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Platte River Ecology Study



During the past 50 years several species of waterfowl have undergone notable shifts in migrational and wintering distributions in response to manipulation of coastal and inland water resources and upland habitats (Buller 1975). This process has been particularly evident in the Central Flyway where agricultural development has caused the loss of extensive wetland habitat and the creation of new food resources. Habitat alteration has caused increased waterfowl use of the remaining traditional habitats and occupancy of man-made habitats such as reservoirs.

Waterfowl use of wintering and staging areas at middle and northern latitudes has received limited study. Little is known about the tens of thousands of mallards now wintering at midcontinent latitudes although these birds represent an increasingly important part of annual harvests in the central states (Grieb and Boeker 1954; Geis 1971). Of particular concern is the influence of wintering at northern latitudes on the physical condition of birds and the potential impact on reproduction. Krapu (1981) has shown that the physical condition of breeding female mallards is an important determinant of their capacity to reproduce. Physical condition is even more critical for species such as the white-fronted goose which arrive on arctic breeding grounds before food resources become available. It has been shown previously that stored nutrient reserves contribute substantially to the reproductive needs of lesser snow geese (Ankney and MacInnes 1978) and Canada geese (Raveling 1979).

The effects of weather on behavior, physical condition, and mortality of wintering and staging waterfowl in the Great Plains have received limited attention. Elsewhere, weather has been shown to affect the availability of open water and food resources and to influence movements and behavioral patterns within and among various habitats (Lefebvre and Raveling 1967; Reed 1971; Raveling et al. 1972; Bennett and Bolen 1978).

Because of the importance of the study area as a wintering and staging site for certain species of waterfowl, studies were undertaken to determine distribution and abundance patterns, to identify food habits of certain key species, and to assess the adequacy of the available diet in meeting nutritional requirements.

Distribution and Abundance

The Platte River Valley is an important early spring staging area for the midcontinent populations of mallards, white-fronted geese and Canada geese. Several thousand mallards and Canada geese also winter along the River. Spring migration begins in late February and early March with the exact arrival date dependent on weather conditions and availability of open water. Waterfowl tend to utilize the Platte River early in the season and move to the Rainwater Basin Area when it becomes ice-free.

Mallards and Canada geese account for most of the winter waterfowl use on the study area. Mallards winter at several sites along the Platte River between Chapman and Kingsley Dam; over 85% of the use occurs between Grand Island and Lexington. Largest numbers occur during years of mild weather and little snow cover. Mallard distribution and abundance during the winter months of 1979 and 1980 are shown in Appendix E. The distribution of mallards varies with the extent of ice-free riverine habitat. During the extremely cold winter of 1978-79, mallards were dispersed along much of the river during December but by February many of the birds had departed to areas farther south; most of the remaining birds were in drainage canals near Overton, with smaller numbers found about 11.3 km (7 miles) east of Kearney. Milder weather during the following winter caused much of the river channel to remain open. Mallards were more dispersed along the River during the 1979-80 winter but the major concentration remained in the Overton area.

Canada geese also winter along several reaches of the Platte River (Appendix E). The primary wintering sites are between Gibbon and Lexington with the highest numbers of geese occurring between Elm Creek and Lexington where warm water discharge from power plants adjacent to the Tri-County Canal maintains open areas throughout the winter months.

Habitat Use

Mallards utilize wetland habitats on the study area during most of the year. In winter, use is largely restricted to major channels of the Platte River and those nearby irrigation canals which remain open. Seventeen radio-marked individuals monitored in the Overton-Lexington reach during January and February 1980 either roosted in wooded river channels or in irrigation canals, depending on weather conditions. Under favorable conditions, birds preferred riverine habitats with extensive open water areas, however, some individuals utilized canals throughout the winter. Most mallards left the roost sites once a day, usually in late afternoon, and flew to forage in cropland. When a storm was imminent, feeding flights would begin earlier in the day and continue until the onset of the storm. These birds apparently attempted to secure adequate food to compensate for the diminished feeding opportunities during and after the storm.

