Experimental Parasitism of American Coot Nests

Abstract

BROOD parasitism has evolved in seven groups of five families of birds involving 80 or more species (Lack, 1968: 82). Many complex structural and behavioral adaptations have resulted, but the selective forces that have induced this mode of life are obscure. It is obvious that the host is an important factor in the successful evolution of this behavior (Hamilton and Orians, 1965; Lack, 1968: 96-97). Only one completely parasitic species has precociat young, the Black-headed Duck (I-Ieteronetta atricapilla) of temperate South America. The major hosts of this species are coots, especially the Red-fronted Coot (Ful•ca rufifrons) (Wetter, 1968). Young Black-headed Ducks are hatched successfully and rear themselves so that differences in feeding and parental behavior are insignificant. Some species of waterfowl lay parasitically but also build nests; these have been termed semiparasites (Wetter, 1959). Those with strong parasitic tendencies are the Redhead (Aythya americana) and the North American Ruddy Duck (Oxyura jamaicensis) (Low, 1941, 1945). Their hosts are almost exclusively other ducks with plain, lightcolored eggs, whereas Black-headed Ducks parasitize hosts with eggs of any size or color. Ruddy Ducks only rarely parasitize nests of American Coots (Fulica americana) (Welter, 1959; Fredrickson, pers. comm.), and there is one record of a Redhead parasitizing a coot nest (Bryant, 1914). The great abundance of nests of American Coots in marshes frequented by semiparasitic Redheads and Ruddy Ducks would appear to be a perfect evolutionary inducement for successful parasitism. Moreover coots seem well-suited as hosts by their nesting behavior. They are extremely broody and both members of the pair incubate with the result that nest and egg success are extremely high. Coots have large clutches, and incubation does not start until the 4th or 5th egg, so there is considerable time when parasitic eggs could be deposited and be incubated fully. That these species occur together without the evolution of a host-parasite relationship might be a result of: 1) lack of response of the parasite to the cooifs spotted, tan egg so that parasitic eggs are not deposited, 2) a negative response of the host to the parasitic egg s.o that the egg does not hatch, or 3) that the young do not survive because they are not completely independent as are young Black-headed Ducks. Because these possibilities have a