Nutrient Deposition in Cattail Stands by Communally Roosting Blackbirds and Starlings

Abstract

Nutrient additions to five cattail (Typha spp.) stands in central New York State from droppings of roosting red-winged blackbirds (Agelaius phoeniceus), common grackles (Quiscalus quiscula), brown-headed cowbirds (Molothrus ater) and European starlings (Sturnus vulgaris) are reported. Bird numbers were estimated by counting birds as they left their roosts in the morning. Red-winged blackbirds were mistnetted as they entered a roost and were held until morning in cages lined with aluminum foil of known mass to determine the quantity of droppings excreted per bird-night. These data were used to develop a regression model to predict, from existence energy, the quantity of excreta input per bird per night. The amounts of nitrogen (N), phosphorus (P) and potassium (K) in the excreta were measured for red-winged blackbirds fed a simulated August diet and a simulated October diet. Red-winged blackbird droppings averaged 9.2% N, 1.41% P and 1.35% K. Nightly excretions of N, P and K averaged 59, 9.0 and 8.7 mg for a 39.0-g female and 75, 12 and 11 mg for a 56.0-g male. Nutrient loadings from blackbirds and starlings to the most densely populated roosts ranged up to 28, 4.3 and 4.1 kg*ha'eyear'I of N, P and K, respectively. These nutrient loadings are greater than those coming into the stands via precipitation and may be of similar magnitude to runoff loadings in some systems. INTRODUCTION Little is known about the role birds play in nutrient cycling (Sturges et al., 1974; Wiens and Dyer, 1977). Wiens (1973) suggested three possible roles for birds in ecosystems: (1) they may directly affect an ecosystem through a major influence on the flow of energy or nutrients; (2) they may act as controlling factors helping to maintain stability in the system without playing a major role in energy or nutrient cycling, or (3) they may be frills in the ecosystem living off its excess production and having no influence on the system. Population bioenergetics has been the primary means of assessing the role of birds in ecosystems. In general, this approach has shown that birds act only as frills, or at most play only a minor role in energy cycling (Holmes and Sturges, 1973; Wiens, 1973; Weiner and Glowacinski, 1975). Because of the necessary link between energy and nutrient cycling in birds, one might conclude that birds play at most a minor role in nutrient cycling. We present evidence to the contrary. Few rigorous studies have directly examined the role of birds in nutrient cycling. Bedard et al. (1980) found that nutrient importation by seabirds to an area of the St. Lawrence estuary was negligible relative to nutrients already present in the water or nutrients flowing into the estuary from the Mitis River. Likewise, Sturges et al. (1974) reported that birds played only a minor role in the nutrient budget of a northern hardwoods ecosystem. These studies support the idea that birds are of little consequence in nutrient cycling, but studies by Manny et al. (1975) and McColl and Burger (1976) came to different conclusions. Manny et al. (1975) suggested that inputs of N (12.9 kgeha-lyearA1) and P (3.9 kgeha-lyear-l) from migrant Canada geese (Branta canadensis) were the principal cause of hypereutrophic conditions in Wintergreen Lake, Michigan. McColl and Burger (1976) found that Franklin's gulls (Larus pipixcan) contributed 36% of the total annual P inputs to a cattail (Typha spp.) pool in Minnesota. These studies indicated that birds were potentially important nutrient cyclers. 1 Present address: Dept. of Biology, University of California-Riverside, Riverside, California 92521