seasons (1993-99), using virus isolations and serological analyses. It was apparent that influenza A viruses were constantly circulating in wild birds during all the seasons considered. Moreover, seroconversion rates (obtained from ducks recaptured during the same season) suggest a frequency of influenza infections higher than expected on the basis of the virus isolation rates.

Comparative dispersal effectiveness of wigeongrass seeds by waterfowl wintering in south‐west Spain: quantitative and qualitative aspects

Summary The effectiveness of a seed disperser depends on the quantity of seed dispersed and the quality of dispersal provided to each seed. Relationships between the quantity and quality components and their dependence on characteristics of the dispersers remain largely uninvestigated. The effectiveness of different waterfowl species at dispersing seeds of Ruppia maritima was evaluated in a wetland in south‐west Spain. Droppings were collected during autumn and spring waterfowl migrations and the number of seeds ingested (estimated from seed fragments), undigested and viable in germination trials were determined. Ingestion by waterfowl enhanced the rate of germination and, for several duck species, it also had a positive effect on germinability. Both the presence of seeds in the diet and the effects of gut passage showed high interspecific and temporal variance. Some of the interspecific variation in dispersal quality was related to gut structure: species with heavier gizzards destroyed a higher proportion of seeds and undigested seeds ingested by species with more grit in the gizzard germinated better. In the waterfowl community studied, the quantity and quality components of seed dispersal effectiveness were positively correlated across species.

Hunting disturbance on a large shallow lake: the effectiveness of waterfowl refuges

Disturbance is most likely to have an impact during those periods of the annual cycle when food is scarce and birds have difficulty in meeting their energy requirements. The provision of disturbance‐free refuges has been shown to enhance the abundance and diversity of waterfowl wintering on such sites. Lough Neagh, a large shallow lake in Northern Ireland, hosts internationally important numbers of wintering waterfowl, the most numerous being Common Pochard Aythya ferina, Tufted Duck A. fuligula, Greater Scaup A. marila and Common Goldeneye Bucephala clangula. Hunting is the predominant winter recreational activity. A small number of refuges have been provided to limit the effects of shooting disturbance on the wintering populations. The use of one of these refuges (Doss Bay) was studied during the winter of 1997/8 and compared with a non‐refuge site (Brockish Bay) during the winter of 1998/9. Shooting intensity was greater at weekends than midweek, and significantly more birds used the refuge at weekends than midweek during the shooting season. In contrast, significantly fewer birds were observed in Brockish Bay at weekends. This trend ceased when the shooting season closed, suggesting that the effect was due to shooting disturbance. Furthermore, significantly fewer birds used Doss Bay after the shooting season had closed, whereas significantly more birds were observed at Brockish Bay. Shooting disturbance had the greatest impact on dabbling duck species and rails at both sites. Diving ducks moved away from shoreline disturbance to shallow areas where they could still feed. The value of shoreline refuges for waterfowl populations on lakes is discussed.

Hunting disturbance and the timing of autumn migration in Anas species

In Europe, waterfowl hunting is a popular recreational activity, especially in waterfowl staging and wintering areas, but also in breeding areas such as Finland. This paper gives an account of hunting disturbance, timing of autumn migration and refuge use in Anas species in eutrophic wetlands in central Finland (63°N, 27°E) during 1995–1999. Based on the timing of migration, duck species may be divided into early and late migrants. In two lakes (total area 2.2 km2) which were monitored on a weekly basis during 1995–1998, numbers of late migrants, i.e. mallard Anas platyrhynchos, teal Anas crecca and wigeon Anas penelope, increased towards the opening of hunting season on 20 August, whereas shoveler Anas clypeata, pintail Anas acuta and garganey Anas querquedula migrated early, and peak numbers of these species occurred already in the beginning of August. Consequently, in Finland's inland lake area, hunting disturbance relates differently to early and late migrating species. Immediately after the opening of the season the numbers of dabbling ducks collapsed and the average numbers in the wetlands were only 9.3% of the numbers present at the end of the protection period in 1995–1998. A corresponding decrease in numbers occurred in 13 lakes (total area 8.5 km2) in the Finnish inland lake district in 1999. After the beginning of the open season numbers of wigeon, teal and mallard decreased in unprotected areas, but increased in a refuge area. In Finland, refuge areas with high‐quality foraging and moulting habitats for waterfowl are few, so a network of waterfowl reserves could mitigate the effects of hunting disturbance.

