Historical spring bird migration dates for arrivals can be a fascinating black hole to disappear into — the median date for the loon's sighting in southern Minnesota is March 23? — but nothing compares to the tangible evidence building just now throughout the state.

For proof check out the Minnesota River where it flows under the Old Cedar Avenue Bridge in the Minnesota Valley National Wildlife Refuge in Bloomington. Northern shovelers. Hooded mergansers. Tundra swans. Canada geese. These and other early spring arrivals are having a time.

Craig Mandel expects Ross's geese and snow geese, among others, on the list next. Mandel, a volunteer naturalist at the wildlife refuge, would know. He leads about 50 bird walks a year, beginning in 1989.

The one scheduled April 8 at Bass Ponds stands to be like preceding excursions: popular, with a maximum capacity of 25. Mandel said several years ago he was encouraged by refuge staff to have a cap. Some walks had grown to 60 information-seekers.

"Fortunately I have a lot of friends on my walks," he said. "Four to six are avid birders who help me."

By early April, Mandel and his followers might encounter song sparrows, bluebirds, winter wrens, and the sap-harvesting yellow-bellied sapsucker woodpecker, to name a very few.

Waterfowl migrants also are descending upon open water in the Wilkie Unit in Shakopee, one of eight units in the refuge, which spans 70 miles along the Minnesota River from Bloomington to Henderson.

Mandel's broad interest in the natural world is both piqued and sated by the arrival of so many spring migrants.

"I have my wall of bug and bird books and flowers," he said. "Pretty much anything you see in Minnesota, I've got a book."