Clive Bowley came across this super rufous morph (sometimes called ‘hepatic’) female Cuckoo near the feeding station at Summer Leys today.
One was also seen in the vicinity about two weeks ago. I’ve heard it said about 15% of female Cuckoos are rufous but I have no references on this and the proportion may well vary with geographical location. Rufous morph juveniles are much commoner, of course.
Northants birds 
A much higher proportion of birds in eastern Europe are rufous morph. Nowhere near as rare along the Black sea as they are here in the UK
Thanks, Steve. Useful to know.