|A winter's evening at Arrocampo Reservoir (Martin Kelsey)|
At a quarter to six the action starts, flagged by the arrival of a band of forty Cattle Egrets, pushed by a sense of purpose and giving their craggy calls as they pass. A guttural wrenching call incongrously emergences from the elegance of a Little Egret. Why is it that a family of birds so suave that demand for their nuptual plumes decimated their populations over a hundred years ago, are the authors of such coarse squawks? My musing is quickly overtaken by the next ribbon of dusk activity as I count over two hundred Jackdaws, lining-up and all facing the same direction along high-tension cables. These are suspended from pylons that cut a tangent along the eastern fringe of the Arrocampo Reservoir. Unlike other reservoirs, the water level of Arrocampo barely fluctuates during the year. It acts as the source of coolant water for the Almaráz nuclear power plant, the white domes of which I can see across to my left. The combination of shallow water, absence of changing levels, a fertile catchment and sightly warmer than average water temperature has resulted in a wide margin of emergent vegetation, dominated by reed mace.
These Typha beds support a rich avifauna (as well as being superb for dragonflies). All of Europe's species of herons, egrets and bitterns have bred here and there are populations also of Purple Swamphen, warblers such as Savi's and Great Reed, and Penduline Tit. Crakes sneak through on passage. But in winter the Typha beds are where an extraordinary mixture of birds bed themselves down for the night. Before their descent, some of the birds are making pre-roost gatherings, such as the Jackdaws on the cables. As I watch them, a group of Lapwings cross my field of view, a staccato of black and white. Also heading for roost, their choice venue will be a shallow pool nearby.
Many species of birds roost communally, especially in the winter. The functions of coming together at night include the hypothesis of the information centre, where information may be transferred about the best foraging areas. If birds huddle together close enough this may help to reduce heat loss. Being together will also enhance viligence against predators and, in the event of an attack, increased likelihood for the predator to be confused and distracted by the multitude. As if to prove the point, there is a sudden frenzied eruption as the egrets and Jackdaws take off. A hunting Marsh Harrier in the twilight makes a couple of wavering banking manoeuvres in the midst of the commotion before straightening its trajectory and drifting off and away, doubtless to its own harrier communal roost.
As the cocktail of birds calm down and disappear once more in the vegetation, they start to produce a rather enchanting cacaphony of sound: growls from the egrets and the sharper calls from the corvids. Mixed together it takes on a gentle bubbling character, rather like a thick soup simmering. I imagine the gathering of now several hundred birds fidgeting as they settle down, a mysterious myriad of interactions between neighbours.
As I leave, with the night closing in, another sound emerges. It is the gruff "waaaa" of Black-crowned Night Herons. A string of these chunky birds fly high across the marsh from right to left. Unlike all of the activity so far, they departing from their roost and heading off to forage. The changing of the guard perhaps, or the ebb and flow of the tide.