Great Northern? by Arthur Ransome
Flat on his front, binoculars to his eyes, alone at dusk, Dick makes a remarkable discovery: two rare birds, never before seen in the British Isles. Captain Flint and his crew decide to consult an expert to confirm the discovery. But when the man they ask turns out to have his collector's eye on the birds' eggs, not to mention skins, an enjoyable voyage around the Outer Hebrides becomes a desperate race to save the birds, and themselves...