When Bob Goetz was growing up in his hometown of Renville, Minnesota, he recalls being fascinated by the activity around his family’s purple martin house, in his family’s backyard.

It seemed, at that time, that everyone had a martin house, that provided nesting areas for the native bird, whose adult plumage is a bright, vibrant purple.

That was then.

Today, the population of purple martins had declined dramatically — by some two-thirds since 1970, said Goetz, of the Austin chapter of the Izaak Walton League. Predatory birds such as starlings, sparrows, owls and hawks are enemies of the martins. The proliferation of trees in the U.S. has provided those enemies with maximum nesting areas. The result has been a dramatic decline in martin numbers.

That’s part of the reason the league is hosting an open house at 2 p.m. on Saturday at the East Side Lake parking area. After a brief history of the birds, their nesting habits, their migration — they fly to Brazil — and other information will be discussed; then, a nest tray with babies will be removed from one of the martin houses, for all to see.

Providing safe habitat for the birds is key to their survival, said Goetz. There are about 30 birds in the colony at East Side Lake Park, where there are three houses. Some have been starling-sparrow-proofed so the renegade birds do not attack the martin’s eggs, as is their nature. They are known to even pull baby birds out of their nest, Goetz said.

The work has brought success; in 2005, there was one colony of a dozen birds in Austin; recently, he said, he counted 56 babies and 10 eggs at East Side Lake Park; at Village Co-op, another site, there were 36 eggs.

Still, he knows the battle is a constant one. Although there are many babies right now, he also knows history. One season, of the 76 eggs hatched, only 38 babies survived. The rest of the eggs were pecked and destroyed by the predatory birds.

The recent number, however, makes him “cautiously optimistic,” he added.

Everyone is invited to come and hear about the birds. Those attending may also want to bring a lawn chair.

Anyone with questions should contact Goetz at 507-440-4400.