Researchers hope geolocators reveal Common Tern stopovers, migration routes

Our April issue included an in-depth feature story about geolocators, tiny battery-powered gizmos that scientists use to track birds’ movements. In a follow-up article, I described significant discoveries about 10 North American species that researchers have made with the devices.

Now ecologists in Wisconsin are adding Common Tern to the list of species tracked with geolocators.

The species has been listed as endangered in the state since 1979, and in the last three decades, the recovery or recapture of terns banded in Wisconsin has revealed that they have wintered in Guatemala, El Salvador, Colombia, and Peru.

“But we don’t know where they stop over and where they stage,” says Sumner Matteson, an avian ecologist with the state’s Department of Natural Resources.

Answers to such questions are within reach. Earlier this month, Matteson and DNR wildlife manager Fred Strand joined scientists from the Natural Resources Research Institute in Duluth, Minnesota, at Interstate Island in the Duluth-Superior Harbor, where 200 to 300 pairs of terns breed. They outfitted 15 Common Terns with geolocators — the first members of their species in the Midwest to receive the devices.

The researchers selected adult terns that had been banded in previous years. The chosen birds were nine years old or younger and had a track record of returning to Interstate Island — factors that make them a good bet to return and be recaptured. After trapping a bird and checking its age from a log maintained by Strand since 1989, the investigators handed off selected birds to NRRI scientists Gerald Niemi and Annie Bracey, who attached geolocators to the birds’ legs. Niemi and Bracey also measured and weighed each tern and collected blood samples for DNA analysis.

In the coming weeks, Bracey and Strand will monitor the terns that wear geolocators. A year from now, when the birds return to Interstate Island, they will be trapped so the geolocators can be removed. Data stored in the devices will be downloaded and decoded using specialized computer software that will reveal the birds’ movements.

The geolocators used in the project weigh about four hundredths of an ounce (1.1 grams). They record the level of sunlight every five minutes for a year, permitting the location of a bird to be calculated, since day length varies with latitude and solar noon varies with longitude. In addition, a wet-dry sensor can tell how often a bird is in the water.

“The exciting part will be one year from now, when we see how many of the outfitted birds come back,” Matteson says. “Because of high colony site fidelity, chances are that most of the 15 adults will survive their long migrations to Central and South America and return to their Lake Superior home in the harbor.

“We’ll get a greater understanding of their ecology and can work proactively with partners in states and countries along the way to aid the species.” — Matt Mendenhall, Managing Editor 

Originally Published June 27, 2013