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 (Oxyura jamaicensis) nesting in southern Manitoba in 1988. Seasonal n utrient dynamics differed between breeding and nonbreeding females. No nbreeders had less fat and protein reserves than did breeders early in the nesting season, but these then increased and subsequently decline d, whereas nutrient reserves of breeders declined linearly with date f rom the outset of our study. Onset of breeding was related to a nutrie nt 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 sho wed greater propensity to initiate breeding (73% of breeding populatio n) than did yearlings (68% of nonbreeding population), and we relate t his 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 committ ed to eggs. Early nesters initiated and ended egg laying with signific antly more fat compared to late nesters, although rate of fat use was not different. Protein and mineral reserves were used for clutch forma tion by early nesters (b = -0.24 and -0.08, respectively), but not by late nesters. Among females nearing end of laying, number of developin g follicles was related to mineral, but not fat or protein reserves, i ndicating that mineral reserves were a proximate factor constraining c lutch size. Our findings confirm that, for Ruddy Ducks, somatic tissue s are important sources of nutrients for clutch formation, and that cl utch size is directly related to mineral reserves. Of importance to li fetime reproduction, and thus fitness, is our finding that minimum lev els of nutrient reserves appear critical for initiation of breeding by female Ruddy Ducks.