Complex sentence: The vomit oil of northern fulmars is made from fish oils from their aquatic diets and has a deadly effect on predator birds, because the oily vomit causes the predator's feathers to mat, leading to the loss of flying ability and water repellency.
Split into simpler sentences: The vomit oil of Northern Fulmars is made from fish oils from their aquatic diets and has a deadly effect on predator birds. This is because the oily vomit causes the predator's feathers to mat, leading to the loss of flying ability and the loss of water repellency.