Complex sentence: Some thought the bird was a hybrid or variant involving the similar hyacinth macaw, but this idea was soon abandoned, as plumage, size, and proportions of Lear's macaw differ from those of its close relatives.
Split into simpler sentences: Some thought the bird was a hybrid or variant involving the similar hyacinth macaw. However, this idea was soon abandoned, as both plumage, size, and proportions of the Lear's macaw differ from those of its close relatives.