Here are more flowers of sandbar willow. Willow whistles are a childhood memory; an incomplete one. Somewhere along the way I forgot how to make them. Then I found a book of nature lessons for kids that had instructions. Then I loaned it out and it was gone. But now of course all you need to do is Google. Still a fun activity with kids.
Salix exigua might be Salix interior. Depends on whether you are a splitter or a lumper. If split, eastern plants are S. interior. The geographic overlap of the varieties is small enough to warrant splitting. But the traits that vary between the two also overlap. For example, stigmas are shorter in S. exigua, but the range of length for S. interior can be just as short. Just seems simpler to lump them. Whistles work either way.