Great River Road through the Upper Mississippi River National Wildlife Refuge from far southeastern Minnesota into northeastern Iowa. We were looking for areas where the river spills over and floods woodlands -- the ideal habitat for the prothonotary warbler, whose breeding range finds its northwesternmost point, and its only Minnesota location, along this area of the Mississippi. (That's what the range maps in two of my bird guides indicate, at least, though the map on the Cornell Lab of Ornithology page linked to above shows the Minnesota range extending farther west across the southern part of the state.) We saw miles of the right kind of swampy habitat in the NWR and found easy access to a suitable spot for observation (above) just off the highway, where a road to a boat launch was closed after a few hundred feet due to high water.
We were lucky that our trip wasn't later or that this year's late spring hadn't come earlier, since the trees were just starting to leaf out. Another couple of weeks of leaf growth and it's unlikely we would have been able to see these birds at all. As we drove further south and east, as far south as Indianapolis and Columbus, Ohio, spring advanced until we were seeing trees almost fully leafed out and lilacs and many other flowering shrubs and trees in bloom.