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Alternative reproductive tactics in a precocial bird : the ecology and evolution of brood parasitism in goldeneyes Eadie, John McAllister

Abstract

Facultative brood parasitism is common among waterfowl (Anatidae), but we have limited understanding of the ecological or evolutionary basis for this behaviour. I studied facultative brood parasitism in two species of cavity-nesting ducks, the Barrow's goldeneye (Bucephala islandica) and the common goldeneye (B. clangula). During a four year study in central British Columbia, I used field experiments, observational studies of marked individuals, and simulation models to (i) examine the consequences of brood parasitism to hosts, and (ii) identify the factors that promote and maintain parasitic behaviour. In order to assess the costs and benefits of brood parasitic behaviour, I first examined proximate influences on reproductive performance of goldeneye females. Variance in reproductive success among females was substantial and some females were consistently more successful than others. Reproductive success was also influenced by breeding experience, time of breeding, and by the availability and quality of nest-sites. Circumstantial evidence suggests that females compete exploitatively and aggressively for nest-sites and brood territories. I argue that such conditions favour the evolution of parasitic reproductive behaviours. Parasitic egg-laying occurred frequently during the four years of this study; 35% of all nests were parasitized, while 17% of all eggs were parasitic. Parasitism had few deleterious effects on the reproductive success of hosts. On average, parasitized females produced as many of their own young as non-parasitized females, and there was no effect of parasitism on female survival or on the growth and survival of host young. Hatch success was reduced in some host nests when the frequency of parasitism was high, but such levels of parasitism were uncommon. These results suggest that the costs of brood parasitism to precocial hosts are low. My findings do not support recent speculations that hosts benefit from being parasitized. Goldeneye females exhibited few defences against parasitic intrusions. Females were more likely to desert their nests when clutch sizes were extreme (i.e., > 16 eggs), but desertion rates did not differ significantly between parasitized and non-parasitized nests. I found no evidence that hosts reduce the size of their own clutch when parasitized, contrary to Anders son & Eriksson's (1982) findings for common goldeneyes in Sweden. On some occasions, hosts removed eggs from their nests, but this appeared to be a response to damaged eggs, rather than a defence against parasitism. Finally, females with territories adjacent to their nest sites were parasitized as often as females with non-adjacent territories, indicating that site-specific territoriality in goldeneyes does not serve to guard nests from parasites. I tested three hypotheses that have been proposed to explain parasitic behaviour. Brood parasites did not attempt to incubate clutches to which they had contributed, even when host females were experimentally removed from those nests. I therefore reject the hypothesis that brood parasitism is an inadvertent consequence of contests among females for the same nest site. In contrast, parasitic behaviour was more frequent in young females, and was strongly related to the availability of nest-sites. These results support the hypothesis that parasitic laying is a conditional strategy pursued by young females when nest-sites are limited. However, other evidence was consistent with the hypothesis that 'parasitic' and 'parental' behaviours are alternative tactics in a mixed strategy. Estimated lifetime reproductive success was nearly identical for the two groups of females, and the relative reproductive success of parasites was negatively frequency-dependent. A simulation model incorporating the effects of both density-dependence and frequency-dependence resolved this paradox. The model revealed a density threshold below which frequency-dependence effects were negligible, but above which frequency-dependence played a prominent role. I show that results consistent with both hypotheses are possible when the effects of population density are included. Current theories for the evolution of alternative reproductive tactics focus primarily on the frequency-dependent components of fitness and ignore the effects of population density. My results indicate that density-dependence and frequency-dependence can interact in an unanticipated way to maintain alternative nesting tactics in goldeneyes.

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