Introduction

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Miller, J.H., and D. Bosanko. 1989. Status and reproductive success in 1988 of common terns and ring-billed gulls at Leech Lake, Cass County, Minnesota. Final report submitted to the Nongame Wildlife Program, Minnesota Department of Natural Resources. 12 pp.

Introduction:

Invasions and population explosions of Ring-billed Gulls (Larus delewarensis) in Common Tern (Sterna hirundo) nesting colonies in Minnesota have resulted in concern about the future of terns throughout the state. The Leech Lake larid colony has been monitored since the summer of 1976 by Miller. This site has apparently been the largest and stablest colony of Common Terns in Minnesota at least since the 1920's and 30's. Miller's research suggests that the reproductive future of Common Terns in this colony is in serious jeopardy because of intruding Ring-billed Gulls. Since 1976 gull numbers have increased dramatically from less than 100 nests during the entire season to over 500 nests on 14 June 1986. During the same period tern numbers have declined, and as much as 50% of the space previously occupied by terns has been used by gulls.

The major problem to terns in this colony is loss of suitable nesting space. Terns are very aggressive and are able to defend nesting territories from gulls; however, they rarely seem to be able to evict gulls from gull territories. Because gulls begin nesting several weeks earlier than terns, terns have been steadily losing prime nesting space to gulls. Consequently, it may be necessary to manage this colony to prevent further losses, if not the complete disappearance of terns from the colony.

This study was conducted to provide additional background data for a possible project in which gull nests would be removed from preferred tern habitat to determine if the colony can be effectively managed to favor Common Terns. The objectives for the 1988 breeding season were to: (1) continue monitoring population trends in the Leech Lake colony by conducting a colony-wide census, (2) to measure reproductive success of terns and gulls in an area where an egg removal project would be conducted in the future, and (3) monitor the nesting chronology of Leach Lake larids. This report summarizes the results of our study.