Effects of weather on habitat selection and behavior of mallards wintering in Nebraska

-Sex and age ratios, habitat selection, spatial characteristics, and time budgets of Mallards (Anas platyrhynchos) wintering on the Platte River in south central Nebraska were studied from mid-December to early April 19781980. The proportion of females and subadults in the population increased substantially from a cold to a mild winter. Radio-tagged Mallards shifted from riverine to canal roost sites during the coldest periods of the winter, seemingly because of more favorable microclimatic conditions there. Subadults ranged over larger areas during winter than did adults. Activity patterns varied with weather conditions, time of day, and habitat type. During cold periods, energetically costly activities such as aggression and courtship decreased at roost sites and the intensity of foraging activities in fields increased. Mallards were more active at riverine than canal sites during both years. High energy requirements and intense competition for scarce food appear to be primary factors limiting the northernmost distribution of Mallards in winter and causing their skewed sex and age ratios. Severe weather conditions, especially low temperatures, are known to cause stress and affect the distribution and movements of waterfowl during winter (LeFebvre and Raveling 1967, Reed 1971, Cain 1973, Bennett and Bolen 1978, Prince 1979). Some Mallard (Anas platyrhynchos) populations winter farther north than most other waterfowl species (Bellrose 1980). How these birds have adapted to northern environments during winter and the factors that limit their winter distribution in North America are poorly understood and were the subjects of our study. More specifically, we examined habitat use and behavior of Mallards on a site north of the traditional winter range of the species. Our objectives were to (1) study the effects of weather patterns and microclimate on daily movements and habitat selection of Mallards, and (2) record nocturnal and diurnal behavior of these birds in several habitats during variable climatic conditions. STUDY AREA AND METHODS The study area was located along the Platte River in south central Nebraska (Fig. 1). Data were collected principally in areas near the river from Gibbon to Kearney and Elm Creek to Lexington. These areas contain ice-free channels and canals used by wintering Mallards. Most of the surrounding land is privately owned and used for corn and livestock production. Much of this land is irrigated and several canals traverse parts of it, particularly between Lexington and Elm Creek. Data were collected from mid-December to mid-March in 1978-1979 and 1979-1980. We observed the ducks' diurnal activities using a 20 x spotting scope and Questar 1300-mm scope. Nocturnal activities were observed with a Javelin Model 226 long-distance night-viewing device equipped with a 300-mm lens. We recorded the ducks' activities at 12-s intervals with the aid of a metronome timer. Individual Mallards were randomly selected by locating a group of birds in the scope's field of view and, after looking away and slightly moving the instrument horizontally and/or vertically, observing the individual nearest to the point where the cross hairs intersected. Data were collected continuously for 1-60 min for each individual observed. Nocturnal activities were only recorded during clear or nearly clear nights so that we could determine a duck's sex at distances up to 100 m. Snow on surrounding areas and/or moonlight greatly improved visibility. The male's dark head was the most prominent characteristic at night. Activities identified in this paper are: Sleeping. Resting, with the head usually held close to the body or lying on the back with bill tucked into the feathers. Swimming. Aquatic locomotion not directly associated with feeding, but including bathing. Walking. Terrestrial locomotion not directly associated with feeding.