Complex sentence: A unique characteristic of the "Apapane" is the white under tail coverts which can be seen clearly when the tail is cocked, which identifieses the "Apapane" from other similar native birds.
Split into simpler sentences: A unique characteristic of the "Apapane" is the white undertail coverts which can be seen clearly when the tail is cocked. The white under tail is a distinctive feature that separates the "Apapane" from the other similar native birds.