Too Rare to Measure: What Great Blue Herons Are Telling Us About Minnesota’s Lakes

Great blue herons are more than familiar shoreline birds. Like loons, they are indicator species, reflecting the health of Minnesota’s lakes, rivers, and wetlands. That is why one observation in Carrol Henderson’s A National Legacy: Fifty Years of Nongame Wildlife Conservation in Minnesota is especially troubling:

“Like loons, herons are an indicator of healthy lakes and waters. Unfortunately, they are now seen in such low numbers that it is difficult to assess how seriously their populations are declining.”

This is a late-stage warning sign in conservation biology. When populations fall low enough, traditional monitoring begins to fail. Colonies disappear quietly. Trends become harder to document. The absence of data can then be mistaken for the absence of a problem.

Henderson notes that in recent years, formerly large heronries—including a major multi-species colony in St. Paul—have shown no nesting activity. At the same time, statewide heronry monitoring has been inconsistent, making it difficult to determine whether observed losses are isolated or part of a broader decline.

Herons do not simply respond to one stressor. They integrate many: shoreline development, water quality, prey availability, and disturbance at long-standing nesting sites. When herons decline, it signals broader ecological stress—often long before those changes are obvious to people.

Henderson’s warning: If indicator species become too rare to measure reliably, conservation efforts shift from prevention to post-hoc explanation. By the time declines are undeniable, the systems that supported them may already be gone.

Herons do not disappear suddenly. They fade—colony by colony—until even their absence becomes easy to overlook.

Photo by Lynn Cornell