Christmas Shopping in York tonight? Look up for a spectacle worthy of Planet Earth II

At this time of year York is humming with shoppers, heads bowed as they plough up and down the streets fixated on the job of getting ready for Christmas. But if you happen to find yourself amidst the throngs at St Nicholas market on Parliament Street at dusk, it’s worth taking a moment out from the hubbub to look up past the brightly-lit stalls and up into the trees.

Up there you’ll find the branches abuzz with a very different sort of get together. Hundreds of pied wagtails gather every evening in the London Plane trees outside Marks and Spencers  – right above the heads of the shoppers. Drawn by the warmth generated by the lights of the city, these small black and white birds flit about overhead, chittering noisily and wagging their long tails in a huge communal gathering before settling down to roost. At first glance they look like Christmas decorations hanging in the trees.

Like the shoppers, these tiny birds come from miles around for this annual winter get together. They flock together above the street lights where it is warmer and, again like the shoppers beneath them, tuck into extra snacks available - in their case snacking on the insects that are also drawn by the warmth of the city centre.

Pied wagtails weigh on average just 21g and during the cold winter nights can lose up to 20pc of their body weight. So huddling together at this time of year is a vital survival technique. I spotted them for the first time some years ago while I was late night shopping with my wife in York. We arrived just as dusk started to fall and looked up to see a flock of more than 20 on top of the roof of Marks and Spencers.

It wasn’t long before a second and then a third large flock joined them. These elegant birds like to roost communally so that they can keep warm and these huge congregations are quite sociable occasions for them. In a short while there were more than 200 pied wagtails noisily chittering amongst themselves as the shoppers below them walked past seemingly oblivious.

As I watched the birds, I noticed the sound of their affable song change tone. What had been an easy, social chit chat turned into higher, faster-pitched, noisy calling. The flocks began flitting uneasily from one rooftop to the next as they let out these sharp warning sounds. Then I realised why. A sparrow hawk suddenly appeared out of nowhere and immediately set to, chasing wagtails before him. There was chaos in the sky as the black and white birds flew and swerved in all directions.

Some wagtails even dared to try chase the hawk away and the tactic worked monetarily as after a while the hawk retreated. The wagtails, now all grouped together in one flock, settled uneasily back down onto the roof of Marks and Spencers.

But before long the hawk was back. It flew flat out across the facia of the Halifax bank building, banking away when it was just inches away from the wall. As it flipped up over the roof top, clouds of pied wagtails took to the air. Again the hawk failed to catch a wagtail. But then on the third fly-by it was successful and plucked one from the flock with its sharp talons.

The wagtails were very flighty after this aerial assault and flew around frantically, landing on different roof tops intermittently before finally settling on top of Barclays bank. Then when they thought the danger had passed the flocks swooped back down into the London Plane trees that line this popular shopping street and at last settled down to roost for the night.

The noise of all the birds during this aerial attack had been incredible – in my opinion the scene wouldn’t have looked out of place on a Planet Earth II episode - and yet not one of the hundreds of people in the street below even looked up!

I was surrounded by folk and felt like shouting out, ‘Wow did you see that!’ but thought better of it. The shoppers, focussed on their own world, had missed this incredible drama in the sky taking place right above their heads in the centre of York.

I will be selling mylimited-edition prints, tableware and greeting cards at a stall opposite the Disney Shop in York's St Nicholas market this year. If you are passing at dusk let a member of staff on my stall point out the pied wagtail roost to you. They begin to congregate at 3.30pm and are usually settled into the roost by about 5pm. Bring along your binoculars!