Complex sentence: Apart from the smaller size and the all-red colouration of the tail feathers, it resembled the Scarlet Macaw of tropical South America and may therefore have been a close relative of that species, or possibly captive Scarlet Macaws which had escaped on the island.
Split into simpler sentences: Apart from the smaller size and the all-red colouration of the tail feathers, it resembled the Scarlet Macaw of tropical South America. It may therefore either have been a close relative of that species, or possibly captive specimens of it which had escaped on the island.