Why do pied wagtails gather in city centres?
The sight of pied wagtails roosting in the centre of a busy city is one of our more unusual wildlife spectacles, most likely seen during the coldest months. The birds are taking advantage of the heat lost from homes, shops and offices, which can lift the ambient temperature to several degrees Celsius above that of the surrounding countryside, a pattern known as the ‘urban heat island effect’.
The pied wag feeds mainly on small invertebrates, which are acutely vulnerable to winter temperatures. This is why much of the northern part of the species’ breeding range within Britain is abandoned outside of the breeding season. Many birds move south to the lowlands, forming large roosting flocks that gather in low bushes, reedbeds and, on occasion, city-centre trees. One flock once even found its way into a commercial greenhouse.