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Bittern displays |
10.12.2002 |
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Ó
Jane Burton |
The bittern seen recently at the
Wetland Centre near Hammersmith created a bit of a flurry in
the daily press. Our few remaining bitterns are joined in
winter by immigrants from the Continent and three had been seen
standing disconsolately on the frozen surface of our gravel
pits. I went along next day and scanned the reedbeds, more
in hope than expectation. Eventually I noticed that a
bittern was virtually waving to me. It was a quarter of a mile away on the far side of the pit (so much for 'perfect
camouflage'!), perched on a dead limb in the reedbed busy
preening,
a few feet above the water.
A few minutes later, a heron
swooped to land nearby. As it passed over, the bittern
adopted its famous 'sky-pointing' display with its bill
directed vertically. I had never seen this display before
but know it well from photographs. The streaked
brown plumage of the throat is said to merge with the vertical lines of
the reeds and enhance the bittern's camouflage. Amazingly, it even
sways to match the reeds shaking in the wind. I can see the
thinking behind this remarkable piece of evolution: there is
no point matching the background if you are given away by movement,
or lack of it.
The photographs also show that the display probably has a
second function. When 'sky-pointing' the bittern's eyes are
turned inwards, so that it squints under its chin, so to
speak. This is a trick occasionally used by other birds.
Their eyes are set to the sides of the head so there is
normally a
blind spot to the front. By lifting the bill and peering under
it, they can focus accurately on prey or danger dead ahead.
Less well known is the bittern's behaviour if it is
seriously disturbed. It turns into a formidable adversary by
crouching with wings spread and feathers raised. It may even
advance slowly, like a galleon in full sail, its dagger
bill poised to strike.
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