Great Northern?

The last book of the Swallows and Amazons series. The cast is large – the Swallows, the Amazons and Dick and Dorothea Callum. They sail in the Hebrides with Uncle Jim and put their borrowed boat ashore in a cove for a scrub down. During their stay Dick discovers a rare nesting event of the Great Northern Diver but an egg collector appears on the scene. The children prevent him from stealing the valuable eggs from the nest, which is on a rocky islet in a Scottish loch. Illustrations except front cover, by Ransome.