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For the next month, the upper Mississippi River between Wabasha and La Crosse will be the temporary home to as many as 15,000 tundra swans. They stop off here each winter en route from their breeding grounds along Alaska's north slope to their wintering grounds on the Chesapeake Bay. Scientists know a fair amount about swan behavior at the breeding and wintering grounds, but they're just beginning to study where the swans spend their time on the river and what impact they have on the Upper Mississippi River Wildlife Refuge.

ON A RECENT EVENING AFTER THE MOON HAS SET behind the Mississippi River bluffs, University of Minnesota graduate student Erik Thorsen and his field assistant set off from Stoddard, Wisconsin on a mission to catch tundra swans. Thorsen perches in the front of a small canoe-like skiff with a flat stern. He's holding a big net on a pole and wearing a miner's light on his head. An assistant steers the boat with a small electric trolling motor. Thorsen gives hand signals about which direction to go as he listens for tundra swans. He seems to have a sixth sense about where the swans are and turns on the bright light strapped to his head. Two swans and a cygnet appear in the beam. Thorsen follows one bird, but it clearly knows something's coming; it paddles quickly, with its head low to the water, kicking up a tiny wake.

Thorsen wrestles the bird into the boat and prepares to fit it with a radio collar. He wants to know if swans stay on one part of the river or whether they move around.

Thorsen: So now what we're going to do is process and band this bird. First of all, we're going to sex it to determine whether it's a male or a female. We already know it's an adult because it's in full white plumage. Juveniles are gray and have a pinkish bill. So we'll sex it first, and weigh it, and take five body measurements. Then we'll put a fish-and-wildlife metal leg band on it and also put a neck collar on with a transmitter attached, and that will be our last step. And then we'll release him back into the wild so he can find his family again.

What I'm doing now is tucking her head under her wing and she'll kind of go to sleep a little bit, and that way she'll be a little less resistant when we sex her. Yep, this is a female.

Thorsen: Swans are primarily aquatic foragers, especially on the river here. We think they're eating wild celery tubers and arrowhead tubers. They do eat a lot of aquatic plants.

Thorsen says he's surprised some of the swans he collared near Wabasha are already on the move.

Thorsen: Two of those birds have moved quite a ways south, probably 60 or 70-miles south into the Wisconsin islands area. It's very interesting to see movements down the pools, and also we can see kind of the daily movement patterns of birds. There seems to be large scale movements and daily movements out from the roosting sites.

Birds like geese tend to stay in one area until it freezes over.

The probing, measuring, and collaring of this swan is almost done.

Thorsen: O.K., putting the radio transmitter on, and it's just held in place with two rivets.