Body Weight and Composition Changes and Adaptations for Breeding in Wood Ducks

The weight and carcass composition of Wood Ducks (Aix sponsa) collected in southeast Missouri were analyzed during fall and at specific stages of the reproductive cycle. Males lost 4% of their body weight between fall and spring. Decreases in the weights of digestive organs were primarily responsible for the change. The body weights of hens increased 14% between fall courtship (618 g) and the maximum attained during laying (706 g). Nearly all of the weight gain of females occurred on the breeding grounds and was the result of fat deposition and increases in the weights of the reproductive organs. Body weights of hens decreased 164 g during laying and incubation. Utilization of stored fat for egg production accounted for most of this weight loss. The lack of significant changes in the ash-free lean dry mass of the carcass as well as a change in food habits indicated that the protein requirements for the clutch are obtained by foraging for invertebrates during laying. Weight losses during incubation were small, indicating that hens are able to meet nearly all incubation energy requirements by foraging during inattentive periods. Female Wood Ducks satisfy their nutrient and energy requirements during the reproductive cycle by foraging and the use of endogenous reserves. Adaptations that maximize the use of stored lipids for egg production and facilitate the conservation of endogenous protein and the acquisition of dietary protein enable hens to produce and incubate large clutches of relatively large eggs without significant changes in body protein. Studies of weight changes in breeding ducks (Weller 1957, Folk et al. 1966, Harris 1970, Korschgen 1977, Landers et al. 1977) have shown that the body weights of females increase prior to clutch initiation and then decline rapidly during laying and incubation. These weight changes have been attributed primarily to the growth and subsequent involution of the ovary and oviduct and fat deposition and mobilization. Changes in the weights of digestive organs and atrophy or mobilization of muscle tissue have also been implicated, but to a lesser degree. Although previous investigators have identified the variables that influence changes in body weight, these variables (especially changes in carcass composition) have not been well measured at specific stages throughout the reproductive cycle for ducks. Information of this kind is nevertheless needed to meaningfully interpret weight data collected in the field and to better understand adaptations for breeding and the impact of reproductive efforts on the physiological condition of hens. The results reported here represent a portion of the data collected during a three-year study of the feeding ecology, nutrition, and reproductive bioenergetics of Wood Ducks (Aix sponsa; Drobney 1977). The purpose of this paper is to quantify the variables responsible for changes in body weight during the reproductive cycle and to relate changes in carcass composition to the breeding biology of Wood Ducks. STUDY AREA AND METHODS Body and organ weights were obtained from 159 Wood Ducks (84 females and 75 males) co lec ed during 1975-1977 on the Duck Creek Wildlife Management Area in southeastern Missouri. All birds except for six hens that were removed from nest boxes, were obtained by shooting in flooded timber foraging areas during morning (06:00-11:00) and afternoon (15:00-19:00) collection periods. Carcass analyses were performed only on the 1976-1977 sample consisting of 43 females and 38 males. Following collection, birds were taken to the laboratory, weighed, and dissected. Fat attached to mesenteries and internal organs was removed and weighed, but not included in the carcass composition analyses. These fat deposits are referred to as visceral fat in this paper. Internal organs (contents removed) were weighed to the nearest 0.01 g (wet weight). Birds that were to be analyzed for carcass composition were plucked, sealed in plastic bags, and frozen. Carcass composition was analyzed using methods described by Rogers and Odum (1964), Ricklefs (1967), and Brisbin (1968). Before processing, birds were removed from the freezer and thawed for approximately 30