Autumn time is party time for our birds. Simon Barnes explains why.

The canopy the leaves change colour and start to fall and then from the heights a sudden sound that seems to be calling the world to order. Sisisi! Sisisi! No question about it: autumn is an accomplished fact and the birds respond to it by – well, more or less by turning into something else entirely.

What must it be like, I wonder, to spend half your year as one kind of person and half your year as someone completely different? We humans don't do like that: we're more or less the same no matter what the season.

Our rough and basic plan – a theme with many variations – is to reach maturity, pair up, breed and raise young, in theory with the same partner, and when that part of our lives is done, to pass on such wisdom as we have acquired to the next lot. It's smooth curve.

Not for these little woodland birds. In the spring time their lives were built around a keep-out message: we're a pair, this place is our place and the rest of you can get stuffed. The mad intensities of getting partnered up and raising a brood are done in territories deliberately isolated from others of their kind: places where they can forage and find enough to feed their young.

Then suddenly the brood is fledged. Some will breed again, others not, but no matter what their strategy, there comes a point when the breeding frenzies are over and done for the year. And so, more or less at the throwing of a switch, they change from in-looking anti-social beasts for whom family is everything, to madly social beings in love with one another's company.

The hormonal levels in the adult birds drop like a brick, the urge to sing and fight and mate is gone from the males, the same-but-different intensities are gone from the females, and suddenly a bird's best friend is a lot of other birds.

Instead of feeding in separate territories they pool resources and form temporary flocks. The best way to appreciate this is in a small wood, when you hear that sisisi-ing. That's the long-tailed tits telling each other to stick together. It means I'm here, where are you?

They seldom shut up unless there's a predator about, sparrowhawk maybe, and the sudden silence is as loud as a shout, the best of warnings. And in those flocks of long-tailies there are very often other species: great tits and blue tits, wrens and dunnocks, all moving together from tree to tree.

They each benefit from the disturbance the others cause, and from the many-eyed vigilance of the flock. But I don't suppose they think that through and ponder out the advantages: my guess is that they take sudden real pleasure and comfort in the presence of each other.

It's what's known as a bird party or a feeding party: and it seems to me that the word party is chosen not just because there are a good few birds in there, but because they are having a party in the other sense of the term: having a high old time up in the canopy, enjoying the food and the company.

Long-tailed tits have been called the convenors of the flock: the great party animals. They are the most intensely social of woodland birds, and the most vocal, and their sisisis summon the flock and keep it together. You'll hear it ringing out in patches of woodland, urban parks, tree-lined suburban streets, along railway lines.

Everybody is everybody's best friend as the party continues out of sight but not out of hearing. Odd to think that those that make it to the spring will be telling each to get stuffed with the same vigour that brought them all together. But that's for then: for now the flock's convenors are calling and the rest join in, glad to be there, part of the party that offers the finest going-home present of them all: survival for another day.