The First Big Chartwell Birdwatch

"CRONK ... CRONK". In the sky above Chartwell, on a startlingly sunny day, a raven is mobbing a common buzzard; flying above it and swooping at it, wings stiff, dipping from side to side like a Spitfire pilot saluting a fallen enemy. 

The birds are about the same size and, at a glance, it looks like two ravens tumbling in the air, which ravens sometimes do. When the buzzard peels away, spiralling west towards Mariners Hill, the difference is immediately clear. The raven, with its diamond-shaped tail and signature 'cronking' call, flies north towards Westerham in a straight line.

It's a good sighting, especially since this is the day of the first official Big Chartwell Birdwatch. Three teams of two - each made up of an experienced birder and an apprentice - are working the site: two teams clockwise and one anti-clockwise. 

It is horribly warm. Only a few weeks earlier, there had been snow on the ground. Just a week ago, parts of the woodland had been a quagmire with mud deep enough to make Wellington boots essential. Today, my Wellies are miniature rubber saunas in the heat: one for each foot.

But the count is on and in spring Chartwell is blossoming with birdsong. The most elaborate comes from a summer migrant, one of the boom boxes of the bird world: a willow warbler. Its huge sound belies its tiny size.

This one is somewhere in the bushes on the east side of the lower lake, audible from at least 50 metres away. Its song is a series of rapid, descending trills. We search the bushes and see nothing until a leaf moves in an unleaflike sort of way, and then we spot it. It's about four inches (10.4cm) long and will only recently have made its extraordinary journey to Chartwell from wintering grounds in southern or central Africa.

At the end of the lake, close to the statues, goldcrests barely bigger than butterflies flit in the shrubbery, and a pair of nuthatches get on with the serious business of making more nuthatches. Spring is in the air.

Sometime later, a sparrowhawk glides silently into the bushes around a pool on part of the site closed to the public. There is no sound, no flapping of wings: sparrowhawks are the stealth fighters of the raptor world.

Green woodpeckers make their laughing calls in the woods - their country name is a 'yaffle' - and mobs of jackdaws 'chack' to each other, a call that is said to be like the sound of two flints being chipped together. Chartwell's jackdaws are worth a separate blog.

By lunchtime, we have seen 38 species, including five not previously recorded. The cumulative list is boosted to 49 (53 if you break the rules and include introduced species, like the black swans, white ornamental ducks, a single mandarin duck and pheasants). 

We have all had a grand morning's birding in convivial company - but there is a lot more to it than that. By October, we should have enough data to begin to compare Chartwell's species list with those of similar sites nearby. That will tell us whether the countryside team needs to do more to encourage certain missing birds to the site - so the data we collect will lead to active conservation on the ground, which is a good thing.

By the way - does anybody want to buy some Wellies? Size 11. Only slightly sweaty. I'm going back to lightweight, breathable hiking boots for the summer.