The Sea of Adventure by Enid Blyton

After a bout of measles, Jack, Philip, Dinah, and Lucy-Ann look forward to a seaside holiday studying birds on remote islands off Scotland. But when their guide falls ill, the children find themselves stranded on the lonely Isle of Wings with talking parrot Kiki. Soon they unravel a dangerous secret and acquire a new feathered friend. Can four resourceful kids outwit ruthless smugglers amid the puffins and cormorants of this wild seabird paradise?

Their greatest adventure yet unfolds on the beautiful but treacherous Sea of Adventure.