All I remember about this book is that a group of kids (who were maybe related. There were two brothers?) ended up in this absolutely empty valley - there was a plane involved. And they explore this valley - find a cave to live in. And at some point they solve a mystery - a chicken is involved - and they find gold. And then get rescued. This was is the Hardy Boys/Nancy Drew genre. All I remember about the cover is that it had a numeral of sorts on it. Also, in another book in the same series one of the brothers finds a rare bird - a great auk or something.
Stump the Bookseller
New format, same book stumper solving.
Could be The Valley of Adventure, one of a series by Enid Blyton. Two sibling pairs get on the wrong plane by mistake and end up stranded in the valley with enemy agents.
Possibly the Secret Circle Mysteries? A 10 book series, published between 1962 and 1965. Mystery of Monster Lake has a gold discovery, and Valley of the Vanishing Birds a crane rescue. Google it, and see if the cover art looks familiar!
Definitely sounds like Enid Blyton Valley of Adventure. An earlier adventure has someone saying they thought they’d discovered a Great Auk. Also they hide in a cave behind a waterfall at one point.