When roosting on winter nights, some birds choose to do so alone but many species congregate in communal roosts.

For example, such species include starlings; crows; gulls and redwings while tiny wrens have been found huddled together in nest boxes to conserve heat for the smaller the body so heat loss is more rapid.

Another communal rooster is the pied wagtail. Every winter, many town centres act as dormitories for large flocks of these attractive black and white long-tailed birds because such situations offer shelter protection and warmth on bitterly cold nights.

I have been watching one such roost for decades in Wimbledon town centre where a group of tall plane trees stand and this year's roost is the largest I have ever seen with up to two hundred wagtails present every night.

In late afternoon the birds begin to arrive from far and wide and perch on nearby buildings. Then, as the light fades, they fly into a tree chattering excitedly to one another as they jostle for the best positions on the bare branches.

Looking up, we can see their white undersides reflecting the glare of street lamps resembling tennis balls in rows perched close to each other. (pictured).

Some wagtails remain by the station precinct throughout the day, running around picking up crumbs dropped by commuters  hurrying to and from the station, with most people oblivious to their presence.