Here's what happened:
I pulled off the highway to have a closer look. The lake was actually in a small park called Mormon Island State Park (and I don't know why they call it that because I didn't see any islands OR Mormons there, but whatever). I parked my car at the lakeshore and from here I could see that the birds were geese. Snow geese to be exact. Snow geese are migratory, but they start their migration earlier than the cranes, so I had never seen them before. In fact, I've never seen so many geese in one place before. There must have been several thousand of them. It was an incredible sight!