When you see a pair of Great Crested Grebes displaying busily in winter, generally the reason is not hard to find ...

 ... there is another pair of grebes doing the same thing not far off.

This display of simultaneous affection and xenophobia is not confined to the breeding season. Grebes need their territories to fish in. The more distant pair in this photograph, in monochrome winter plumage, are based on the eastern end of the Serpentine island, where this year they successfully brought up two youngsters. The nearer pair are interlopers trying their luck. Grebes mate for life, and their constant displays help to bond the pair. Sometimes you see a pair displaying simply because one of them has gone fishing around the other side of the island and they have lost sight of each other for ten minutes. The only time they abandon their displays is when they are about to fly somewhere with some other grebes; they melt quietly into an amicable flock, close together for safety. But when they arrive at their destination, within a day or so they will have split into pairs again.

Elizabeth asked me whether birds that have their eyes on the side of their head have true binocular vision. They answer is yes, certainly. They have to, because otherwise they could not judge the distance of things when pecking at them or seizing them. Many birds, such as the Great Crested Grebes above, have 'lores' -- bare strips of skin in front of their eyes pointing at the tip of the beak. The absence of feathers gives a clear sight forward, and the line of the lore helps the aim, rather like the notch in a gunsight. Here a Herring Gull stares directly at me as I photograph it. There can be no doubt that it is looking with both eyes.

This young Mute Swan is looking a bit dishevelled as the wing blows up its feathers the wrong way.

Notice that it is not wearing a ring. When the cygnets and their parents were captured and taken temporarily to Esher to stop them from beating up the Olympic swimmers, they were all ringed. This young swan has been hatched somewhere else, and has flown in to spend the winter in the park.