As I was passing the Serpentine Gallery there was a furious rattling sound and two Carrion Crows erupted out of a tree, chased by a pair of Song Thrushes who were clearly building a nest. The thrushes then turned on a Green Woodpecker, which retreated around the back of the trunk.

But that was to no avail, and soon the woodpecker was routed as well. There is also a pair of Mistle Thrushes in a tree nearby, which attack Magpies furiously. The male is singing but there is no sign of a nest yet.

Later, when I was coming home along the south side of the Serpentine, the same thing happened again. Here is one of the thrushes shouting abuse at a couple of Magpies and four Carrion Crows, all of whom left the scene, if only in the hope of a bit of peace and quiet.

It is amazing how bold these normally timid birds become when there is a nest to defend. Both Song and Mistle Thrushes have been singing loudly in many parts of the park, and for the past two days a Blackbird has been singing from a tree north of the Flower Walk. Blackbirds start singing later than the other thrushes.

Two pairs of Starlings that nest in the plane trees on the edge of the Serpentine have started work on whatever it is that Starlings do to make their nests habitable -- you never know what this is, because you can't see into the hole.

A few years ago I tried to photograph the inside of a Starling's nest containing young in one of these trees, holding the camera at arms' length above my head and using flash to illuminate the inside of the hole. I got a clear shot of the inside of the hole, but no sign of a living creature. The nest was farther inside the hole, out of site round a corner.

When these nests are in operation, you often see a parent Starling fly out of the hole and straight as an arrow across the lake to the Lido restaurant to scavenge some food for the youngsters. It is less work that poking around in the nearby grass for wireworms.

Near the Serpentine island, a Great Crested Grebe was enjoying a wash.

In a week from now it will be time to start searching seriously for the Tawny Owls and their owlets. Last year they turned up late, in a completely unexpected place 200 yards from their nest tree. It is a challenge, but a lot of practised owl watchers will be searching, and as soon as anything is found it will be noted on this blog.

There is a mixed pair of gulls in the southeast corner of the Serpentine: a second-winter Herring Gull paired with an adult Lesser Black-Backed Gull. They have been together for at least a week. Today I found them displaying to each other.

Mixed pairs of gulls are not uncommon. In the case of Herring Gulls, Lesser Black-Backs and Yellow-Legged Gulls, the offspring are fertile, so that there are quarter- and eighth-breed guls before the minor species becomes unnoticeable. All these species are so closely related that it is tempting to consider them different races of a single species. But even the official species are divided into races: most of the Lesser Black-Backs we see are Larus fuscus graelsii, with slate-grey backs, but there is also the dark grey Scandinavian race L. f. intermedius and the black Baltic race L. f. fuscus.

Just along the shore from this couple, a foolish woman had let her expensive Italian greyhound go into the lake where there was a group of Mute Swans. The swans united to repel the intruder.

Dogs often kill swans on the grass around the Round Pond. But if the dog is in the water, the swan is boss -- and you can see that these birds know it. Swans hate dogs, for obvious reasons, and will make a beeline for any dog that has had the temerity to intrude on their patch.

The solitary Barnacle Goose is still here, and I found it walking down from feeding on the Parade Ground to have a drink in the Serpentine.

It is very tame and took a piece of biscuit from my hand. No one knows where it came from.

The east side of the Vista is sometimes called the Rabbit Paddock, and rabbits were numerous here until the outbreak of myxomatosis a few years ago. So, of course, were foxes. The rabbits have been very slow to recover from the outbreak. But today there were three young ones on the Vista.

They may be cuddly fluffy bunnies, but if you are used to the alertness of birds they seem terribly inert and dull. They just sit there chewing.

A report from Des this morning of a male House Sparrow singing in the shrubberies near the bridge brought me out eagerly searching but, despite three tours of the area over three and a half hours, I didn't hear a cheep or see a feather. For those who don't live in Central London, and are wondering why anyone would bother to search for one of the commonest birds in the country, the answer is simple: they have simply disappeared from Kensington and Chelsea and Westminster. The population in the park was gone by 2000, which was also the last time I saw any: three, coincidentally in exactly this spot. One was seen in 2006. The nearest I have seen sparrows to the park myself was in the Portobello Road, just north of the Westway, in 2011.

No one is sure why the sparrows left, but one plausible theory is that they are sensitive to the benzene in unleaded petrol. The statistics for their disappearance and the introduction of this highly toxic fuel show a remarkable correlation, not just in London but in other places that have lost their sparrows.

Three of the Red Crested Pochards that have been hanging around the Serpentine island decided to visit the Long Water, and started behaving cutting up rough. First two males engaged in a long fight.

When a winner had been declared, he came over to join the female near the Peter Pan statue. Soon the loser turned up too, and this time it was the female who saw him off.

It would be good if this behaviour were a sign that they are thinking of breeding. The last time they did this was 2010, and one pair did manage to raise and fledge four ducklings. These beautiful birds are not originally natives, and the population is founded on escapes from ornamental collections. However, they are so well established that they are now recognised as a resident breeding species.

