I think it's just a dog being a dog

I wouldn't quite call it retriever instinct. They weren't bred to hunt the game, only retrieve it, and she sounded pretty reluctant to give up her treasure!

Timber loves to chase birds, but she's never caught one so far! She doesn't have the patience...just goes all gun-hoe and chases after it like crazy. She also barks at them when they fly above her

Leather doesn't even bother lol. She's just like, whatever