I've seen quite a few Red Kites over recent years on the South-East Kent coast and have yet to see one come in/off. I'm not aware that others have either, whilst Marsh Harrier and Sparrowhawk are regularly observed arriving in/off. Most of the Red Kites are usually coasting (flying upchannel), turning West when the hit the North Kent coast.
Yesterdays birds are unlikely to have arrived from Northern France, as there is no evidence of any movement from across the channel. Of course they could have arrived further South (as eluded to in a previous post). For me the now annual movement of Kites in the South West with increasing numbers seem a better fit with the increasing British population than arrival from the continent.
Having a brief google search shows a number of discussions of the summer migration and most seem to argue that this is related to British birds, with some arguing that the birds are of continental origin. An interesting blogpost here:
http://cornishringing.blogspot.co.uk/2015/06/wandering-red-kite.html
Occurance of the odd Black Kite with Reds does not necessarily point to continental origin of the Reds for me. If there is an increase in British birds moving at this time of year, then it seems not unlikely that the occasional Black gets caught up in this.
