Agreed, there is nothing more peaceful then dusk or dawn on a lake when the loons start to call.

From the description, it doesn't sound like the idiom has to include the word "loon". I think you could use any idiom you want as long as you put a loon in your quilt.

For example, the idiom "all that glitters isn't gold" could be expressed with a loon that was thread painted with metallic thread -- or that has rhinestones or glitter on it.

Or the idiom to "dance with death" could have a loon dancing with the Grim Reaper. "Have a cow" could be the loon sitting next to its egg that has a miniature cow hatching out of it.

There are tons of sites that list idioms on the web. Here's one if you're interested. http://www.idiomsphrases.com/

There is also a Loon Lake in the province of Saskatchewan in Canada. It's a very popular and gorgeous fishing lake. The loon song is magical, especially in the evening.

Bree123, I love your answer. Guess it depends on how the rules are worded. My first thought was Looney Tunes, too.
How about substituting "Loon" for a rhyming work in an idiom, such as "Harvest Loon"?

Oooh, that could be fun, quilting cat. I'm not sure harvest moon is an idiom but that would definitely work! :)