What could be the reason that some birds hide their eggs on high trees on a certain island while others hide them between rocks on another island?

Different places, different predators, different environmental occurrences, which means the bird have to change their ways in order to protect their young.

that would be called plucking : )

explanation:

''plucking, also referred to as quarrying, is a glacial phenomenon that is responsible for the erosion and transportation of individual pieces of bedrock, especially large "joint blocks". as a glacier moves down a valley, friction causes the basal ice of the glacier to melt and infiltrate joints (cracks) in the bedrock."