Kleptoparasitism of American Coots by Gadwalls and Its Relationship to Social Dominance and Food Abundance
The AUK: A Quarterly Journal of Ornithology
Kleptoparasitism of American Coots (Fulica americana) by Gadwalls (Anas strepera) 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 kleptoparasitic; the relative frequency and rate of kleptoparasitism varied monthly. The proportion of individuals engaged in food stealing and the rate of
... e 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. FOOD STEALING, or kleptoparasitism, has been reported in many species of birds (Brockmann and Barnard 1979). It is especially prevalent in seabirds including gulls (Larus spp.; Hatch 1970, Schnell et al. 1983, Carroll and Cramer 1985, Rice 1985, Hesp and Barnard 1989), skuas (Stercorarius spp.; Andersson 1976, Furness 1977), and boobies (Sula spp.; Duffy 1980, Tershy and Breese 1990). Waterfowl (Anatidae) are hosts to at least 47 species of kleptoparasites, but only 3 species of ducks have been reported as kleptoparasites, despite the fact that waterfowl frequently occur in mixed species flocks (Brockmann and Barnard 1979). Gadwalls (Anas strepera) and American Wigeons (A. americana) are frequent parasites on American Coots (Fulica americana) and Eurasian Coots (F. atra) in areas of distributional overlap (Knapton and Knudsen 1978, Ryan 1981, Amat and Soriguer 1984). Kleptoparasitism can be important for meeting energy requirements, but benefits depend on cost of attack and profitability of food items • Present address: Florida Game and Fresh Water Fish Commission, 1239 S.W. 10th Street, Ocala, Florida 34474.