A cold day with a biting northeast wind, and after a brisk trot round the lake I was glad to hurry home. Snow flurries didn't deter the Shovellers on the Long Water from their endless circling.

In the leaf yard the Song Thrush was rather subdued but sang a phrase or two from time to time. There was also a Nuthatch singing, and Great Spotted Woodpeckers calling to each other, but not drumming. On the edge of the enclosure a patch of Feral Pigeon feathers showed that the female Sparrowhawk had been on one of her frequent raids.

Male Sparrowhawks are considerably smaller than the females, and not large enough to deal with a full-sized pigeon. I haven't seen the male for some time, though a few years ago an entire family with two young flew over the Long Water. With Sparrowhawks, Peregrines and Lesser Black-Backed Gulls -- and occasionally a Great Black-Back -- after them, the park pigeons have plenty of enemies. But they are so prolific that these predators never make a noticeable reduction in their numbers.

This unusual Mallard drake has been on the lake for several years, and is one of two such ducks, probably brothers. His back is lighter than usual, almost ivory, and his head is brown with a faint reddish iridescence and no trace of the usual green.

The iridescent green of the head of a normal Mallard is not a normal colour, but is an interference effect, caused by tiny ridges on the feathers having a spacing related to the wavelength of green light. We see the sides of the head as blue because the feathers here are at an angle to us, making the ridges appear closer together; blue light has a shorter wavelength than green. Since this duck's head shows no green or blue, its feathers must therefore have a slightly different structure from normal ones. The reddish colour of the slight iridescence shows that the ridges on the feathers must be coarser than normal, since red light has a longer wavelength than green.

A Black-Headed Gull in full breeding plumage was standing on top of the marble fountain in the Italian Garden. This is quite an old bird, as can be seen from the deep beetroot colour of its legs and bill. Although these gulls develop adult plumage in their first year, it takes their legs several years to change from juvenile marmalade colour through orange-red to beetroot.

Still no sign of the male Tawny Owl. I also went around the place north of the Albert Memorial where the family unexpectedly showed up on 23 March last year, but there was nothing there either.