Bird arrivals signal the changing of the seasons

They are less familiar to most people, but they signal the approach of winter just as swallows and cuckoos announce the coming of spring. Redwings and fieldfares, colourful thrushes from Scandinavia and Russia, pour into Britain in enormous numbers every autumn to escape the harder winter further north, just as Britain's summer visitor birds are heading back south for an easier life in Africa.

They are less familiar to most people, but they signal the approach of winter just as swallows and cuckoos announce the coming of spring. Redwings and fieldfares, colourful thrushes from Scandinavia and Russia, pour into Britain in enormous numbers every autumn to escape the harder winter further north, just as Britain's summer visitor birds are heading back south for an easier life in Africa.

Now, for the first time, you can watch the arrival of the redwings as it happens. Reports from birdwatchers in Britain and Ireland are being co-ordinated on the internet to produce an unfolding, animated picture of the advent of these two cold-weather visitors, from the first trickle of birds in September to the great mass migration happening now. You can watch maps of the country being slowly covered in redwing and fieldfare dots, sightings by observers, every week.

Eventually, the countryside will be covered, with perhaps more than a million of each species, usually most visible when they are stripping the bright-red winter berries off trees such as rowan and holly.

The place to watch the phenomenon of their arrival is BirdTrack, the online bird recording scheme of Britain's leading bird research organisation, the British Trust for Ornithology (BTO). Run in partnership with the Royal Society for the Protection of Birds (RSPB) and BirdWatch Ireland, Birdtrack is the first attempt at capturing a mass picture of bird migration as it happens.

This spring (under its earlier name of Migration Watch) it illustrated for the first time the arrival of millions of summer visitors, such as chiffchaffs and willow warblers, as well as swallows, swifts and cuckoos. Much less is known about the timings of arrival and departure of winter visitors, but a picture is emerging.

For example, there were no redwing sightings across Britain by the observer network until the week of 19 September, then there were three: a group of three birds at Aberffrwd, Ceredigion, on 19 September, a single bird at Washington Wetlands Centre, Tyne and Wear, on 22 September, and three more at Hungerford Marsh in Berkshire on 25 September. The next week, the influx proper began, with sightings in 24 spots, and they have since become a flood.

Fieldfares began to arrive later that same week too: six were spotted at Bungay in Suffolk on 23 September, and a further seven birds at Gallt, Powys, two days later. There were only six sightings in the following week, but the week after that the rush began, with sightings from 41 locations.

Now the birds are pouring in, hordes crossing the coast after their long flight across the North Sea. At North Ronaldsay bird observatory at the north-eastern tip of Orkney, 7,000 redwings and 15,000 fieldfares were counted one day last week.

Both birds, members of the thrush family and close relatives of our song thrush and blackbird, are frequently seen in gardens. The redwing, Turdus iliacus, is similar in size to the song thrush but has a white eye-stripe and rusty-red patches on its flanks. The fieldfare, Turdus pilaris, is the same size as the mistle thrush but has a grey nape and rump, with a red-brown back.

BirdTrack can be accessed at www.bto.org/birdtrack