Wednesday, 30 January 2013

Nocturnal movements

There have been some nights recently when, taking the dogs out just before bed, I have heard the calls of golden plover and lapwing from high above me in the dark winter sky. These are birds on the move, perhaps local flocks moving between feeding sites within Breckland or recent arrivals from the Continent, pushed west by changing weather conditions. Such movements are common at this time of the year but they largely go unnoticed as birds pass overhead in the darkness while we are, for the most part, tucked up in the warmth of our homes.

Evidence of the range of species on the move comes not only from the occasional calls heard and noted down by birdwatchers, but also from the corpses that are sometimes found beneath office block windows in our urban centres. It is thought that some of these nocturnal migrants collide with lit buildings, perhaps even drawn to the light of illuminated office windows. Other individuals may be taken by urban peregrines, hunting at night with the aid of urban light pollution. Such behaviour has been little studied here in the UK but a growing interest in cities like Bristol is beginning to reveal the extent of the behaviour and the range of species taken. As BBC Winterwatch revealed, the Bristol peregrines have taken woodcock, snipe and even little grebe.

Many other birds are on the move at the moment, though not always at night. Changing weather conditions across large parts of continental Europe, with temperatures falling and waterbodies freezing over, may see an influx of waterbirds, including geese and swans, into the UK. The warming influence of the Gulf Stream means that Britain and Ireland present more favourable wintering conditions than those of the near Continent and east towards Russia. A flight of swans over Thetford just the other may have been such an arrival.

Of course, some birds are leaving our shores at the same time. Wintering blackbirds and other thrushes may move further south or southwest with the arrival of snow. Redwings, in particular, find cold conditions challenging and so tend to be very mobile. Some of those to have wintered here may move some way south into Iberia, perhaps even reaching the Mediterranean. The waxwings that have been such a feature of this winter are also very mobile, although in this case the movements are made in response to the availability of (or lack of) the suitable berry crops on which they depend.

If you can spare time to wrap up warm and stand in your garden on a still, clear night then you too might be treated to the soft calls of birds, passing overhead in the dark of night.

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