This comment has been bugging me for a while.
I think what you must be seeing are non-breeders, Briony. Let me explain.
Imagine there's a population of say 500 red kites, which might be around right for oxfordshire? That might consist of say 150 adult males, 150 adult females and 200 birds too young for breeding or otherwise unpaired.
During early spring the adult breeders (male + female) will be displaying over their territories, and very easy to see. By about now, each pair will have eggs, and at any one time one adult will be sitting. This cuts down the number of possibly viewable birds by 150, (or 30%) even assuming no change in behaviour.
However it is also a fact that once a pair of birds has eggs, they will immediately stop displaying so that the nest site is no longer being advertised to predators.
This is the case with all birds of prey that I know of, and most songbirds, which stop singing for the same reason. A piece of woodland which is bursting with song in May will be almost silent by early June, even though the birds are still there.
Back to red kites. It is now April 19th, and the number of kites I am seeing casually here in mid-wales is very much smaller than it was a couple of weeks ago, for the reason mentioned above. I'd be surprised if it was not the same in Oxfordshire.