Author Neil Fletcher
For their diminutive size, wrens are incredibly loud. Even if you don't take their size into account, they are incredibly loud. It's a bit of a challenge to make a recording, since some of the notes within the song are much louder than others and cause the levels to ĺpeakĺ above the tolerance of the recorder, creating little moments of distortion. How these tiny birds achieve this level of amplification is a wonder.
Each burst of song usually lasts four to six seconds, and is easily distinguishable on two counts. First, it's remarkably loud, even from a distance. Second, the high pitched notes are repeated very rapidly, about ten a second, and there is almost always a central trill lasting around half a second, a little like a high-pitched version of a football refereeĺs whistle (the sort with a pea inside), at about thirty five beats a second.
More bird song here