Diet and nutrition of breeding canvasback and redhead ducks at Ruby Lake National Wildlife Refuge, Nevada

Graduation date: 1983Diet, nutrition, and foraging strategies of\ud canvasbacks (Aythya valisineria) and redheads (A.\ud americana) were studied by collecting actively feeding\ud birds throughout the reproductive season in 1980 and 1981;\ud samples of available foods were taken each time a bird was\ud collected. Nutrient composition of the aggregate diet was\ud determined for each of 6 reproductive stages (Females,\ud prelaying through brooding; Ducklings, Age Classes I and\ud II). Body composition (lipids and protein) of males and\ud females was determined at each stage of reproduction.\ud Emphasis was placed on the female segment of the redhead\ud and canvasback populations. Canvasback females pursued a\ud conservative foraging strategy, consuming pondweed tubers\ud (84% of diet) prior to laying, but invertebrates\ud (principally caddisfly larvae, dragonfly nymphs, and\ud snails) throughout the remainder of the summer. Preferred\ud food items made up a large part of the diet. With the\ud exception of snails, canvasback females generally did not\ud consume the most abundant foods available. During the\ud prelaying stage, the diet contained high levels of NFE\ud (nitrogen free extract) (68%), but low levels of protein\ud (17%) and lipids (1.5%). Nutrient content of the diet was\ud constant throughout the remainder of the reproductive\ud season (NFE, 30%; protein, 50%; lipids, 4%). Female\ud canvasbacks had large lipid reserves during prelaying and\ud retained those reserves during laying. However, they used\ud 72% of their lipid reserves during incubation, probably\ud because of high attentiveness to the nest. Redhead\ud females tended to be more opportunistic foragers than\ud canvasbacks. Redheads fed largely on benthic aquatic\ud vegetation (mostly Chara) and seeds except during the\ud laying stage, when invertebrates and bass (Micropterus\ud salmoides) eggs made up the bulk of the diet. The food\ud item composition of the diet varied among all stages and\ud preferred items often were only a small portion of the\ud diet. Nutrient composition of the diet also varied among\ud the reproductive stages and was high in lipids (15%) and\ud protein (50%) only during the laying stage. Energy\ud content of the diet was quite variable (4.4 to 2.4 kcal/g)\ud among the reproductive stages, but was highest during\ud laying. However, female redheads used 34% of their lipid\ud reserves during the laying stage and 56% during the\ud incubation stage. Redheads are parasitic egg layers and\ud apparently devoted much of their energy to locating host\ud nests and laying large numbers of eggs. Such a strategy\ud requires a high energy, high protein diet, but such foods\ud may be difficult for redheads to obtain. As a result,\ud redheads had reduced energy reserves to expend during\ud incubation and brooding. If redheads had expended energy\ud during incubation at the same rate as canvasbacks,\ud redheads would have depleted their lipid depots 1/2-2/3 of\ud the way through incubation. Thus, these two closely\ud related species pursued quite different reproductive and\ud foraging strategies in the same habitat