If you have ever watched eastern meadowlarks or grasshopper sparrows singing from atop a fencepost, their bills will be wide open.  However, if you take the time to watch a male eastern bluebird singing from a branch in your backyard, its beak will seem to be completely closed.  The truth the matter is, if you are able to view the bird through a pair of binoculars or spotting scope you will learn that its bill is actually barely open.  I think that is truly an amazing feat!