I think it's a generalisation to say that gulls are winter visitors to Deep Bay but spring migrants to the rest of HK. The peak counts in Deep Bay are usually also in early spring - February and March. This may be because there are more suitable tides at these times so birds are easier to count, but I suspect it could equally reflect a peak in abundance at this time (certainly there seems to have been increasing numbers in the last few weeks). there is a good chance that a high proportion of gulls in Deep Bay are also spring migrants.
