22 July 2011
Song thrushes are relatively short-lived birds. They live an average of 3-4 years, but a few can reach quite an advanced age. The oldest known wild individual was 13 years, 9 months old. Mortality is high and its causes many and varied. Only 20 per cent of fledglings and 60 per cent of adults survive to breed the following spring.
Long-term monitoring carried out by the British Trust for Ornithology shows that the population in England declined by more than 50 per cent between 1970 and 1995. This decline was most pronounced on farmland, where the population decreased by about 70 per cent. Because of this decline, the song thrush is red listed as a bird of serious conservation concern. There has been a partial recovery in numbers during the last decade.
The decline in song thrush numbers has probably been caused by the loss and degradation of preferred feeding and nesting habitats.
Loss of hedgerows and wet ditches removed feeding and nesting sites, while increased land drainage and tillage are likely to have reduced the number of earthworms and other crucial invertebrate prey available to song thrushes on farmland.
Grazed permanent pasture (especially cow pastures) and woodland are important habitats with plenty of food for song thrushes. Both of these have been lost or degraded in many lowland areas.
In many areas of intensive farming most song thrushes now breed in, or close to, gardens. RSPB research has compared a declining population on intensive arable farmland with a stable population on mixed farmland.
Two major differences were found. Thrushes on intensive arable farmland made only 2-3 nesting attempts each year, compared to 4-5 attempts for birds in the stable population. Few fledglings on intensive farmland appear to survive their first few weeks after leaving the nest.
These differences are large enough to have caused the population decline on arable farmland and are probably caused by lack of food (earthworms and snails) during spring and summer. Farming measures likely to help song thrushes include sympathetic hedgerow management (with tall, thick hedges), planting new woodlands on farmland, and planting wild bird seed mixtures including leafy cover. Government grants are available for all these measures. Any measures that prevent soil from drying out during summer are also likely to be beneficial.
Some people suggest that increases in the numbers of magpies and sparrowhawks may be causing song thrushes and other songbirds to decline.
Two pieces of evidence suggest that this is not true. First, the proportion of thrush nests which are predated has actually fallen during the last 30 years. Second, changes in the number of breeding thrushes measured across several hundred farms across lowland Britain are not correlated with changes in hawk or magpie numbers on the same farms. So thrushes are just as likely to have declined on farms which have lost hawks or magpies. (Source: British Trust for Ornithology).