I think Swinhoe's Egret and Watercock are fairly unlikely in August. Amur Falcon has not been recorded in this month, and Black-headed Munia was an introduced species not a migrant (and is no longer regularly present in HK).
There are a number of shorebird species that will be migrating through August - Green and Wood Sandpipers, Terek Sandpiper, Grey-tailed Tattler, Red-necked Stint, Curlew Sandpiper, etc. There are also a few passerines that should start turning up by the end of the month - there could be the first records of Amur Paradise-flycatcher, Arctic Warbler, Eastern Crowned Warbler, Brown Shrike, Yellow-rumped Flycatcher, Asian Brown Flycatcher, Eastern Yellow Wagtail, Forest Wagtail, Stejneger's Stonechat, etc.
You may be interested to look at the seasonal charts of records on eBird:
http://ebird.org/ebird/GuideMe?r ... .y=8&continue=t
There is now sufficient data in the eBird system that these are starting to look reasonably representative of the seasonality in HK for many species (although some species are still a bit misleading and need more data)