New Important Wintering Areas of Waterfowl in Ukraine

The status, distribution, and numbers of 27 wintering waterfowl species are reviewed for the last 50-year period in Ukraine. The main shifts that occurred are discussed along with hydrological and global climate changes.

Impact of Disturbance on Waterfowl Wintering in a UK Dockland Redevelopment Area

One of the UK's largest flocks of pochard Aythya ferina and tufted duck A. fuligula winters in Manchester's busy dockland redevelopment area. I examined the effects of human disturbance on the population, and used this information to recommend minimal land-use restrictions that will help ensure the population's continued use of the site. Birds fed at the docks every night, but on 75% of days, the flock flew to suburban or rural refuges in response to disturbance. The common causes of disturbance, particularly pedestrians, did not affect the duck greatly, but disturbances associated with building and redevelopment of the site (e.g., machinery) often made duck evacuate the dock. While birds were no less likely to evacuate the docks as the winter progressed, they did show some short-term flexibility in their responses to disturbance. For example, birds appeared able to sit out disturbances throughout the afternoon if they had avoided evacuation of the docks during the morning. Birds spent only a small proportion of time feeding, and feeding activity was not heightened following periods of exclusion from the docks due to disturbance. Daytime feeding activity was elevated during periods of lowering temperatures, and despite heavy disturbance at the site, the impact of disturbance may only be significant during the coldest period of the winter. During very cold spells, it is important to limit the incidence of unusual disturbances, such as evening concerts and waterborne activities which tend to make duck evacuate the site.

An evaluation of waterfowl sampling methods for the national census on river and dam lake environments.

A national census of river and dam lake environments (NCRE) conducted by the Ministry of Construction of Japan has been underway since 1991. Information on the variation in number and habitat use of waterfowl is important in determining the optimum sampling effort necessary for accurately estimating the population density. However, this variation is not sufficiently considered in the census methods used in the NCRE. In this paper, we report the variation in number and habitat use of waterfowl, especially ducks, over short and long temporal scales (daily and monthly changes, respectively) at Takayama dam lake in central Japan. Between December 1998 and March 1999, we counted wintering waterfowl from a small boat and from the lake shore, and analysed the results in relation to the valid range of census methods adopted by the NCRE. General results of the field observations were : (1) the waterfowl assemblage was stable in January and February ; (2) the waterfowl assemblage was stable on any given day and only slightly varied during five consecutive days ; (3) the locations of ducks on the dam lake remained almost the same during both short and long temporal scales ; and (4) terrestrial observations were not useful for counting mandarin ducks and greenwinged teals resting on the surface of the dam lake. These results suggest the following: (1) the winter waterfowl census in central Japan should be conducted in January or February ; (2) the area surveyed in the census should be large enough to encompass the large home ranges seen in several waterfowl species (e.g. common cormorants, mallards and European wigeons) ; (3) routes for line transect and positions for point counts should be chosen carefully when observers count waterfowl on land ; and (4) boat-based observations are recommended for waterfowl censuses.

Factors Affecting the Distribution of Waterfowl Wintering in the Inland Water of the Saijo Basin in Western Japan.

Effects of human disturbances on the distribution of wintering waterfowl were studied in 64 irrigation ponds in the Saijo basin, western Japan, during 1998-99. Thirteen waterfowl species were recorded in 27 ponds. 100% of the waterfowl were distributed in ponds located in residential areas in 1998. Small number of the residential species, the Spot-billed Duck (Anas poecilorhyncha), used a single pond in forest in 1999. Waterfowl in residential ponds were regularly disturbed by natural and human disturbances except for hunting. Pedestrians, anglers and bird watchers caused the highest number of disturbances in both years. We suggest that waterfowl are capable of habituating to frequent and regular non hunting disturbances, which helped them to occupy the ponds in residential areas. Hunting was the most critical factor on the distribution of waterfowl in Saijo basin. Potential risk of shooting caused waterfowl to forgo using the ponds in forests. In addition to hunting, construction work and dry out of ponds caused waterfowl to abandon ponds.

