Complex sentence: Caves are found in limestone formations and in the lave flows which cover much of western Saudi Arabia, and their ceilings are a favoured location for nesting Pale Crag Martins, Red-rumped Swallows, and the Little Swifts which may appropriate the hirundines' nests.
Split into simpler sentences: Caves are found in limestone formations and in the lave flows which cover much of western Saudi Arabia. Their ceilings are a favoured location for nesting Pale Crag Martins, Red-rumped Swallows, and the Little Swifts which may appropriate the hirundines' nests.