Complex sentence: They are almost completely arboreal, but very rarely descend to the ground to pick up fallen fruits, to dust bathe, or to pick up mud pellets to seal the nest cavity during the nesting period.
Split into simpler sentences: They are almost completely arboreal and very rarely descend to the ground where they may pick up fallen fruits or dust bathe. They descend to the ground also to pick up mud pellets for sealing the nest cavity during the nesting period.