Seven is the magic number for starlings

ANOTHER LIFE:A CONVULSION WOULD twist the cloud into an hour-glass, or clench it into a tight black fist. It would writhe and split and rejoin, but the egg shape would reassert itself like the physical form of a corporate intelligence. The flock would pour down finally into a conifer shelter-belt at the farm down the hill, "like a genie back into its bottle".

This winter, a small share of the great influx of starlings pressed on westwards to Thallabawn, roosting in another neighbour’s conifers and settling occasionally in the old, gaunt spruce at our gate, smothering it top to bottom in twittering black leaves. (A “murmuration” of starlings, the plurality invented, allegedly, by a 15th-century English prioress, Dame Juliana Berners, is a pleasant, but not specially appropriate, literary whimsy).

Our current local flock, perhaps a thousand, is minuscule, however, compared with gatherings at some east coast roosts. For a full-blown spectacular of pre-roost display, you should, it seems, head for a conifer plantation near a Tesco car park at Bettystown, Co Meath: "a truly astonishing sight" as vouched on the Irish Birds Network. For some mesmerising, armchair images, including video (made last March in neighbouring Laytown) go to akellyphoto.com/html/recent/starlings/starlings.htm

Fifty thousand birds, half a million . . . it needs an expert.

But Birdwatch Ireland doesn’t balk at the higher number for some winter gatherings here, drawn from Britain, Scandinavia and the Baltic states, as far as Poland. Looking at recent weather maps, new records seem inevitable.

Given such figures, it can be hard to think that starlings are on the decline. But many European populations have fallen drastically over the past 30 years. In Britain, where numbers have more than halved, some 8.5 million birds now keep mostly to the warmer south and find their nest-holes in the suburbs.

In Ireland, an old estimate of about 360,000 breeding pairs may need revising. Starlings need short grass and permanent pasture in which to probe for leatherjackets – the larvae of cranefly and their most substantial prey. Their appetite for this leathery pest, a root-chewer damaging roots of grass and arable crops, has made the birds an agricultural ally in many northern regions of Europe. But in Ireland, as elsewhere, the intensification of silage grass, where neither starlings nor lapwing like to walk, and other cattle-farming changes, must have taken its toll on breeding.

Meanwhile, the perennial human awe at how starling flocks manage their pre-roost aerobatics (meant to fox the intentions of any predatory hawk) may be nearer a persuasive resolution.

Starlings in a flock are keeping station with companions in three dimensions, all changing direction unpredictably, in a fraction of a second, without collision. The birds have remarkably mobile eyes, tilting forwards, upwards and backwards and able to focus on near and far objects in the same field of vision. For a long time, it was thought that they kept together on a follow-my-leader principle, like squadrons of fighter planes.

Then frame-by-frame analysis of filmed flight sequences suggested something more like the corner-of-the-eye awareness mastered by the chorus-line of Riverdance.

A new answer has come from Italy, where one team in a European research project known as Starflag (starlings in flight) has been led by Andrea Cavagna of the Italian National Institute for Condensed Matter Physics. Using two digital cameras, positioned precisely 25 metres apart on the roof of the National Museum of Rome, he filmed starlings wheeling around the sky above Rome’s Termini railway station. A new computer software technique allowed him to integrate one image with another to calculate an individual bird’s 3D position in the tightly-packed flock. He could process 2,700 starlings in under three hours.

Current computer models assume that each bird interacts with all its neighbours within a certain distance. But Cavagna’s observations and analysis seems to show that each bird keeps an eye on a fixed number of neighbours ­ seven other starlings ­ regardless of distance, and this, he suggests, gives the flock the fantastic cohesiveness that never leaves a bird on its own.

We always knew that seven was a magic number (Google offers “about 67,900,000” references to the fact), so that finding it as the key to one of nature’s most mind-blowing spectacles can come as no great surprise.

Eye on Nature

What causes mountains to turn blue when viewed from a distance? Are they bluest before, during or after rain? Does the type of rock in the mountains have any bearing on the blueness?

Cecilia McGovern, Ballsbridge, Dublin, 4

The air in the atmosphere scatters the blue light in the spectrum. That is why the sky is blue and also distant objects such as mountains, and the further away they are the more blue they appear, because there is more atmosphere between you and them. It has nothing to do with the composition of the mountains. Moisture in the atmosphere also refracts the light.

I found a dead goldcrest with a bloated tick attached to its throat, the same as those on dogs. I presume it killed the bird. How can one protect the birds?

Colette Ware, Skibbereen, Co Cork

Bird ticks resemble those that attach to dogs and sheep, but are specific to birds. The female tick attaches to the bird for a feed of blood and while feeding introduces a chemical in her saliva that paralyses it. In the wild there is nothing you can do, but it is important to keep nest boxes cleaned out after each season so as not to provide quarters for the ticks.

Michael Viney welcomes observations at Thallabawn, Carrowniskey PO, Westport, Co Mayo viney@anu.ie. Please include a postal address