Kleptoparasitism of american coots by gadwalls and its relationship to social dominance and food abundance

Abstract

Kleptoparasitism of American Coots (Fulica americana) by Gadwalls (Anas strep-era) was recorded from October 1991-February 1992. We used these data to test whether frequency and rate of kleptoparasitism were related to variation in food abundance and dominance status of Gadwalls. Overall, 30% (n = 423) of Gadwalls observed were kleptopar-asitic; the relative frequency and rate of kleptoparasitism varied monthly. The proportion of individuals engaged in food stealing and the rate of kleptoparasitism were greatest for subordinate sex/pair status classes of Gadwalls. When comparing diurnal time-activity budgets, kleptoparasitic Gadwalls spent less time feeding (56 vs. 68%) and more time searching for food (37 vs. 24%) than did individuals not participating in kleptoparasitism. Eurasian watermilfoil (Myriophyllum spicatum) was the primary food of Gadwalls and American Coots. As this aquatic macrophyte declined in abundance over the winter as a result of the effects of herbivory and natural senescence, so did numbers of coots and Gadwalls. When coot and Gadwall numbers were held constant statistically, the rate of kleptoparasitism was inversely related to milfoil cover. Food-stealing behavior of Gadwalls was influenced by social dominance and food abundance; it was mediated by host/parasite numbers. Results suggest that kleptoparasitism is an alternative foraging strategy used by subordinate Gadwalls that do not have access to good feeding areas.