I doubt that you will get a clear date for the start of the Autumn migration from any group of South Coast UK Birders, as it will depend on their Birding preferences. Many will say the end of June or start of July when the first returning Waders appear. But that's subjective as the first heading South are likely to be failed breeding Birds. By the second half of June I was seeing good numbers of Curlews back in Poole Harbour. Around that date, I found a local Great White Egret which could have been a non-breeding individual or a post breeding individual wandering across from the continent or down from the Somerset Levels. Perhaps the date be based on returning Waders from further afield? Maybe, it should it be around the start of July when the Swifts start to depart & the first Sand Martins start gathering to head South. Alternatively, should it be when the Passerines start moving towards the end of July. Personally, I like to work on the start of Aug as that is when the Passerines start moving, although I do enjoy the arrivals of the early Waders. On the first of Aug I was Birding around Littlesea as I was still trying to find the Purple Heron. I had no joy with the Purple Heron, but I was happy to find both a Pied Flycatcher & a Spotted Flycatcher at Littlesea. Both were skulky & not easy to get views of them which is typical when they are on their breeding grounds. So, they were still behaving as if they were on their breeding grounds. Both species are migrants through the Studland area. Spotted Flycatchers are relatively regular and Pied Flycatchers are always scarce with just a small handful of records mainly in the Autumn. So, it was good to see that the Autumn Passerine migration had started.
Pied Flycatcher
Spotted Flycatcher
Robin: Juvenile. A locally breed individual close to where the two Flycatchers were feeding
Robin: Juvenile
Red-eyed Damselfly: Pair. Preparing for next year's generation (3 Aug 18)
Water Strider: The large size & upturned spurs at the end of the abdomen confirms the identification (3 Aug 18)