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..

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