A nightly gathering

The other afternoon, on my way to give an evening lecture, I made a brief stop at Waitrose in Swaffham. The crisp, clear day was coming to an end, as the sun slowly slipped towards the horizon, dipping behind the cathedral-like spire of the wind turbine that now dominates the local landscape. From all around me I could hear the loud ‘chis ick’ flight calls of pied wagtails, as individual birds moved between the young trees that dotted the car park. Others stood, silhouetted against the sky on the roof of the supermarket, marking the beginnings of a gathering that would, most likely, be repeated each night throughout the coming winter.

Pied wagtails are well known for their communal roosting behaviour, perhaps because such roosts are often formed on or around man-made structures. These small, insect-eating, birds can find winter a difficult time; food is scarce and energy expenditure is high. In the short-days of mid-winter they may spend 90% of the daylight hours feeding, collecting a prey item every four or five seconds. Even this high rate of feeding is only just enough to balance their energy expenditure. In particularly cold winters, many individuals will move further south, deserting southern and central Britain, to winter in France and Spain. Even so, winter mortality can be very high.

It should be no surprise then, that pied wagtails gather to roost communally, seeking out the warmest sites. These include factories, hospitals, commercial glasshouses and trees along major roads or in supermarket car parks. Not only do such roosting sites provide much-needed warmth, they can also act as information centres. Birds that have not fed that well during the day are able to spot individuals that have been more successful; the following morning they will follow these individuals out of the roost and, with luck, have a better day. One final benefit of roosting communally is safety in numbers. While the presence of many small birds at a roost might attract predators, the individual’s chances of being taken by a predator remain lower than they would be if the bird were alone.

Most pied wagtail roosts within Norfolk number just a few tens of birds but roosts in excess of a hundred strong are not uncommon and a small number have been known to hold over 1,000 birds. For example, a roost in Brigg Street, Norwich, was reported to have held 2,213 individuals on the night of 12 January 2001. Pied wagtail roosts lack the frenetic energy of starling roosts and, because of the choice of roosting site, are often more readily viewed. Next time you are out shopping late afternoon keep an eye out for these delightful little birds.