Field Notes

Learning from Night-Herons

As ecotourism continues to grow, questions arise: Just how close can we get to wildlife without causing harm? How often, really, can we poke our voyeuristic binoculars into animals’ lives? New research on the famed black-crowned night-heron colonies of the Calumet region offers some answers.

Patrick Zollner, an ecology professor at Purdue University, spent two recent summers with colleagues spying on a group of young black-crowneds in a pair of Calumet marshes. While parents were away, 14 infrared cameras recorded the effect of “planned disturbances” on the nestlings’ behavior.

By both canoe and foot, Zollner’s team approached the birds from a variety of directions, at varying speeds and noise levels. They measured the frequency with which the birds stopped their preening and resting to freeze, stare, or otherwise express discomfort. Their data reveals that while visitors in general are a nuisance, the number of visitors is of less concern to the birds than how close these intruders come to their nests.

The study — a partnership of the USDA Forest Service, the Southeast Environmental Task Force, BP, and California State University — has created a model of the relationship between the birds’ stressed and relaxed states. It recommends that land managers keep trails and canoes at least 164 feet from black-crowned night-heron colonies, and that visitor activities be restricted during the early part of the nesting season so fewer birds abandon their nests. The Forest Preserve District of Cook County is incorporating the findings as it plans public access at Indian Ridge Marsh.

In a separate study of black-crowned herons, researchers have found that pollutants banned in the 1970s exist in Calumet birds in greater concentrations than in birds from Minnesota or Virginia. Lake Michigan alewives are the link.

“In other places, black-crowned night-heron populations have recovered from the days of DDT. But that doesn’t seem to have happened in Illinois,” says Jeffrey Levengood, a wildlife toxicologist with the Illinois Natural History Survey. Levengood and colleagues studied the tissue of birds from northwest Minnesota, coastal Virginia, and Lake Calumet.

They discovered the Lake Calumet colony had greater concentrations of PCBs, DDE (a byproduct of DDT), and a variety of toxic metals — byproducts of industry and urbanization. It’s no surprise that the pollutants reach the herons through the food chain. Alewives, an invasive fish, pick up the pollutants from the plankton they eat. The fish’s spawning attracts hundreds of herons to Lake Michigan feasts each spring — just when adult birds are feeding their young. The toxins then concentrate in the birds.

Thankfully, though toxin exposure is elevated here, it is not causing deformities in the Calumet birds. Nor is it lowering their reproductive success.

“The higher exposure is a red flag,” says Levengood. “It tells us that our work still isn’t done. There’s no immediate ecological crisis, at least not for the black-crowned night-heron. But for more sensitive species it’s still an open question.”

— Alison Carney Brown