Complex sentence: Unless seen very close, it is almost indistinguishable from the common diving petrel; the common petrel has brown inner web primary feathers, whereas the South Georgia petrel has light inner web feathering.
Split into simpler sentences: Unless seen very close, it is almost indistinguishable from the common diving petrel. The common petrel has brown inner web primary feathers, whereas the South Georgia petrel has light inner web feathering.