Great Northern?

As this classic series continues, adventuring children off the western coast of Scotland spot a strange bird and soon land themselves in danger.
While on a sailing cruise with Cap. Flint exploring in the Outer Hebrides, the Swallows, Amazons, and the Ds spot a mysterious bird nesting on an island in a loch. Could it be a great northern diver, never known to nest in the British Isles? They tell their discovery to a local expert who secretly collects birds' eggs and stuffed skins of birds. The collector sets off with a gun—and the children set off to stop him.
Friendship and resourcefulness, dangers and excitement: Arthur Ransome's Swallows and Amazons series has stood the test of time. More than just great stories, each one celebrates independence and initiative with a colorful, large cast of characters—and children are the heroes. Great Northern? (originally published in 1947) is the twelfth and final completed title in the Swallows and Amazons series, books for children or grownups, anyone captivated by a world of adventure, exploration, and imagination.
"[Ransome] makes a tale of adventure a handbook to adventure." —Observer (UK)