[IsTom]

Thump two round rocks together -> sharp noise

Thump pointy branch against a tree -> dull noise

And chickens aren't using tools.

[blurbleblurble]

They're scratching for gizzard stones and food though, with their built-in beak, fwiw

[sixsevenrot]

I challenge you to find two objects of a similar size and cut them into shapes that would produce a sharper sound for the rounder object.

In your example it's obviously the round tree trunk that produces the dull sound.