I think that if the occurrence of these birds was weather-related we could expect to see a change also in commoner bird species. There have been no arrivals of e.g. Teal or Garganey associated with these birds. This suggests to me that it is not due to the weather.
Both the Oriental Stork and the Greylag Goose appear to be adults. I'm not sure about the duck. This would seem to go against Captain's theory (although perhaps not entirely - they could be young adults).
I suspect a different origin for all three birds, with their occurrence at a similar time being partly coincidental and partly the result of increased observers at Mai Po compared to previous Julys. The duck was only seen by an observer who had come to Mai Po to see the stork.
Vagrancy by storks in late summer is an established phenomenon elsewhere I think. (European) White Storks that have not bred will wander widely, away from the usual breeding range. There seems to be no real reason that Oriental Stork should not do the same. With a breeding population now in the Yangtze Valley, I suspect that this is a bird from that population that has not bred (perhaps too young or could not find a mate) and has wandered around south China.
Geese should now be moulting a long way to the north of Hong Kong. Theb Greylag Goose seems to be an unlikely vagrant at this time of year, especially an adult which should know its migration route. Of course, there was a known escape in Deep Bay last winter - I suspect that this is the same bird, which has subsequently moulted feathers back and has healed on the bill.
That just leaves the duck, which I am still puzzling about. There is a previous mid-summer record of Baer's Pochard. The first sighting of Red-crested Pochard was also mid-summer - this is currently treated as an escape, but with a breeding range similar to Ferruginous, could there be a possibility of both bineg wild?