There were plenty of pairs of Long-Tailed Tits all over the park, rather than the large flocks typical of their winter habits.

Several Coal Tits were singing their distinctive song, faster and higher pitched than a Great Tit's and with the stress on the even-numbered notes. They sing perfect iambic pentameters again and again, with an occasional alexandrine by way of variation.

Beside the path, a Grey Heron was waiting patiently for a rat to poke its head out of the shrubbery and achieve the status of lunch.

A pair of Great Crested Grebes has been hanging around the west end of the Serpentine island for several days, and it looks as if they are going to build a nest behind the wall of floating baskets.

This spot has been tried before, unsuccessfully -- I think it was raided by Grey Herons which, from a grebe's point of view, are the main hazard of nesting on the island. But the Serpentine is so poor in suitable nest sites that there is little choice.

Meanwhile the grebes at the east end of the island have abandoned a nest that they built behind the baskets, but may be expected to go to an overhanging bush, a site that has been used before, sometimes successfully.

The idea of these floating baskets was that they would make a wall of water plants around the island, which would both purify the water and look pretty. It hasn't worked very well. Any plants that grow beyond the mesh are promptly chewed off by the waterfowl, and they have died down to a stunted remnant. Some new plants were put in the baskets in the autumn, and it remains to be seen if they will do any better. The horizontally laid twigs in the background of the picture are another water purification measure: according to the men who put them in, they are supposed to stimulate the growth of beneficial organisms (they didn't say which ones).

A Blackcap was singing on the fence at the back of the Lido. Annoyingly, it flew off as I raised my camera.

For some time the Mute Swans have been using the patch of water near the Serpentine outflow as a kind of marriage market, with displays between prospective partners and occasional outbreaks of aggression. Here a male has a rather violent bath right next to the group; I don't know whether this is a kind of display or whether he just felt like having a wash.

And, photographed straight down from the parapet, a young and low-ranking swan ignores the kerfuffle and looks for something to eat.

In the Dell, a Wren was poking around under a holly bush, dwarfed by the leaves.

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.

A pleasant sunny day, and more signs of the arrival of spring. A pair of Wrens were chasing each across the Flower Walk, and so were a pair of Long-Tailed Tits. I also saw one Long-Tailed Tit on the ground under a bush, a place they go only when they are searching for the thousands of little feathers they need to line their intricate nests.

A Greenfinch was singing near the Serpentine bridge, where I just managed to get a very distant shot of him.

There was also loud drumming from a tree north of the Albert Memorial where Green Woodpeckers nested last year, but when I got there it was a different tree and a Great Spotted Woodpecker -- I am not sure if you can tell the sounds of these two species apart. Here is another excessively distant shot of him hanging nonchalantly upside down from a branch while he searches the bark for insects.

Last year's Green Woodpecker hole in the nearby tree looks deserted, but I am keeping an eye on it. The damage caused to the tree by the birds' excavation has allowed a large bracket fungus to grow above the hole like a black porch.

The Feral Pigeons were up to their usual tricks.

The male's pouting, tail-dragging display accomapnied by enticing coos may seem irresistible to him, but the female pigeon always look unimpressed and tries to leave, which is made more difficult by the male waddling round and plonking himself in front of her the better to appreciate his pouting. This female did get free, only to be pouted at by a different male. So she went and stood in the water at the edge of the lake until he went away.

The last survivor of the Egyptians at the Round Pond is still there.

It looks very runty, although its wings have grown straight. I don't think it's ever going to be a fine figure of a bird. It was chased twice by a dog which some fool had let loose and was running around in an ecstasy of bloodlust, so it went and joined its parents in the pond.

There was a Wren poking around on the path in the Flower Walk, not an unusual sight.

But I had been reading the February issue of British Birds, which contains population estimates of the bird species found in Britain, and this rates the Wren as the most numerous breeding bird in the country, with 8.6 million territories -- that represents considerably more than double that number of birds, since Wrens are polygamous and may have several mates in separate nests in one territory.

This compares with 6.7 million territories for the Robin.

As you walk around the park you might see 10 or 15 Robins, but only one or two Wrens. And this is true even in spring when the male Wrens are singing (Robins, of course, sing all year round). It is remarkable that such a common bird should be so little seen, even allowing for its small size, unobtrusive brown plumage and shy habits.

The Robins have mostly stopped defending their individual territories, and some have mates. You can tell when two of these fierce little birds are a pair: they will come within ten feet of each other without trying to kill each other. Later you will see the male giving food to the female as a token that he will look after her properly when she is on her nest.

However, there are three Robins in the shrubbery between Buck Hill and the Long Water, near the bandstand where people practise martial arts. And they are all perfectly easy with each other, and come to my hand in turns to be fed. Can they be a ménage à trois?

One of the Mute Swans on the Serpentine, a rather skinny female, is a visitor. She has an orange plastic ring numbered 785 in black, and a metal ring with the number W34225.

On the other side of the lake a Pied Wagtail looks out over a bubble bigger than its head.

 

It is always a delight to see these birds racing up and down the shore at an incredible speed on their neat black legs, and launching themselves into the air uttering their cheerful flight song.