This winter, the common land near my home has been home to a roost of starlings and to the daily spectacle of them wheeling and flowing across the sky, like smoke in caught in the wind. An hour or so before sunset, small groups gather on rooftops, then they wing their way across the winter tree tops, the numbers of birds slowly growing to several hundred, as more and more groups arrive from the fields and gardens where they have been feeding by day. Sometimes small groups break away, dart off on a circuit of their own, then return to a flock that one moment spreads out like a streamer, then clusters tightly, constantly twisting and flowing in forms that never quite resemble any nameable shape. As I watch, the birds shoot over my head chattering quietly, with a whisper of wings like soft rain.
One reason for these circling flypasts, before settling roost in the reeds of the pond, may be the local sparrow hawk. Scientists studying the phenomena of starling murmurations think that the flocking and ever-shifting shapes are largely a response to predation, as targeting one bird out of the mass becomes impossible. I’ve watched the sparrow hawk fly up out of the reeds into the flock, or try flying at them from above; mostly it sits in a tree, watching.
The science of murmurations is incomplete, but researchers have employed sophisticated video analysis and computer modelling to study how the birds achieve the spectacular moving sky patterns. To date, the thinking is that the flock is like a liquid turning to gas, or snow before an avalanche. It is a system poised to tip, with the movement of every part affecting the whole and every shift known as a critical transition. This is a science closer to physics than biology and the evidence seems to be that each bird reacts to even the smallest movement of the birds closest to them, this movement rippling rapidly through the flock in groups of seven – each bird affecting the seven closest to them. Research continues, for the exact science of how the changes ripple through the flock without accident or confusion remains a mystery.
Flocks of thousands of starlings are known from large reed beds around Britain, with notable winter roosts on the East Anglian coast. Many of the birds will be winter migrants, boosting the numbers of a species that has suffered a dramatic decline in recent decades; long-term monitoring by the British Trust for Ornithology (BTO) shows that numbers of breeding starlings have fallen by 66 per cent in Britain since the mid-1970s. The roost on the common is tiny compared to some, but it demonstrates the value of even small areas of habitat and is a reminder that observing the natural world is vital in informing the management of sites. It would be easy to look at the reed covered pond as “choked” – untidy and overgrown, but it is one of several on the common and the only one to support common reed. If clearance of the ponds is considered in the future, retaining some of the reed will mean there is always a winter roost for the starlings.
As evening grows darker, the starlings circle ever closer to the reeds, swooping down close, then rising again, until at last, as if on some unseen signal, they pour into their roost in a single black flow, like dark liquid through a funnel. They are easily unsettled at first, fluttering and chattering amongst themselves, rising uneasily if I walk too close. In the morning they fill my garden hedge with the fizz and buzz of their song, reminding me that spring is around the corner and that the flock will soon disperse – until next winter.
This post will also appear on the Norfolk Wildlife Trust blog – see norfolkwidlifetrust.org.uk