Changes in nutrient reserves and organ size of female ruddy ducks breeding in Manitoba

-Female Ruddy Ducks (Oxyura jamaicensis) were collected at specific stages of reproduction to investigate the role of endogenous nutrients in breeding. Pectoral muscle weight decreased between prelaying and laying, suggesting the transfer of labile protein to the developing reproductive organs. Liver weights peaked during laying, reflecting increased metabolic activity associated with hyperphagia. Body weights increased 123 g (21%) between arrival and prelaying, primarily as a result of the deposition of stored lipids. Between laying and early incubation, body weights declined 145 g (20%), primarily as a result of the regression of the oviduct and lipid loss associated with egg formation. Body weights declined approximately 111 g (18%) during incubation, reflecting the catabolism of lipid reserves to supplement energy requirements not met by foraging during incubation recesses. Dietary intake provides the majority of the energy necessary for incubation, because lipid catabolism supplies only one-fifth of the basal metabolic requirements. Female Ruddy Ducks, therefore, depend almost exclusively upon dietary intake to meet the energy and nutrient requirements of reproduction. This pattern of energy acquisition for and allocation to reproduction differs markedly from that of other small-bodied anatids studied thus far. Received 17 May 1982, resubmitted 31 October 1983, accepted 1 March 1984. RECENTLY, the reproductive energetics of waterfowl have been intensively investigated. Owen and Reinecke (1979) summarized four general patterns of energy acquisition for and allocation to reproduction. Waterfowl species studied thus far have exhibited patterns of energy acquisition and allocation that range from utilizing exclusively endogenous energy accumulated either on or away from the breeding grounds to depending upon dietary intake plus a small but significant energy component accumulated either on or away from the breeding area. The breeding biology of the Ruddy Duck (Oxyura jamaicensis), the only oxyurid to nest regularly in North America north of Mexico, differs markedly from that of other North American Anatidae. Ruddy Ducks arrive unpaired on the southwestern Manitoba breeding grounds in late April. Monogamous pair bonding and other male-female associations, including promiscuous and polygynous behavior (Gray 1980), are exhibited, although Siegfried (1976a) reported that most Ruddy Duck pair bonds in Manitoba are monogamous. Egg lay' Present address: School of Natural Resources, Samuel Trask Dana Building, The University of Michigan, Ann Arbor, Michigan 48109 USA. ing begins approximately 4 weeks after the arrival on the breeding grounds (Siegfried 1976b). Oxyurids produce eggs that are the largest (proportional to body size) of any produced by Anatidae (Lack 1968), and a completed clutch may weigh more than the female. Furthermore, female Ruddy Ducks do not insulate their nests with down (Low 1941), and they take frequent, short, incubation recesses (Siegfried et al. 1976a, Tome 1981). Although several aspects of the breeding biology of the Ruddy Duck have been described (Low 1941; Misterek 1974; Siegfried 1973, 1976a; Siegfried et al. 1976a; Joyner 1977, 1983; Gray 1980), no one has determined the significance of stored nutrients for reproduction in this species. The objective of this paper is to discuss the significance of nutrient reserves to reproduction in female Ruddy Ducks by examining changes in carcass composition and selected organ weights. STUDY AREA AND METHODS I collected female Ruddy Ducks in the prairie-pothole region of southwestern Manitoba approximately 16 km southeast of Minnedosa. Birds were collected from 1979 through 1981 from early May through the first 10 days of brood rearing in late July. Using the following criteria, I assigned each specimen to 1 830 The Auk 101: 830-837. October 1984 This content downloaded from 207.46.13.113 on Thu, 06 Oct 2016 04:36:34 UTC All use subject to http://about.jstor.org/terms October 1984] Nutrient Reserves of Female Ruddy Ducks 831 of 6 reproductive categories on the basis of ovarian development or stage of incubation. Arrival.-No follicles in the rapid development stage, which is characterized by yolk deposition; largest follicle <8 mm (n = 15). Prelaying.-Follicles yolky in appearance and -8 mm; no ruptured or regressing follicles present (n = 9). Laying.-An egg present in the oviduct or ruptured follicles and large, developing follicles in the ovary; no regressing follicles present (n = 6). Early incubation.-Females in the first 12 days of incubation as determined by comparing an embryo from the clutch with preserved embryos of known age (n = 9). Late incubation.-Females in the last 12 days of incubation determined as above (n = 8). Brood rearing.-Females accompanying broods < 10