Nutrition of Breeding Female Ruddy Ducks: The Role of Nutrient Reserves

We determined fat, protein, and mineral content of somatic tissue, and estimated amounts of these nutrients committed to eggs by Ruddy Ducks (Oxyurajamaicensis) nesting in southern Manitoba in 1988. Seasonal nutrient dynamics differed between breeding and nonbreeding females. Nonbreeders had less fat and protein reserves than did breeders early in the nesting season, but these then increased and subsequently declined, whereas nutrient reserves of breeders declined linearly with date from the outset of our study. Onset of breeding was related to a nutrient reserve threshold (early RFG [rapid follicle growth] ducks had 22.7 g more fat and 5.1 g more protein than did non-RFG ducks). Adults showed greater propensity to initiate breeding (73% of breeding population) than did yearlings (68% of nonbreeding population), and we relate this to greater fat and protein reserves of adults. Adult breeders had more fat and protein reserves than did yearlings at every stage in the breeding cycle but no age-related differences in use of fat, protein or mineral reserves for clutch formation were found. When controlling for date effects, fat reserves declined (b = -0.49) as fat was committed to eggs. Early nesters initiated and ended egg laying with significantly more fat compared to late nesters, although rate of fat use was not different. Protein and mineral reserves were used for clutch formation by early nesters (b = -0.24 and -0.08, respectively), but not by late nesters. Among females nearing end of laying, number of developing follicles was related to mineral, but not fat or protein reserves, indicating that mineral reserves were a proximate factor constraining clutch size. Our findings confirm that, for Ruddy Ducks, somatic tissues are important sources of nutrients for clutch formation, and that clutch size is directly related to mineral reserves. Of importance to lifetime reproduction, and thus fitness, is our finding that minimum levels ofnutrient reserves appear critical for initiation of breeding by female Ruddy Ducks.

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