Having spent endless nights working on Nightjars all over Wales I'm in perhaps an unusual position to be able to add something to this. Nightjars in Wales frequently still have dependent young from second broods well into September but most will have finally departed by Mid September. Looking back through the Mid Wales Ringing Group records, the last handling date I can find for Ceredigion was a nesting female caught attending a recently fledged chick between Borth and the Leri on 2nd September 2018. I did however see a Nightjar hawking moths over Glandwr Pools near the Leri on 10th September 2021. I believe there are records from Alan Lowe (who used to have a caravan at Llanrhystud) of Nightjars still in their South Wales plantation breeding sites in early October!
