Complex sentence: Nine subspecies of Hoopoe are recognised by the Kristin 2001 (in the "Handbook of the Birds of the World"), with these subspecies varying mostly in size and the depth of colour in the plumage.
Split into simpler sentences: Nine subspecies of hoopoe are recognised by Kristin (in the 2001 "Handbook of the Birds of the World"). They vary mostly in size and the depth of colour in the plumage.