Complex sentence: The most important prey item in their diet are Hymenoptera, mostly "Apis mellifera"; a study in Spain found that these comprise 69.4% to 82% of the European bee-eaters' diet.
Split into simpler sentences: The most important prey item in their diet is Hymenoptera, mostly "Apis mellifera". A study in Spain found that these comprise 69.4% to 82% of the European bee-eaters' diet.