Hi Bart
THX for your painstaking and hard works on seabirds
About your data of streaked shearwaters and our sightings in recent oil rig trips (very common in Sept but uncommon in May), it is very likely that the Streaked Shearwater is a regular winter visitor to the northern South China Sea. My speculation is that it arrives in Sept and stays off shore. It becomes more "on shore" during winter monoons, in particular in March as your observations shown. It departs and migrates back to the breeding grounds (The East Sea?) in April
About the regular signting of Brown Bobby in May, maybe some young guys wander around this area, or this area falls within feeding range of adult birds that nesting in nearby areas (Xi sha!?) during the late breeding season.
Anyway, just my thoughts .....