Establishing a reserve network for waterfowl in Denmark: a biological evaluation of needs and consequences

Denmark is of major international importance for a number of waterfowl populations during migration and in winter. As part of the revision of the Danish Hunting and Wildlife Management Act, 1992, it was decided to establish a network of reserves by creation and extension of reserves in 46 EU Special Protection Areas for birds (SPAs). This paper summarises an overall, biologically-based strategy for a reserve network in Denmark, as a basis for sustainable development of wetlands for migratory and wintering waterfowl. As well as securing the future for vulnerable populations, the reserve network should improve conditions for waterfowl staging and wintering in Denmark. The groups potentially most exposed to disturbance from hunting and other leisure activities, i.e. in greatest need of a reserve network, are the geese and the dabbling and small diving ducks. Swans, seaducks, sawbills and coots have less need for such a network. On this basis, deficiencies in the existing system of reserves are identified. Human recreational activities are considered compatible with sustainable development if the use by waterfowl of important wetlands is primarily limited by available food resources. Within each of the principal waterfowl migration routes and wintering areas identified, there should be a network of reserves which provides adequate opportunities for the birds to feed and rest. The proposed network will amount to a total shooting-free refuge area of ca. 650 km 2, representing almost a doubling of the refuge area in Denmark. The shooting-free area will be supplemented by ca. 870 km 2 with regulation of recreational activities or certain hunting methods. The extension is expected to result in a considerable increase in the numbers of dabbling ducks and geese, with the potential of holding back substantial parts of some populations at a more northerly position on their flyways than before. The area available for hunting in SPAs will be reduced but hunting opportunities in adjacent areas could be expected to improve because the numbers of birds there will also increase.

High Prevalences of Lead Poisoning in Wintering Waterfowl in Spain

Some Mediterranean wetlands are found between the areas with the highest prevalence of lead pellet exposure for waterfowl in the world. To assess the situation in Spain, availability of pellets in sediments and rates of ingestion by waterfowl were determined in four important wetlands: Albufera de València, El Fondo, Tablas de Daimiel, and Doñana (Guadalquivir Marshes). Lead pellet density in sediment was maximum in the Albufera, with 2.8 million pellets/ha. In Tablas de Daimiel, where hunting was banned in 1965, about 1.0 million pellets/ha were found. Percentages of shot waterfowl with ingested pellets were maximum in the wetlands on the Mediterranean coast (El Fondo and Albufera de València): 87.5% of common pochard (Aythya ferina) and approximately 33% of mallard (Anas platyrhynchos) and shoveler (Anas clypeata) contained ingested pellets. It was found that mallards with > 5 µg/g dry weight of lead in the liver had 8.5% lower body condition and 7.7% less body weight than birds with lower lead concentrations. Moreover, lead poisoning was diagnosed as the cause of death in 16 out of 75 birds, mainly greylag geese (Anser anser), found dead in some of these zones.

Activity Budget of White-Faced Whistling-Ducks during Winter and Spring in Northern Kwazulu-Natal, South Africa

