Foods of Juvenile Ring-Necked Ducks: Relationship to Wetland pH

Abstract

Foods of 37 juvenile ring-necked ducks (Aythya collaris) from 16 different wetlands were examined in eastcentral Maine in 1983-85. Invertebrates made up 70% aggregate dry weight (100% occurrence) of the foods of Class Ia-IIa (-24 days old) ducklings and 32% (86% occurrence) of Class IIb-III (>25 days old) ducklings. These percentages may be as high as 85% for Ia-IIa ducklings and 47% for IIb-III ducklings after adjusting for insect sclerites. Ducklings ate invertebrates from 44 taxa and seeds or fibers from 23 plant taxa. Freshwater sponges (Porifera) were the most important invertebrate and constituted 23% of the foods of all ducklings. Seeds of pondweeds (Potamogeton spp.) were the most important plant foods. Diets of ducklings from high-pH (26.1) wetlands were more diverse (t = 2.54, P = 0.021) than those from low-pH (<6.1) wetlands and consisted of 33 invertebrate taxa. Only 17 taxa occurred in ducklings from low-pH ponds. Class Ia-IIa ducklings from high-pH wetlands ate more invertebrates (77%) than ducklings from lower-pH wetlands (61%), although the difference was not significant (P = 0.21). Sponges made up the largest percentage of the diets and occurred in similar amounts in high(34%) and low(31.5%) pH wetlands. J. WILDL. MANAGE. 52(2):177-185 Aquatic invertebrates are important foods for young ducklings (Bartonek and Hickey 1969, Sugden 1973, Reinecke 1979), and retarded growth and low survival have been associated with low invertebrate abundance (Street 1977). Aquatic invertebrates are important in the diet of ring-necked duck ducklings in Minnesota (Hohman 1985). Mendall (1958) reported the foods eaten by ring-necked ducks in Maine, but his data included only gizzard contents, which bias results toward the less digestible plant foods (Swanson and Bartonek 1970). Ring-necked duck habitats in Maine consist primarily of permanent wetlands that have lower productivities than newly-flooded or seasonally-flooded wetlands (Whitman 1974). Many wetlands in Maine are vulnerable to acidification, a process that can adversely affect invertebrates (Bell 1971, Haines 1981, Eilers et al. 1984). Reinecke (1977) reported no difference in the nutrient content of duck foods in Maine wetlands versus richer, prairie wetlands, and he believed that food quantity rather than quality limited waterfowl in the Northeast. Low invertebrate diversity or biomass in wetlands could adversely affect duckling diets. The purpose of our study was to document food habits of ring-necked duck ducklings in Maine and to determine if diets differed between wetlands of high and low pH. We thank A. G. LaRochelle, P. E. Malicky, D. D. Eggeman, and R. R. Roy for their assistance in the field. T. M. Mingo identified invertebrates and J. E. Longcore identified sponges. K. L. Stromborg analyzed water samples. M. A. Howe, K. J. Reinecke, T. W. Custer, and 2 anonymous referees gave helpful reviews of the manuscript. STUDY AREA AND METHODS Our study area consisted of isolated broodrearing wetlands in a 17,800-km2 area (McAuley 1986) that extended from eastern Waldo County through Hancock and Washington counties and into northern Penobscot and southwestern Aroostook counties in Maine. This area included most of the wetlands that were studied by Mendall (1958). Bedrock types (i.e., sensitivity class 1 and 2 [no-low and low-medium acid neutralizing capacity]) (Norton et al. 1982a,b) indicated wetlands were vulnerable to acidification (alkalinity 5200 geq/L [Hendrey et al. 1980]). Ducklings were collected from 16 wetlands. Most water samples were collected in July 1983-84. Measurements of water chemistry followed Haines and Akielaszek (1983), except alkalinity was determined by the Gran plot method (Stumm and Morgan 1970) only. During 1983-85, we attempted to collect Class I (?16 days old), II (17-38 days old), and III (39-49 days old) ducklings (Gollop and Marshall 1954) from as many wetlands as possible. We tried to observe ducklings feeding before collecting (Swanson and Bartonek 1970), but because broods were sparsely distributed and often