Queen Mum (QMR) is a massive area of water at the western extreme of London which attracts lots of vagrants throughout the year. Many of the rarer birds on the Berkshire list can only be found from the 5 km circumference of the reservoir
You'll need to get a birdwatcher's permit from the BOC and accept the very busy weekends and much fruitless, scanning through the thousands of gulls
But you might find flocks of Velvet scoter & Brent geese, Spoonbill, Great, Pom, Arctic & Long-tailed skua, Great northern diver ...
If you can't be bothered to lock/unlock the gate you can always go to the sailing club bar to enjoy a cup of tea and ask for a "day pass" to get out of the gate
Queen Mother Reservoir
