The functional significance of parasitic egg laying and typical nesting in redhead ducks: an analysis of individual behaviour

At the population level, redhead ducks, Aythya americana, lay as many as 75% of their eggs parasitically but at least some females lay and incubate eggs in their own nests ('typical nesting'). Parasitic egg laying by redheads was documented with remote, time-lapse photography of potential host nests, allowing histories of parasitic egg laying and typical nesting to be compiled for individual females. In 1986 and 1987, years of favourable environmental conditions, many adult ( ~> 2 years old) females laid parasitic eggs prior to initiating their own nests in the same season. This dual strategy of individuals was reflected at the population level: the seasonal peak of parasitic egg laying preceded that of typical nesting and per caput rates of both parasitic egg laying and nesting were high. In contrast, redhead females either laid parasitic eggs or nested but did not do both during a drought in t 988. It is suggested that all redhead females employ a flexible, conditional reproductive strategy with four options of increasing reproductive effort: (1) nonbreeding, (2) parasitic egg laying, (3) typical nesting, and (4) a dual strategy of parasitic egg laying prior to nesting. When environmental conditions are favourable, a dual strategy enables redheads to increase their fecundity above the normal limits to clutch size. When prospects for successful nesting are poor, females reduce reproductive effort and employ parasitic egg laying as a low-cost alternative to nesting. Age-related differences in reproductive tactics were consistent with this model. Avian brood parasites are well known for the remarkable adaptations that enable them to obtain parental care from individuals of other species (e.g. Friedmann 1955; Wylie 1981). Although freedom from parental care should allow a dramatic increase in fecundity (e.g. Scott & Ankney 1980), relatively few species employ this seemingly ideal reproductive strategy. Recently, however, there has been growing evidence that parasitic egg laying by species that typically care for their own eggs and young is a widespread phenomenon among birds (MacWhirter 1989; Rohwer & Freeman 1989). Although it is more difficult to detect than obligate parasitism because host and parasite are often conspecific, facultative parasitism is continually being documented in additional species (e.g. Gratson 1989; Petter et al. 1990). Several recent reviews have proposed hypotheses for the functional significance of facultative parasitic egg laying (Yom-Tov *Present Address: Conservation & Research Center, U.S. National Zoological Park, Smithsonian Institution, Front Royal, Virginia 22630, U.S.A. 1980; Andersson 1984; Eadie et al. 1988; Sayler, in press), but only a few researchers have collected enough information about the context in which individuals lay parasitic eggs to test these hypotheses (Brown 1984; Emlen & Wrege 1986; Gibbons 1986; Moller 1987; Eadie, in press, see also Lank et al. 1989). Parasitic egg laying has been documented more often and is probably more common in waterfowl (Anatidae) than any other group of birds (Weller 1959; Rohwer & Freeman 1989). Among all birds, the redhead duck, Aythya americana, may exhibit the highest rates of facultative parasitic egg laying: at the population level, up to 50% of redhead ducklings may hatch from parasitically laid eggs (e.g. Olson 1964; Sugden & Butler 1980). Redheads also parasitize other species more frequently than other facultative parasites. In one 12-year study, redheads parasitized 57% of canvasback, Aythya valisineria, nests, laying an average of 3.1 eggs in each (Stoudt 1982). Most importantly, however, in all populations studied, at least some redhead females lay and incubate eggs in nests of their own 0003 3472/91/110771 +26 $03.00/0 9 1991 The Association for the Study of Animal Behaviour