I find Green-backed Flycatcher interesting counterexample to Geoff's theory. It seems most of our spring migrants come from the Philippines while most of our autumn migrants are headed to South East Asia. This species winters in Vietnam and Malaysia as far as I have read. Green-backed and Yellow-rumped Flycatchers are scarce winter visitors to Singapore while Narcissus is only a vagrant. So I would expect this species more in the autumn. Seems, however, that the opposite is true.