Reproductive Success of Forster's Terns on Natural and Artificial Substrates

Three colony sites of Forster's terns were studied at Lake Poygan, Winnebago County, Wisconsin, during 1990 and 1991. Two of the colonies existed on artificial platforms designed to provide much needed nesting substrate. Reproductive success on natural and artificial substrate was compared. In 1990, 25% of 52 platform clutches hatched but produced no fledged chicks, while none of 88 natural nest clutches hatched. In 1991, 70% of 43 platform clutches hatched, but only 1 chick may have survived to fledge. At least 82% of 65 natural nest clutches hatched, and 61 chicks survived to fledge. Nest loss was caused by wave action, egg predation, and abandonment. Chick mortality was caused by sibling rivalry and parent neglect, attacks by adult terns, and aquatic predators. Human disturbance, including research activity, did not appear important. Reproductive success on platforms was much lower than originally anticipated. Future success of the colonies will depend on achieving the summer water level goal of the Winnebago Comprehensive Management Plan in mid-May, and maintaining a stable water level throughout the breeding season. Modifications recommended for the platform program include improving chick access to platforms with plastic mesh ramps, and fencing platform colonies to exclude predatory fish and large turtles.WDNR Bureau of Endangered Resources; Wisconsin Wildlife Federation; Wisconsin Academy of Science, Arts, and Letters; UW-Stevens Point Student Research Fund; and the Wisconsin Society for Ornithology