Threats
Song thrush decline appears to be caused by a combination of lack of food and lack of nesting sites, both brought about by the intensive farming methods so widely practices in the UK today.
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% of fledglings and 60% of adults survive to breed the following spring. There are currently serious concerns over the song thrush population in the UK, with anecdotal reports of population declines since at least 1950. The long term monitoring carried out by the British Trust for Ornithology shows that the population has been in a major decline since 1970. This decline has been most pronounced on farmland, where the population has decreased by about 70%. Because of this decline, the song thrush is red listed as a bird of serious conservation concern. The decline appears to be caused by a combination of lack of food and lack of nesting sites, both brought about by the intensive farming methods so widely practices in the UK today. 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 ideal habitats with plenty of food for song thrushes. Both of these have been lost or degraded in most lowland arable 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 farmland with a stable population on mixed farmland. Two major differences have been found. Thrushes on intensive arable farmland make only 2-3 nesting attempts per year, compared to 4-5 attempts for birds in a 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. 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: the proportion of thrush nests which are predated has actually fallen during the last 30 years, and changes in the number of breeding thrushes on 250 individual study farms across lowland Britain are not related to 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).
|