The availability of suitable feeding conditions influenced the distance mallards traveled during feeding flights. During the mild winter of 1979-80, mallards generally remained within 3.9 km (2.4 miles) of the river, whereas during the harsh winter of 1978-79, with food scarce because of heavy snow accumulations, mallards flew up to 21 km (13 miles) from the river to feed. Snow accumulation that winter made corn inaccessible to mallards, except where grazing cattle broke up the snow and ice. There was little snow cover during the 1979-80 winter, markedly increasing access to food supplies; this change was reflected in time spent foraging. Mallards spent 78% of the time feeding while in grazed cornfields during 1978-79, and only 52% during the following year. During the first year of the study birds spent more time feeding to compensate for heavy snow cover that reduced access to corn.

Roost fidelity was strong during cold weather when fewer suitable habitats were available; movement among and within habitats increased during warm weather. Yearling ducks tended to change habitats more often and have larger home ranges than did adults, presumably reflecting inexperience in securing their needs.

Mallards preferred rainwater basins when wetlands became ice-free. The birds generally utilized the deeper basins for nocturnal roosting and moved to nearby agricultural fields for diurnal roosting and foraging. Mallards moved back to the Platte during intermittent cold periods when the basins froze. In March 1980, several radio-marked individuals, which had taken up residence in the Rainwater Basin Area in late February temporarily returned to their winter roost sites on the Platte. While in the Rainwater Basin Area, mallards made relatively short daily feeding flights from wetlands to surrounding cornfields. Mallards typically fed with other dabbling ducks and geese. The length of feeding flights gradually increased as the staging period progressed, presumably because nearby food resources became scarce due to intensive foraging activity by waterfowl.

White-fronted geese typically arrive in south central Nebraska in late February when snow cover is present and basins are frozen. Under these conditions, geese initially congregate in open reaches of the Platte between Lexington and Grand Island and make daily morning and afternoon flights to adjoining cornfields to feed with the large numbers of Canada geese-and mallards that are present at that time. With opening of the Rainwater Basins, most of the white-fronted goose population shifts to the Basin Area. Feeding flights radiate out from the basins to nearby corn and winter wheat fields. White-fronted geese typically make a morning and afternoon feeding flight.

Feeding Ecology

Waste corn appears to be the dietary staple of most dabbling ducks and geese during the annual stopover in south central Nebraska. The winter diet of mallards consisted principally of foods taken on agricultural lands. Five plant foods (foxtail, sorghum, corn, smartweed, and barnyard grass) accounted for over 99% of their diet (Fig. 14). Corn accounted for 97% and 94% of the diet of drakes and hens, respectively. Unharvested sorghum became a relatively important item in the diet during the 1978-79 winter when seed stalks were accessible above the snow cover and access to corn was restricted.

Animal matter, primarily snails, formed less than 1% of the diet of mallards. Birds fed almost exclusively on corn when foraging in grazed corn stubble, whereas aquatic plant seeds and animal matter came principally from riverine habitats. About 55% and 45% of the daylight hours were spent feeding while in riverine habitats during the 1978-79 and 1979-80 winters, respectively. These data suggest that mallards fare well on a diet that is principally corn but small quantities of animal matter are needed to meet nutritional requirements.

The diet of female mallards during the spring staging period differed only slightly from the winter months with almost no change in the proportion of animal matter consumed (Fig. 14). These data support previous evidence that mallards rely principally on food acquired on the breeding grounds to meet the substantial protein and calcium requirements for reproduction (Krapu 1979).

Field feeding by white-fronted geese occurs primarily in corn and winter wheat fields. Corn occurred in the following proportions in the diet by sex and age group: adult males, 76.9%; juvenile males 98.0%; adult females, 90.9%; and juvenile females, 98.5% The corn in the diet is from waste kernels remaining in the fields after harvest; green shoots of winter wheat form the other principal component of the diet. The birds probably take some animal matter from Basin wetlands to meet calcium and protein requirements but further study is needed to identify the contribution of wetland foods to the diet of white-fronted geese.

Fat Storage

The study area serves as a major conditioning site for spring staging waterfowl. White-fronted geese, for example, arrive relatively lean but undergo a marked increase in body weight and fat content during March and early April (Fig. 15). Fat levels among males and females increased at a daily rate of 12.5 and 17.6 g, respectively. Assuming a 30 day stopover, lipid deposition among males and females averaged 375 and 528 g, respectively.

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