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Fraser, G. 1994. Feeding ecology and nesting success of Forster's terns on lake Osakis, MN. Final report submitted to the Nongame Wildlife Program, Minnesota Department of Natural Resources. 35 pp.


A colony of Forster's Terns (Sterna forsteri) nesting on Lake Osakis, Minnesota was studied in 1992 (132 pairs) and 1993 (158 pairs). Objectives were to: 1) identify the major prey species on the terns, 2) determine whether food availability was limiting, 3) measure reproductive success, and 4) identify the types of disturbances to the colony. Average foraging distance and group sizes were measured by weekly shoreline surveys. Prey identification, size, and feeding rates were measured using half-hour observation blocks, during morning, afternoon, and evening time periods. Average foraging distance from the colony site varied from 1.4 to 3.5 km. The average foraging group size was 2.8 in 1992 to 2.1 in 1993. Courtship feeding rates ranged from 0.53 to 1.32 fish per hour. As chicks aged, feedings rates decreased, and the size of the prey increased significantly. The main prey species were yellow perch, shiners, sunfish, and northern pike. Reproductive success was 0 for 1992 and between 0.126 and 0.323 for 1993. The main disturbances to the colony were boats, Great Blue Herons, a mink, and possibly a Great Horned Owl. Predation pressure rather than food availability appeared to be limiting reproductive success at the Lake Osakis colony. Recommendations include annual monitoring of major regional nesting sites to provide long-term population trends, protection of these sites, and distribution of educational materials to the public, particularly in areas that support a large sport-fishing community and with large, piscivorous, waterbird colonies.

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