Wednesday, September 8, 2021

Canada’s Loon Nesting Survey Analysis

There is renewed interest in the nesting success of Common Loons (photos by Mark Peck).

The Canadian Lakes Loon Survey conducted by Birds Canada has analyzed information about a remarkable 35,000 nesting attempts by Common Loon pairs across southern Canada, collected by more than 4,000 volunteer birders. Data collection started in 1981 in Ontario and spread across the rest of southern Canada in the mid-1990s. The huge amount of information provided biologists with an opportunity to test the influence of more than a dozen different factors on Common Loon productivity over an extended period.

The factors included the effect of acid rain, mercury contamination in fish, shoreline development, boating activity, risk of predation by Bald Eagles, competition for forage fish by Double-crested Cormorants, and a suite of weather variables based on temperature and precipitation.

Researchers ruled out shoreline development, human disturbance, eagles, and cormorants as strong factors for the declines in Common Loon productivity across all of Ontario or southern Canada; however, some of those factors certainly play a role on particular lakes. Their analysis suggests that a complex interplay between the effect of acid rain, mercury pollution, and on-going climate heating may be at least partly responsible for productivity declines of Common Loons across southern Canada. Dubbing this the “acid-mercury-climate hypothesis,” biologists are pursuing additional research to test its merits, which will provide much needed guidance about what can be done to best conserve the Common Loon, an impressive symbol of pristine wilderness.

Learn more about the information provided in the Birds Canada report at and view an enjoyable introduction to the Common Loon report on YouTube at