The functional significance of parasitic egg laying and typical nesting in redhead ducks: An analysis of individual behaviour. Anim. Behav

final acceptance 15March 1991; MS. number: A5924) Abstract. At the population level, redhead ucks, Aythya americana, lay as many as 75 % of their eggs parasitically but at least some females lay and incubate ggs 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 nvironmental conditions, many adult ( ~> 2years 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) non-breeding, (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 imits to clutch size. When prospects for successful nesting are poor, females reduce reproductive effort and employ parasitic egg laying as a low-cost alternative tonesting. Age-relate