Despite the fact that white-faced whistling-ducks (Dendrocygna viduata) incur the added costs of flight-feather replacement on wintering areas, little is known about how they allocate their time and energy during this period. We used focal sampling techniques to quantify diel activity patterns of nonbreeding white-faced whistling-ducks using semipermanent pans in northern KwaZulu-Natal, South Africa, during the winter and spring of 1995. A large proportion of early winter (44% of diel activities), late winter (29%), and spring (50%) activities were attributed to foraging. Increased foraging intensity during early winter and spring (P < 0.05) corresponded with peak flight-feather replacement. Birds spent more time in comfort-related activities (sleep, loaf, preen) during late winter (57%) than both early winter (35%) and spring (29%; P < 0.05). Time spent in locomotion (walk, swim, fly) decreased as winter progressed and was lowest during spring, whereas alert behaviors and aggression increased (P < 0.05). No courtship activities were observed. Activities of white-faced whistling-ducks were correlated more strongly with flock size and ambient temperature than several other climatic variables. Relative to waterfowl wintering in north-temperate regions, the activity patterns of white-faced whistling-ducks were probably influenced by either or both the nutrient costs of feather replacement and a reduction in foraging opportunities during the flightless period, as well as a low availability of cereal grains near wintering areas. Fifty-three percent of foraging activities occurred nocturnally, which further ex-emplifies the importance of considering nocturnal behaviors when assessing the activity patterns and energetics of waterfowl.

Wintering waterfowl community structure and the characteristics of gravel pit lakes

Wintering waterfowl community structure and their association with lake characteristics were studied in the Cotswold Water Park, a complex of more than 120 gravel pit lakes in southern England. The major distinction in community types was between assemblages dominated by diving waterfowl and those dominated by dabbling waterfowl. The trophic status of lakes was found to be a major determinant of community structure: young lakes in the early stages of a natural process of eutrophication tended to support the most diverse assemblages of diving waterfowl. The abundance of individual species was strongly related to lake size and assemblage type. Some effects of food supply and the recreational use of lakes were also apparent. The relevance of the findings for the sustainable value of the lakes for wintering waterfowl is discussed.

Dam and geomorphological influences on colorado river waterbird distribution, Grand Canyon, Arizona, USA

Impoundment effects override natural, reach-based channel geomorphology influences on seasonal waterbird distribution in Grand Canyon along the Colorado River downstream from Glen Canyon Dam. Large winter waterbird populations were rare or non-existent prior to completion of Glen Canyon Dam in 1963, and pre-dam summer breeding was rare. Post-dam river corridor surveys of 13 geomorphological reaches from 1973 to 1994 detected 58 species of waterfowl, waders, shorebirds and piscivorous raptors, with a grand mean of 138·2 waterbirds/reach (SE=31·0, n=727 reach surveys), and a mean area-adjusted rate of encounter (AARE) of 372·8 waterbirds km−1 h−1 of observation per reach (SE=69·1). The post-dam assemblage has been dominated by Anseriformes (13 diving and 12 dabbling species) and includes regionally significant populations of wintering waterfowl and bald eagle, and breeding mallard. Most wading birds and shorebirds occur primarily as migrants or summer vagrants. Total waterbird AARE was greatest in the productive clear water (CW) and variably turbid (VT) segments upstream from the Little Colorado River (LCR) (km 98), decreasing downstream on the usually turbid (UT) lower Grand Canyon segment. Mean total winter waterfowl AARE was 1076·8, and decreased by three orders of magnitude from the CW to the UT segments (p=0·0001). Mean total summer AARE was 2·7, and also decreased across the turbidity segments (p=0·066). In contrast, AARE varied little between wide and narrow geomorphological reaches. Total AARE was only 1·4 and 1·3-fold greater in wide versus narrow reaches within the VT and UT turbidity segments, respectively (p<0·0002). Winter AARE was threefold greater (p=0·0002), while summer AARE was equivalent between wide and narrow reaches. These tributary-related turbidity and geomorphological reach width factors contributed to a non-linear, circuitous shift in the waterbird assemblage over distance downstream from the dam, differentially affecting the seasonal distribution of waterbird feeding guilds. We discuss flow regulation and habitat management implications. © 1997 John Wiley & Sons, Ltd.

