Complex sentence: All albatrosses, procellarids (gadfly petrels and shearwaters) and storm-petrels use the oil, the only Procellariiformes that do not are the diving petrels.
Split into simpler sentences: All albatrosses, procellarids (gadfly petrels and shearwaters) and storm-petrels use the oil. The only Procellariiformes that do not are the diving petrels.