Bioenergetics of Breeding Northern Shovelers: Diet, Nutrient Reserves, Clutch Size, and Incubation

Abstract

We analyzed food habits, digestive organs, and nutrient reserves (lipid, protein, and mineral) of male and female Northern Shovelers (Anas clypeata) nesting in southern Manitoba. We studied shovelers because, on an annual basis, they consume more animal matter than do other Anas spp. Thus, they are ideal subjects for testing the hypothesis that the inability of female ducks to obtain animal protein during egg laying limits clutch size. Predictably, we found that, during prelaying and laying, males and females primarily ate aquatic invertebrates. Despite this, the intestine lengths of breeding shovelers, particularly of females, are longer than those of many other waterfowl, which may be a response to an energy-deficient diet. Nutrient reserves of males changed little during prelaying and laying. Lipid reserves of females, on average, declined by 0.72 g for every 1 g of lipid that they deposited in eggs. Furthermore, in females that had laid 2 five eggs, there was a positive correlation (+0.60) between number of developing ovarian follicles and size of lipid reserves. Remarkably, however, the protein reserves of females increased by 0.1 g for every 1 g of protein committed to reproductive tissue. Mineral reserves were unrelated to eggshell pro- duction. The decline in lipid and protein reserves of incubating females could account for 26% of their energy requirements during that period. The protein : lipid ratio in shoveler diets was about 14: 1, whereas that in the reproductive tissue (eggs and oviduct) was near unity. Thus, we argue that lipid reserves, not protein acquisition, limit clutch size of shovelers and also that the importance of protein to tem- perate-nesting waterfowl has been overemphasized. We conclude that most intraspecific variation in clutch size of waterfowl is proximately caused and, thus, that few females lay the optimum clutch size, i.e., the average clutch size is not directly a product of natural selection, and probably reflects nutrients available to the average female.