Effects of food abundance and environmental parameters on foraging behavior of gadwalls and American coots in winter

Diurnal behaviors of wintering gadwalls (Anas strepera) and American coots (Fulica americana) were studied at Guntersville Reservoir, Alabama. Feeding accounted for a substantial amount of time spent by gadwalls (>50%) and coots (>62%). Feeding rate (pecks per minute) of female gadwalls was higher than that of males, and foraging speed (metres per minute) of unpaired gadwalls was higher than that of paired birds. For both gadwalls and coots, foraging speed increased and feeding rate decreased over time. Stepwise multiple regression was used to test the effects of food abundance, day length, ambient temperature, and mean daily wind speed on feeding rate and foraging speed. Foraging speed increased with declining food abundance for both gadwalls and coots. Additionally, day length accounted for a small but significant amount of variation in foraging speed of coots. Moving faster while feeding probably allowed gadwalls and coots to maintain adequate intake of nutritious food items from a declining food resource. Variation in feeding rate of gadwalls was not explained by food abundance or environmental variables. Feeding rate of coots decreased with declining food abundance and increased with decreasing ambient temperature. As food abundance declined, reduced density of suitable food items resulted in fewer surface pecks and more time-consuming dives. The higher feeding rate of coots at lower ambient temperatures probably resulted from increased thermoregulatory costs. Unlike coots, gadwalls typically do not dive, and adjusted to reduced food abundance at the surface by stealing food from diving coots. Because gadwalls exploited coots bringing food items to the surface in January and February, gadwall feeding rate during this time was somewhat uncoupled from variation in food abundance at the surface. Although diurnal foraging time is limiting for some waterfowl species, results from the present study indicate that variation in foraging behaviors of gadwalls and coots wintering in Alabama predominantly was a response to changes in food abundance.