Another bright day and plenty of spring activity, but nothing remarkable to be seen. Several Nuthatches were singing. A pair of Great Spotted Woodpeckers are showing interest in the dead tree they usually nest in, which is near the southest corner of the leaf yard, and there was a great deal of drumming and bustling about.

A dead tree on the edge of the Long Water was thickly festooned with Black-Headed Gulls.

Only a few of them have developed the dark brown heads that are their breeding plumage, but nevertheless the breeding season is coming soon and most of them will have flown away by the end of the month, to Holland, Germany, Poland, Sweden, Finland and Lithuania, among other countries.

Near the Italian Garden a Little Grebe was standing up and flapping vigorously, looking like a tiny angel, while a Moorhen preened and a Mallard dozed.

It is surprising what small wings grebes have, though all the European and North American species can fly perfectly well, and often travel considerable distances on migration. An aerodynamicist would say that they had a 'high wing loading', in contrast to a soaring bird like a gull or a raptor, which has a very low wing loading. It means that they have to move their wings extremely fast to stay in the air.

Peter Scott, the proprietor of the Bluebird Boats for hire on the Serpentine, has presented me with a curious find. During the winter, his pedalos are moored round the back of the island, where gulls perch on them and they get full of all kind of detritus. The staff have been spring cleaning them, and have found small shells of bivalve molluscs in some of the boats. They are ¾ to ⅞ inch (19 to 23 mm) long.

It looks as if gulls have been finding these creatures, eating the contents, and dropping the shells. I think they are Corbicula fluminalis, one of two species of Asian freshwater clam commonly known as Golden Clams (the other is C. fluminea). They have been imported for food, but live clams are also used in koi ponds. And apparently they have somehow made their way into the Serpentine. It is possible that a human family was enjoying a meal of clams and left it unfinished and the gulls found it, but these green-encrusted shells don't look as if they have been boiled, and it seems more likely that the clams are actually alive in the lake. Golden Clams are considered an invasive species, and are already present in Ireland (in the Nore and Barrow River since 2010), Germany, the Czech Republic and Portugal, as well as several places in America.