According to the "Birds of Berkshire" Montagu's Harrier have bred on the Oxon side of the border but no confirmed breeding has been recorded on the Berkshire side

For the past few years there have been annual sightings, however. In each of the recent annual bird races one or other team has managed to record Monty's on the day and anyone with the patience & time to spend on the Berkshire Downs has a fair chance of connecting with this rarest of all British breeding birds 
(7 pairs according to the RSPB)

CDRH published a record of a ringtail at East Ilsley in the June 2004 edition of the Berkshire Bird Bulletin

This bird was not seen at any of the previous sites mentioned in County Reports. Thanks to Dick Forsman, I identified the bird as a 2nd calendar year dark morph male ... which is uncommon and is, I think, only the second for Berkshire - the other being "killed" near Hurley in 1870

I  "found" this bird one evening and went back a week or so later to take these photos ... 

Montagu's Harrier Circus pygargus

 All these photos were taken on the same day in August. The bird was very active quartering the cropped fields frequently dropping to disappear in the heat haze. The photographer was never closer than 250 metres from the harrier