Miller, J.H., and D. Bosanko. 1990. 1990 management plan for common terns and ring-billed gulls at Leech Lake, Cass County, Minnesota. Report submitted to the Nongame Wildlife Program, Minnesota Department of Natural Resources. 5 pp.
The past sixty years have seen a well documented increase in large gulls throughout the Northern Hemisphere. The local populations of great black-backed, herring, ring-billed, and laughing gulls have exploded. During this same time populations of common, roseate and sandwich terns have decreased.
This larger pageant is being played out in the Great Lakes region with ring-billed gulls and common terns. The increasing gull populations if not immediately, then within a few years come in contact with the common terns on the nesting grounds. The most common scenario is that gulls, not depending on open water to feed, arrive on the nesting grounds a few weeks before the terns. The gulls are then able to set up territories on the traditional tern nesting sites. While common terns are very capable of defending their nests, they are not normally able to displace gulls.
The Minnesota common tern and ring-billed gull colonies are typical of hose throughout the Great Lakes region. The terns are decreasing while the gull population expands. The reason for this rapid increase in gulls is poorly understood, but the accompanying decrease in tern reproduction is obviously highly correlated. This is in no place more clearly evident than at Leech Lake, Cass County, Minnesota, where the gulls have almost entirely displaced the common terns.
The Leech Lake tern colony is located on Gull Island, approximately 7 km north of Whipholt, Cass County, Minnesota. This 0.3 ha island is comprised of glacial rubble with one open sandy area beginning in a small bay and extending into the center of the island. The island for the most part is devoid of vegetation.
The open sand on Gull Island (~0.1 ha) has been an active common tern nesting site for many years. In the 1960's and 70's ring-billed gulls began nesting in the rockier portions of the island. Since that time gull numbers have increased dramatically, expanding beyond the rocks into the tern colony. By 1988 fewer than 150 pairs of common terns nested on Gull Island and none successfully fledged chicks.
It is evident from the history of the Leech Lake colony as well as from other Great Lakes colonies that without significant management, the common terns will soon be extirpated from their traditional nesting site. It is also probable that the management of terns will have to be long term, remaining in effect until the gull population decreases.