Thursday, 22 February 2018

Circles

Vortex behaviour by Shoveler (Martin Kelsey)

With an unaided eye, they appear like dense, floating mats. There are four of them visible on this small water body, along with a selection of busy evenly-scattered ducks: up-ending Mallards, pootling Shovelers, diving Pochards and Teal nibbling at the water's edge. But there is something about the motion of these mats that intrigue: they are alive. With binoculars the species responsible is instantly recognisable. With brilliant white bows, toffee-brown flanks and bottle-green heads, the drakes are Shovelers, and they seem to outnumber the females. And I become spellbound at what they are up to. 

These boldy patterned concentrations of Shoveler are circular in form and are spinning anti-clockwise. These duck mats vary in size, but the one I am watching has over 40 birds. Shoveler swim in to join the cluster, becoming tightly embedded into its form. Almost all of the birds have their bills, or even whole heads, submerged, whilst those in the middle are upending. It is almost as if the sheer pressure towards the centre forces those positioned there to be pushed vertically, so tight is the concentration of bodies. The mass continually circles in the same direction, a metaorganism, a vortex.

This pack of spinning Shoveler is creating a swirl of currents under the water, enough indeed to stir up the sediment on the bottom of the pool, bringing it up in a spiralling column to within the reach of the feeding duck. There their spatulate bills, edged with filtration combs, are ideally formed to collect the range of tiny food items they seek: seeds, insects, crustaceans. Biologists have described this as "Collective Vortex Behaviour", indeed there is even a paper about it in The Quarterly Review of Biology, published in 2016 by Johann Delcourt et al. They describe a model of how these spinning arrays of Shoveler form. Shovelers when feeding alone usually move either by simply ploughing forward in a straight line, or going round in little circles. This circular motion is an attempt to stir up sediment. If another joins it they circle close together in the same direction in order to avoid colliding with one another. Then there may be one or two more joining them. The result is a vortex which becomes so effective that it brings to the surface more food than they can eat. This then becomes an attraction for other Shoveler nearby and very quickly the group becomes much bigger.

I watch a place where three vortices were quite close to each other, and there is a steady movement between them of individual birds moving from the periphery of one to the other: sneaky opportunists perhaps.

I have watched this pool over several winters, but never before have I seen so many Shoveler on it and never so many showing this spinning behaviour. But the difference this year is that the water level has reduced hugely because of the severe drought. Perhaps this year the water is shallow enough across the whole pool for the vortex to work effectively, whereas in previous years the Shoveler seemed to spend most of their time snoozing on the surface: dormant and gently rocked by the ripples.
Spiralling Common Cranes (Martin Kelsey)

Above me, as I watch the vortices, birds are spiralling. A multitude of Common Cranes on northward migation rising on a thermal. This is a wholly disorganised circle, noisy and atomised. It rises anti-clockwise too but takes no clear form - each individual bird using the rising air to gain height. At its climax it undergoes an amazing transformation. Somehow they organise themselves into a strewn-out skein and proceed on their north-east trajectory. Having used thermal forces for ascent, they are now pulled by gravity, aided by the slipstream from the shape of the skein. There is beauty as they move, wings held in a glide with the occasional sine wave of gentle flaps. The sky is braided by this long string of birds. 

Braids of Common Cranes (Martin Kelsey)



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