nest, hole, indentation, den
窠 centers on a sheltered hollow space: a nest, a den, or any small cavity or indentation that provides a protected spot.
窠 combines 穴 (hole, cavity) with 果, which likely contributes a phonetic element while also suggesting a rounded, fruit-like hollow. The character originally referred to a bird's nest or a small hollow space.
A hole (穴) that holds fruit (果) like a nest: picture a bird's nest tucked into a tree hollow, cradling berries.
For カ, imagine a crow (カラス) building a nest inside a hollow tree: カ from カラス, and the nest is the 窠.