Cr. Leschack et Gr. Hepp, KLEPTOPARASITISM OF AMERICAN COOTS BY GADWALLS AND ITS RELATIONSHIP TO SOCIAL-DOMINANCE AND FOOD ABUNDANCE, The Auk, 112(2), 1995, pp. 429-435
Kleptoparasitism of American Coots (Fulica americana) by Gadwalls (Ana
s strepera) was recorded from October 1991-February 1992. We used thes
e data to test whether frequency and rate of kleptoparasitism were rel
ated 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 pr
oportion of individuals engaged in food stealing and the rate of klept
oparasitism were greatest for subordinate sex/pair status classes of G
adwalls. 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 klept
oparasitism. Eurasian watermilfoil (Myriophyllum spicatum) was the pri
mary food of Gadwalls and American Coots. As this aquatic macrophyte d
eclined in abundance over the winter as a result of the effects of her
bivory and natural senescence, so did numbers of coots and Gadwalls. W
hen coot and Gadwall numbers were held constant statistically, the rat
e of kleptoparasitism was inversely related to milfoil cover. Food-ste
aling 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 subo
rdinate Gadwalls that do not have access to good feeding areas.