Effects of Brood Parasitism and Nest-Box Placement on Wood Duck Breeding Ecology

Abstract

Intraspecific brood parasitism occurs frequently among Wood Duck (Aix sponsa) populations nesting in artificial structures. To assess the effects of such parasitism on population ecology, we analyzed 12 years of Wood Duck nesting records (1976-1987) from a study site in northeastern Illinois. Hatchability of eggs (ducklings produced/total eggs laid) was inversely correlated with population density, with the frequency of parasitism, and with the number of eggs laid per nest. The hatchability of all eggs laid in nests that had been parasitized (16-44 eggs) was 57.5% vs. 67.3% for eggs laid in "normal" nests (7-15 eggs). The negative consequences of parasitism were due mainly to nest abandonment, damaged eggs, and eggs laid after the start of incubation, and occurred despite the consistent availability of suitable unused boxes. The frequency of brood parasitism was strongly affected by box placement. During 1976-1987, parasitism occurred in 49.5% of boxes erected singly in highly visible locations, in 49.5% of boxes erected in highly visible groups, but in only 29.8% of boxes that had been erected singly in visually occluded habitat. Mean clutch sizes for the visible-isolated (15.7 eggs) and visible-clumped (16.3 eggs) boxes were significantly higher (F = 4.49, P = 0.012) than for the well-hidden boxes (12.4 eggs). Hatchability in successful well-hidden nests was 82.0% vs. ca. 74.0% in successful visible boxes. The data suggest that reduced parasitism and increased hatchability occur when artificial nesting structures are placed in habitats and at densities resembling the natural circumstances in which Wood Ducks evolved. These results have implications not only for the study and management of A. sponsa populations, but also for the placement of nest boxes in behavioral and ecological studies of other cavity-nesting birds.