I regularly see an unusual pair in the soi. They squawk like Mynahs, they strut like mynahs, they are the same size and shape as Mynahs, but they are less reluctant to fly as Mynahs. They have black backs and wings, but white heads and bellies. I cannot get close enough to photograph, nor can I find anything in my book nor on Google which resembles them closely enough. What might they be?