Cold weather acts as a cull

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THE OLD saying: “Green Winter, full churchyard” may be true for humans but not for birds.

A nice spell of frost helps destroy our cold and ‘flu viruses maybe, but sorts out the weaker wild birds and mammals. Starvation for them is not the only problem though. Their battle lines for life are drawn the second they are born.

Dr David Lack, when he was Director of the Edward Grey Institute for Field Ornithology at Oxford came up with some fascinating facts for the expected fate of wild birds. One in three mallard hatched could expect to be shot and eaten, for instance.

Nestlings of most garden birds except those that nest in holes, are most at risk from the seven million cats in the UK. Cats are the chief predator for birds.

Sparrowhawks are a major predator for fledged birds and adults in woodlands and heathland. Millions of birds are struck down by the apparatus of communication (road, rail and air traffic; over head wires).

Garden fruit netting, slug pellets, window glass into which birds mistakenly fly all play minor roles. The old pesticides of the 1960s are now thankfully inside history’s shameful bin of mistakes.

Dr Lack found that from every hundred robin’s eggs, seventy three hatched, and fifty seven would leave the nest.

By August, fifteen of these survivors will be eaten leaving forty two. Twelve of these survivors will be alive a year later. Four will last another year. Two will live to reach the age of three and only one the age of four. Blackbirds have a slightly different version of the same pattern.

Sixty four out of the hundred eggs hatch. Fifty fledge, thirty survive till August and over the following years the survival rate is fourteen, eight, four, two, one, nought. Apparently this expectation of adult life is constant and matched only in the animal kingdom by young male humans of warlike nations or in time of major conflicts among Western civilisations.

Exceptionally cold February weather can kill smaller birds which will not recover for up to four years.

I found that to be true when with my studies of birds over a period of forty years at Kingley Vale nature reserve.

February weather is more critical because food supplies have already been depleted by then so the birds are no longer as strong as they were in December. 1963 gave us a terrible winter in the new year and only one pair of wrens survived it at Kingley Vale.

But within four years the number of pairs had bounced back to their usual twenty which remained fairly constant until the spread of fallow deer destroyed their lower shrub layer mainly the ivy, in which they made their nests.