Silent Winter - Where have all the birds gone?

For the last several years I've spent an hour of the last Sunday morning in January doing the Big Garden Bird Watch. This year the count has been the lowest ever. We've a pretty big plot, over six acres, and over the years the garden round the house has gradually spread into what 40 years ago was a barley field and is not a mosaic of trees, shrubberies, rough grass and flower meadow with ponds. The ecological diversity has increased immeasurably and its value as a wildlife haven has increased year by year over the thirty years we been here.

But this year, where are the birds?

I walked across the field at first light to the village shop and saw overhead perhaps 150 pigeons and 36 black-headed gulls on the football pitch. There were a couple of dozen of each of jackdaws and rooks flapping about the house and tall trees in the garden. But when I went for my one hour wander round the garden later in the morning I was struck by the stillness. I was expecting something like this as I have noticed that the rate at which the peanuts and mixed seed in my bird feeders has gone down more slowly this winter than in previous years. It's been a mild winter here; the lowest temperature I've recorded has been -1°C and there's plenty of natural bird food about so I've not been to concerned.

So what I saw was:

1 Great Spotted Woodpecker
1 Magpie
3 Blackbired
6 Great tits
2 Chaffinch
2 Wren
2 Robin
1 Pheasant

What I heard were 3 or 4 curlew.

But the significant thing was what wasn't there:

Greenfinch, once our commonest small bird but not been around much for a few years
Blue tits, once more common than great tits
Tree Sparrow, once commoner than house sparrow
House Sparrow, we had three pairs nesting last spring but not here this morning
Dunnock, there's usually been a few about but not today