Saturday, 27 June 2015

Mobbing frenzy

Short-toed Eagle with snake (John Hawkins)
The juvenile Blackbird was adopting a curious stance on the ground. It held its body erect and alert, its wings dropped, so that their tips almost brushed the earth and, most striking of all, its dark new tail spread wide like a stiff fan, also pushed downwards. Indeed so widely opened was the tail that the shafts of its twelve pristine-perfect feathers were countable. Adjacent to the young Blackbird was a Nightingale, cocking its tail skywards and giving a dry harsh alarm call. A posse of House Sparrows charged noisily onto the scene, scenting danger. In the bush above an Azure-winged Magpie swooped in to join the frenzy, uttering its menacing nasal drawl. It took me a little longer to figure out what the commotion was about. Just a few centimetres in front of the Blackbird lay the greyish-brown form of a Ladder Snake, almost a metre in length and with the diagnostic parallel dark lines running from head to tail along its upper surface. It was motionless, its head held slightly off the ground and its fixed stare magnet-like as it drew in my focus and that of the birds around it. But, it had merely paused and then glided from the shorter vegetation into a deeper tangle of growth where its presence would be fully hidden from view.

It was evening and the Ladder Snake, one of the most common species here, was setting off to search for prey having spent the day sheltering from the summer heat. Mobbing responses of birds towards snakes are often seen here, indeed most episodes like this that I encounter have been triggered by snakes, rather than say owls or mammalian predators. On several occasions along dirt tracks on the plains I have seen what at first seem to be dancing larks, but on closer inspection have been anxious birds jigging with tails and wings spread, seemingly attempting in a form of distraction display, feigning helplessness, to draw an intruding Montpellier Snake away from vulnerable chicks. What made this event particularly interesting was the protagonism of the juvenile Blackbird, not that long ago fledged from the nest and possibly encountering a snake for the first time in its life. It presumably was displaying a deep-rooted instinctive response, set off by something about the form and motion of a snake, evoking both intrigue and fear, that does not happen when, for example, a bird might see a basking lizard. The bird's response in turn set off a reaction amongst other birds, of different species, in the vicinity, combining curiosity and reciprocity to result in a collective action to force the snake onward.
Ladder Snake (Martin Kelsey)
The changing daily patterns of the snake activity brings out other interested parties as well. In these baking hot summer days there is little movement or sign of life as the temperature rises in the afternoon. Most animals are finding some shelter, some shade. For cold-blooded reptiles there is a real risk of overheating if exposed to the force of the summer sun, so snakes become most active as the sun drops and night approaches. Short-toed Eagles are snake specialists and have timed their return from wintering quarters in Africa to the emergence of snakes from their winter hiberation. In late February and March, snakes need all the sunlight they can find and become easy to see as they bask on the roads in the middle of the day (and became very vulnerable in the process). Then the Short-toed Eagles are hunting at midday, but now in late June, their stationary form fixed in the sky, making occasional slow deep hovering flaps, appears at dusk. With the light so dim that even the acumen of an eagle's eye must be limited, they scan the hillside from their high position, searching for serpentine movement. I wonder if they too track the frenzy of the mobbing response when smaller birds detect a snake..

Monday, 22 June 2015

Opportunity terns

Gull-billed Tern colony (Martin Kelsey)
Hit by the heat of summer, now laden upon us, a soporific solstice, I feel a lull across most of the landscape. There is a tidyness across the sun-bleached plains, with hay stacked in bales and stubble fields in lieu of crops. They are dotted by White Storks, foraging for youngsters which any moment now will vacate their nests. Around the smallholdings and gardens in the village, the withered yellowed stems that remained of the spring flush have been cut and gathered. We are all fire conscious and have used last few weeks in trimming and tidying. The dehesas are neat with each tree defined in shape against the dry golden pasture below. Summer sounds different too. The Nightingale now croaks like a frog, its song period over and we hear merely its call note from the bramble cover. Dawn is quieter, save for year-long House Sparrows' conversations. There are flurries of Spotless Starlings passing by whilst small parties of Bee-eaters slowly wheel high above, reinforcing the sense of the languid summer.

I find an abrupt counterpoint by water. Making my way beside the Alcollarín Reservoir, the sound reaches me from some distance. It is incessant, a cacophonous, rubbing almost nasal chitter. Gull-billed Terns with slow, deliberate flight head towards this sound, bearing small glistening fish fry from their jet black bills. Others, empty-billed, radiate away. On a small, low-lying island, tricoloured with its bands of sandy beach, vegetation green and blond dry stems in the centre, there is sound and motion. Over eighty pairs of Gull-billed Terns have opportunistically chosen this secure but transient site to establish a colony. Crowded mainly at one end, leaving the extreme eastern end of the slither of land for loafing Great Cormorants, the terns produce that wonderful sense of bickering and neighbourly squabbling. A bird arrives with a small fish, hovering briefly over its patch as it checks the best place to land for passing this gift to its mate. Most of the birds seem to be sitting tight on nests, but along the shoreline, there are two large tern chicks begging for food, confirming what I had suspected, that this breeding opportunity had been taken up by some pioneering pairs first with others coming on board later on.  The Gull-billed Terns were not alone. Dwarfed and out-numbered,  Little Terns with their darting jerky flight crossed the island and I reckoned that there were at least five nesting pairs there, away from the Gull-billed hullabaloo. The beach also was liberally dotted by incubating Great Crested Grebes, looking remarkably ungainly the land, their legs set almost as far back along their bodies as could be, gingerly waddling in a slow swaying motion from the water to the pile of weeds which served as the nest. Along the strandline Little Ringed Plovers were scampering, darting between the terns and grebes.

There were Black-headed Gills too and as a Black Kite flew over, sending the terns into an immedate panic, a pair of gulls set off after the kite to mob it. This display by the gulls to ward off such a threat I had not seen here before and it meant just one thing: that the gulls too had nests. On the side of the island in view, I saw two gulls that were sitting tight at the edge of the tall grasses. On the other side another bird was sitting, clearly incubating, whilst nearby there were a pair of adults with the dark brown downy forms of three small gull chicks. Abundant though this species is most of the year in Extremadura, it is a rare breeding bird, with just a small population spread across just a few breeding colonies. This finding represented a new breeding site for the species.