Feeding Ecology of Breeding Gadwalls on Saline Wetlands

Abstract

The feeding ecology of breeding gadwalls (Anas strepera) from saline wetlands in North Dakota was examined in relation to sex, pair mates, reproductive status, food availability, and wetland type during the spring and summer of 1971 and 1972. Esophagi of males and females contained 40.4 and 48.2 percent animal food, respectively, between 17 April and 25 August. Animal foods consumed by paired females varied with reproductive condition and were independent of their mates. Invertebrates increased from 47.7 ? 17.4 percent in the diet during prelaying to 72.0 ? 18.4 percent during laying and declined to 46.3 ? 30.0 percent during postlaying. Aquatic insects dominated the diet during egg-laying and were selected disproportionately relative to their availability. Esophageal contents indicated that diversity of plant and animal foods in the diet varied inversely with specific conductance. Major factors influencing food selection of the breeding birds are discussed as interactions among their physiological status, their anatomical and behavioral characteristics, and the abundance and behavior of food organisms as influenced by chemical and physical features of the environment. The data suggested that these interrelated ecological factors act simultaneously to control the phenology of events and determine the foods utilized. J. WILDL. MANAGE. 40(1):69-817 Waterfowl feeding studies traditionally have provided a general description of foods consumed by a species (Martin et al. 1951:31, Mabbott 1920). Few earlier studies, however, considered the influence of the birds' reproductive status or the specific habitats where feeding occurred. Consideration of these factors in some recent studies has allowed more accurate interpretation of nutritional requisites of breeding birds and has provided insight into trophic relationships to particular habitats (Perret 1962, Bartonek and Hickey 1969, Swanson et al. 1974b, Krapu 1974a). Therefore, information on foods consumed by breeding birds will aid management through maintenance and preservation of essential habitats. Gadwalls have been regarded as vegetarians during most times of the year, yet sufficient information about foods utilized by breeding birds is lacking (Mabbott 1920, Martin et al. 1951:59, Dement'ev and Gladkov 1967:474). Feeding studies that utilized techniques described by Swanson and Bartonek (1970) clearly demonstrated the importance of animal foods in the diet of breeding ducks (Perret 1962, Bartonek and Hickey 1969, Krapu 1974a, Swanson et al. 1974b, Krapu and Swanson 1975). The gadwall, throughout its breeding range, uses a diversity of aquatic habitats ranging from fresh to saline. This study reports the feeding ecology of breeding gadwalls associated with saline wetlands and is designed to understand the gadwalls' relationships with particular habitats. Specific objectives were (1) to determine the foods consumed by breeding gadwalls, (2) to relate food utilization to the reproductive status of breeding females, (3) to assess selectivity by comparing food availability at sampling sites to foods consumed by females, and (4) to relate food consumption to various physical and chemical measurements of saline wetlands. Appreciation is extended to the Northern Prairie Wildlife Research Center for providing support and to J. J. Peterka, E. C. 'Present address: U.S. Fish and Wildlife Service, Northern Prairie Wildlife Research Center, Jamestown, North Dakota 58401. J. Wildl. Manage. 40 (1):1976 69 This content downloaded from 207.46.13.64 on Sat, 03 Sep 2016 04:51:15 UTC All use subject to http://about.jstor.org/terms 70 FEEDING ECOLOGY OF GADWALLS * Serie and Swanson Meslow, H. K. Nelson, and D. H. Johnson for helpful advice throughout the study. We are indebted to D. L. Trauger, G. L. Krapu, and W. R. Goforth for criticism and editorial review of this manuscript.