Grey Wagtail: Seen on most fast flowing streams and rivers in and around Sheffield, and occasionally on our garden pond, these very active birds feed at speed on insects in and above the water. Why do wagtails wag their tails? As well as signalling to each other, It is thought this wagging flushes out the insects all wagtails feed on, and helps power and guide the little flights they do to catch flying prey. Although they have bright yellow rumps, they are easy to distinguish from the much less common Yellow Wagtail, having a grey back. Best time to see them: uplands in summer and lower areas in winter. Conditions: Sunny intervals with a breeze. Temperature: Max 19- Min 13C.