Rapid increase in the lower Great Lakes population of feral mute swans: a review and a recommendation

Mute swans (Cygnus olor) are an exotic species whose population has increased throughout the lower Great Lakes since their introduction during the mid-twentieth century. We used 3 independent data sources to estimate the rate of increase of mute swans on the lower Great Lakes: aerial surveys in spring and autumn at Long Point, Lake Erie, Ontario, Canada 1971-2000; midwinter waterfowl inventory of the north shore of Lake Ontario, 1980-2000; and Christmas Bird Counts on both the United States and Canadian shores of the lakes, 1980-2000. The average estimated population growth rate varied from 10 to 18% per year. The most conservative growth-rate estimate of 10% per year indicates a doubling of the mute swan population every 7-8 years. These high growth rates indicate that mute swans have found a favorable environment in the lower Great Lakes. That area is climatically somewhat similar to their native range in Europe, with low natural predation rates and minimal human interference (the birds are protected in Canada since and the United States under the Migratory Birds Convention of 1916). If the mute swan carrying capacity of the lower Great Lakes is similar to portions of the species' native European range and growth rates continue, the Canadian population could reach as many as 30,000 birds within 30 years. Given that the species is nonnative and its ecological impact potentially could be large, we suggest that control measures be implemented before the population grows much larger.