22/3/25. The Wren. A Biography. They are Britain's most common bird, with 8.5 million breeding pairs and have by far the loudest song in proportion to their size. They also thrive up and down Britain and Ireland: from the smallest city garden to remote offshore islands, blustery moors to chilly mountains. Yet many people are not sure if they have ever seen a wren. Perhaps because the wren is so tiny, weighing just as much as two A4 sheets of paper, and so busy, always on the move, more mouse than bird.