Metal in diet harms Colorado birds

Cadmium, the white metal used in rechargeable batteries, is poisoning the white ptarmigan, a species of grouse, in Colorado. Stretching from Denver to Durango, the state’s ore belt is rich in the toxic metal. Although cadmium concentrations in soil and water are highest downstream from commercial mining operations, it’s not mining activities in this region that endanger the birds, researchers say.

Seeking to discover why the Colorado Lagopus leucurus has fragile bones, a team of ecologists led by James R. Larison of Oregon State University in Corvallis undertook a 2-year study of the foods that the ptarmigan consumes. The researchers also analyzed tissues from 39 birds. “The studies that took us through the food web clearly indicated that we were dealing with cadmium problems,” Larison says.

The willow is a major food source for ptarmigan in winter. All species of willow, the team discovered, take in and concentrate whatever cadmium is available in the soil and water. The trees funnel the metal into their buds and shoots, resulting in significantly higher concentrations than are found in neighboring plants.

Cadmium accumulates in the birds’ kidneys and liver, say Larison and his colleagues. Concentrations above a threshold cause irreversible renal damage. In the study, internal organs of 46 percent of birds examined had cadmium concentrations above the toxicity threshold. “This is truly unusual,” says Larison.

Lacking adequate kidney function to regulate calcium in their blood, the birds lose calcium from bone, say the researchers. Furthermore, they add, the Colorado ptarmigan habitat is naturally calcium poor, placing the birds in a vicious circle: A low-calcium diet increases cadmium uptake, and cadmium ingestion retards calcium absorption.

Poisoned ptarmigans are unlikely to themselves be a major source of concentrated cadmium to predators because they don’t usually eat the birds’ internal organs, the team notes. However, willows in the ore belt may also be deadly to other local herbivores, such as beaver and moose.