I think these would be Great Egrets.
Every year in October large flocks of Great Egrets head out from the Deep Bay area in the early morning (and probably late evening also from your observation) flying south south east. They have been noted before by others (I remember Matthew at Wonderland Villas last year I think) but I see them every year at Po Toi in the early morning in late September and October. From the direction and purposeful flight (exactly as you mention), I believe they are heading for the southern Philippines which would be about 2 days flight away.
The interesting thing is, I've never seen or heard reported a return passage in spring. They must use a different route.
