The earliest known loon population records, from around the turn of the 20th century, document loons nesting as far south as Iowa. However, through the mid-20th century, their breeding range moved northward as lakeshore habitat disappeared and lakes became increasingly polluted.
In the last quarter-century their numbers have stabilized and perhaps even increased slightly in the northern Great Lakes region, though loons are still sensitive to pollutants such as mercury, lead and acid rain. They also will abandon lakes that no longer provide suitable nesting habitat due to shoreline development. In addition, loons are endangered by power boaters or personal water craft users whose wake may swamp nests, wash away eggs or drown newly born chicks.