All the best to you and the Harley if you decide to adopt him

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I have never met a Harlequin Macaw, however, I personally know a Miligold, a Ruby, a couple of Shamrocks, a few Camelots (one of them is mine), a Rubalina (don't ask me--RubyxCatalina??), and several Catalinas. From my experience, they're like purebred Macaws--each individual is different, within the limitations set by genetics, age, health, upbringing and environment. If the Harley likes you and you two have established a bond, you'll probably never regret bringing him home, as long as you're willing to be patient and work on any "issues" he might have.
If you get him, I look forward to seeing photos

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