I am doing some research, and am looking for any resources or information about birds that whistle (worldwide). Is there such a list? Since a "whistle" is defined as making a sound between the lips and teeth, what defines a bird that whistles?
Simply put, Jeff is right, using the standard definition of whistling using lips and teeth - no birds whistle as they have no lips and no teeth.
Merriam Webster's definitions do give you leeway, as the sound made is also a whistle, so by definition anything making that sound is whistling.
Next you have to stop and define what you mean by the word, for us to understand what you are looking for.
Starlings and Lyre Birds are extraordinary mimics and both can sound like a police or referee's whistle, or do you mean the liquid trill of the skylark?