Gone for a harsh winter's song

Cold winters and loss of pasture to new crops are hastening the decline of the song thrush, says Malcolm Smith

At one time it was instantly known for its distinctive medley of shrill and flute-like cadenzas. Not now. Instead, the more melodic, deeper tones of the blackbird ring out from the woods and gardens the length of Britain.

For the song thrush, once so familiar to our parents' generation, is in serious decline. There are, according to data compiled for the New Atlas of Breeding Birds, around 990,000 breeding pairs in Britain compared with around 4.4 million pairs of blackbirds. A human generation ago thrushes were more abundant than blackbirds.

The figure of 990,000 breeding pairs may sound like an awful lot of birds. The British Trust for Ornithology (BTO), however, has documented song thrush population changes - along with a plethora of other bird species as part of its Common Birds Census - and found a 64% decline in song thrush numbers on farmland between 1975 and 1994. In woodland, the thrush numbers fell by 34% between the same dates. The species is more common in the south and east of England than in the north and west.

BTO research has shown that whatever is doing for the song thrush is affecting the adults. Their breeding success - the number of eggs laid and young successfully reared - has not altered. The finger of suspicion has been pointed at slug and snail pellets, copious quantities of which are used on farmland and in our gardens. After all, song thrushes, unlike blackbirds, are particularly fond of these succulent creatures, especially in late summer and winter.

But slug and snail pellets are not a major factor. Song thrushes have been in decline for longer than pellets have been in use. So far, says the BTO's Richard Gregory, there is no evidence that pellets are directly implicated. "Detailed analysis of our census data implicates severe winter weather, to which song thrushes seem particularly vulnerable. They declined after the severe winter of 1962-63, a little in 1978-79 and again with the cold weather of 1990-91 - especially in the south-east. But the pattern isn't crystal clear. The steepest decline, between 1975 and 1978, preceded the cold of the 1978-79 winter when winters were relatively mild.

British song thrushes, unlike blackbirds, are partial migrants, many Scottish and northern English birds zipping off to milder Ireland for the winter. Some even leave the balmy south of England for France and Spain. So winter frosts don't offer a plausible answer either. Vulnerability to continental hunters has been ruled out because levels of hunting haven't increased, and the proportion of our song thrushes on the Continent isn't large.

Declines have been most severe on livestock farms where pasture has often been replaced in recent years by wheat and oil seed rape. Song thrushes favour woodland edges, breeding in the wood but feeding outside it in invertebrate-rich pastures. Loss of pasture has probably taken its toll.

Less intensive agriculture and more crop land set aside could see thrushes recovering somewhat over the next decade. But it is unlikely they will ever again compete in the birdsong charts with that garden success story, the blackbird.