WINTER PERIOD ORNITHOLOGICAL IMPACT ASSESSMENT OF OIL RELATED ACTIVITIES AND SEA TRANSPORTATION IN LITHUANIAN INSHORE WATERS OF THE BALTIC SEA AND IN THE KURŠIŲ LAGOON

Abstract Studies carried out during the last decade indicated that a number of key wintering sites of waterfowl and seabirds of NW Europe arc located in brackish waters of the Baltic Sea. Experience of many industrialized countries shows that human activities are likely to cause harm to waterfowl and seabird concentrations, especially in winter, when these birds aggregate in high numbers and become very susceptible to any human impact. The increase of oil related activities and sea transportation in the eastern Baltic in general and in Lithuanian waters in particular gave rise to concern about the well-being of birds wintering in Lithuanian waters. The current study was carried out in order to assess the magnitude and significance of negative impacts of oil related activities, at present and in the future, on wintering waterfowl and seabirds in Lithuanian inshore waters of the Baltic Sea and in the Kuršū Lagoon. Information about the current level of oil related activities and their prospected development in the future, was obtained from various governmental and literature sources. Data on species composition, abundance and distribution of wintering waterfowl and seabirds in Lithuanian inshore waters and in the Kuršiū Lagoon is presented in this work and is used for identification of the most important wintering sites and most vulnerable bird species. This data was collected by means of bird surveys from the coast and airplanes. Beached bird surveys were performed along the Lithuanian Baltic Sea coast, and their results were used for identification of the areas most affected by oil pollution. Four waterfowl and seabird wintering sites of international importance were identified in Lithuanian inshore waters and in the Kuršiū Lagoon during this study. A considerable proportion of total NW European populations of Steller's Eider, Velvet Scoter, Red-throated and Black-throated Divers, Goosander and Smew were found to concentrate in these wintering sites. Steller's Eider was identified as the most sensitive and the most endangered specics wintering in Lithuanian inshore waters. Operational oil pollution from ships at present represents the greatest threat to waterfowl and seabirds wintering in Lithuanian inshore waters, especially in the wintering site along the Palanga coast. Disturbance by sea traffic was not found to represent any major threat to wintering birds at present, except in the immediate vicinity of the Klaipėda port. Future development of oil related activities in Lithuania is likely to cause a substantial threat to waterfowl and seabirds wintering in Lithuanian inshore waters. Therefore, key bird wintering areas in Lithuanian inshore waters and in the Kuršiū Lagoon, especially the one along the coast of Palanga, are recommended to be assigned a protected status. Strict control of operational oil discharges from ships and development of oil spill combating measures is recommended in order to reduce the threat arising to wintering birds from oil pollution.

POSSIBLE IMPACT OF GILL NET FISHING ON WINTERING BIRDS IN LITHUANIAN INSHORE WATERS OF THE BALTIC SEA

Abstract Spatial distribution and intensity of gill net fishing activities in Lithuanian inshore waters of the Baltic Sea were studied in the winter of 1996/97 in order to identify the possible impact of these activities on wintering birds. Internationally important waterfowl and seabird wintering site along the coast of Palanga was found to overlap with intensive gill net fisheries. Consequently, Steller's Eiders wintering in compact flocks in this site are most likely to be adversely affected by gill net fishing. Other abundant species in this wintering site—Red-throated and Black-throated Divers and Long-tailed Ducks may also be subject to considerable pressure from gill net fisheries. More detailed investigations are needed to quantify these impacts and assess their significance.

Assessing river habitat selection by waterfowl wintering in the South Platte River, Colorado

We assessed river habitat selection of waterfowl wintering in the South Platte River below the Metro Wastewater Reclamation District treatment plant in Adams County, Colorado to determine potential impacts of proposed river channel modifications. Daily mean number of waterfowl was 711 per km of river over the study area, and 19 species of waterfowl were observed. We describe use of a resource selection model to evaluate whether a habitat type is selected more than expected based on its availability. Habitat selection indices indicated that Canada geese (Branta canadensis) used all habitats in proportion to their availability. Dabbling ducks selected large pools, secondary channels, riffles and sandbars, avoided smaller pools and islands, and used runs in proportion to their availability. Diving ducks selected large pools, small pools, and runs; they avoided secondary channels, riffles, sandbars, and islands. Due to different habitat preferences between diving and dabbling ducks, changes that alter river habitat structure may favor some species and not benefit others. The optimum way to maintain diversity and abundance of waterfowl wintering in the South Platte River is to maintain a variety of habitat types.