Great Northern‪?‬

The Swallows, Amazons and Ds are all on a sailing cruise with Captain Flint in the Outer Hebrides. While the older members of the party clean the boat before returning her to the owner, the younger ones explore inland and a mysterious bird is seen nesting on an island in a loch. The question arises whether it is a great northern diver, which has never been known to nest in the British Isles, or a black-throated diver.

As the plot involves more excitement and violence than usual some have classified this book as one of the metafictional stories in the series: a fantasy tale made up by the children themselves. The other two books generally agreed to be metafictional are Peter Duck and Missee Lee.