ABANDONED. YA Mystery. Group of kids explore an empty hidden valley, and find a cave to live in. Plane, mystery, chicken, gold. Read in 2000s.

[Priyanka]

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 somethingEdit | More

[Becca]

It's from way before the 2000s, but The Valley of Adventure by Enid Blyton? There are four characters, Jack, Philip, Lucy and Dinah. Jack is obsessed with finding a great auk throughout the series and I don't think he ever does (they're extinct rather than just rare), but he thinks he's seen one in The Island of Adventure.

[Kris]

Priyanka, are the kids stranded after a plane crash? Are they in a jungle or near mountains?

Where do you think their parents are?

[Daisy]

There's a Trixie Belden book called The Happy Valley Mystery that has a couple of these elements.

[Kris]

No response, moving to Abandoned folder.

Priyanka (OP) last posted in the group in December 2020.