Biologist Finds Shadowy Tern Killer

By late last summer, Brad Semel had hoped to be counting Forster's tern fledglings at the birds' island colony on Grass Lake in northwest Lake County. But he soon found himself investigating the mysterious disappearance of the colony's chicks.

As natural resource specialist with the Illinois Department of Natural Resources (DNR), Semel has been working with others to reestablish a breeding colony of state-endangered Forster's terns at Chain O' Lakes State Park. The island was the species' only nesting site in Illinois from 1976 through 1995, but high water levels and boat wakes eroded the island and forced the birds to abandon the site. Efforts to rebuild the island paid off in the summer of 2003, when the birds successfully raised 149 chicks.

The Forster's terns returned this spring. But the breeding season started badly when two floods destroyed nearly 100 nests in April and May. When water levels dropped, the birds made another attempt, establishing 30 more nests. "Every day we'd watch a number of chicks hatch," says Semel, "and by the next day, they'd be gone. So we knew that predation was occurring." At first, Semel and other biologists suspected predation by gulls or a mink. In order to find out exactly what was happening, Semel needed to keep a 24-hour watch.

"I had a set of night vision goggles and I spent a couple of nights on the island," says Semel. The first night was uneventful, but on the second night, he saw a great horned owl swoop in and chase away the adult Forster's terns. The owl then settled into the colony to feed on the chicks. Semel saw the owl again the next night. In the morning, he and other biologists started discussing plans to protect the colony, but that same day a DNR intern reported seeing the entire group of adult terns fly off. The terns had abandoned the nesting site in Grass Lake and dispersed. As a result, there were no known successful Forster's tern nest sites in Illinois last summer.

Nevertheless, Semel believes there is a good chance that the birds will return to try again in 2005. DNR and Fox Waterway Agency staff members are working to make the island in Grass Lake less vulnerable to motorboats and floods by reinforcing it with stones and adding floating nest platforms to the island's low, swampy interior. In addition, biologists plan to use decoys and recorded tern calls to attract Forster's terns to two other potential breeding sites in Lake and McHenry Counties. Their hope is that if the birds nest at multiple sites, the species will have a greater chance of successfully raising young in at least one location in Illinois next summer.

— Stephanie Folk