Great Northern?

In 'Great Northern?', the twelfth and final book of the Swallows and Amazons series, the Walker children, also known as the Swallows, and the two intrepid Blackett sisters, known as the Amazons, are on holiday in the Outer Hebrides. They encounter a rare bird, the Great Northern Diver, which is being hunted by an egg collector. The children band together to protect the birds and their eggs, leading to a series of adventures and challenges as they navigate the natural environment and confront the egg collector.