Female Hooded Merganser Body Mass during Nesting

Abstract

Body mass of female Hooded Mergansers (Lophodytes cucullatus) nesting in widely dispersed and newly erected wooden nest boxes in northcentral Minnesota was measured in 1982-1985. Median body mass during egg-laying was 635 g. Female mass during incubation varied significantly among years, but decreased monotonically at the same rate (1.0 g day-1) each year. Mass at the end of incubation (519-494 g) was 5.7 to 6.0% less than when incubation began. There was no indication that females having the greatest body mass began nesting earliest. However, females with the greatest body mass incubated the largest clutches and hatched the most young. Comparison of the observed body mass-clutch size relationship with one assumed to exist in the absence of intraspecific brood parasitism indicated that more parasitic eggs were laid in nests incubated by heavier females.