If these birds have made the trip on a boat, I think the question is how do they arrive on this boat. If these birds are resident I see no reason why they have been over the sea for an other reason than migration. So prior to have been on a boat, these birds for a reason or another have been flying over the sea and presumably at the same time as was the typhoon Sanba, then they could have been blown away from their route in the sea.
Now in the case the bird has arrived by itself in Po Toi, its route would have crossed Taiwan or the Fujian coast somewhere, and the chance this bird arrived in Po Toi without seing any piece of earth on its route are very slim. So then the theory of the boat make quite sense, with a bird that had stopped on the boat finding some shelter during the Typhoon and that decided to leave the boat when it cross in front of Po Toi seing the island. Just one thing I don't know, is how long it takes for a boat in front of Zhejiang or Japan to reach Hong Kong, the birds from Zhejiang have been found at the same time, even it does'nt means that they have not been there since a few days, well at least for the Zhejiang birds as the birding pressure is probably not so hight as in Po Toi.I expect that a boat in front of Japan may takes quite a few days to reach Hong Kong, I don't know if the Tit by itself would not have been quicker.
I think something should be good to try is to see if it can be ID at subspecies level to have a rough idea from where this bird is coming from. It should help to examine the different scenario.
Really fascinating on how this bird has reach Po Toi.
As per previous discussion with other birders who's got boat experience, land birds may use boat very often in sea especially by bad weather conditions, with sometimes thousand of birds landing on a boat during a storm or heavy fog, this is very well known. Last year, a Palla's leaf warbler has been found dead on a boat in a few hundeeds kilometer in front of France britany!
All the best,
Jonathan
