The Limestone Islands IBA is located on the eastern coast of Georgian Bay, Ontario, about 20 km west of the small port of Snug Harbour, and 50 km west of Parry Sound. This site is comprised of three islands: North Limestone Island, and a north and south island within the South Limestone Islands group. The islands are quite isolated with the open expanse of Georgian Bay to the west, and the Mink Islands lying about 7 km to the east.
As their name suggests, these islands are composed of mainly flat, open limestone alvar habitat. North Limestone Island has a few deciduous trees in the south-central part of the island, as well as some cedar trees and large areas of poison ivy. There is a large crescent-shaped gravel beach at the southeastern tip. The South Limestone Islands also have some cedar trees as well as some shrub vegetation. They are flatter than North Limestone Island and have several large shallow bays at the north end.
The Limestone Islands support significant colonies of Common Terns, Caspian Terns and Ring-billed Gulls. In most survey years, over a thousand Common Tern nests are recorded, with a high of 1,339 nests being recorded in 1998. A thousand nests represents about 2.5% of the North American population of Common Terns. Common Terns nest only on North Limestone Island.
Caspian Terns are equally well represented. Well over 300 nests are recorded here on average, with a high of 433 nests in 1989. The average number of nests represent just over 1% if the North American population of Caspian Terns. This species does not nest on the same island as the Common Terns, but nests instead solely on the northernmost of the two South Limestone Islands.
The more abundant Ring-billed Gull also nests at this site in significant numbers. In the 1980s, based on two survey years, an average of 16,851 Ring-billed Gull nests were recorded. This is about 2% of the world's estimated population. The Ring-billed Gull nests are located on both of the South Limestone Islands.
In addition to the large colonies of terns and Ring-billed Gulls, smaller numbers of Herring Gulls (227 nests in 1989) and Double-crested Cormorants (33 nests in 1989) nest here as well. In the 1980s, Great Blue Herons nested on North Limestone, but they are no longer present, and it is thought that they moved to the McCoy Islands to the northeast.
BirdLife International (2020) Important Bird Areas factsheet: Limestone Islands. Downloaded from http://www.birdlife.org on 28/01/2020.