Causes of change

Changes in survival in the first winter, and also the post-fledging period, are sufficient to have caused the population decline. The environmental causes of this are unknown but are likely to include changes in farming practices, particularly land drainage and possibly increased pesticide usage.

Further information on causes of change

CES productivity shows an initial decrease, followed by fluctuations around a new lower level, and the number of fledglings per breeding attempt increased during the 1980s and 1990s but has since decreased (see above). There is good evidence to show that changes in survival in the first winter have contributed to the population decline (Thomson et al. 1997, Siriwardena et al. 1998b, Robinson et al. 2004). A more recent integrated analysis also indicated that post-fledging survival also made some contribution to annual population changes (Robinson et al. 2014).

Peach et al. (2004) suggested that loss of hedgerows, scrub and permanent grassland with livestock and the widespread installation of field drainage systems, all of which would act to reduce the availability of good quality foraging areas, have probably contributed to the decline of the Song Thrush in the UK. Similarly, it has been suggested that the species is unable to survive the winter in woodland, due to a lack of food, and a reduction of food supply in other habitat types has also been reported (Simms 1989). It is likely that a reduction in food supply would adversely affect the survival of juvenile birds to a greater extent than adult birds, as appears to be the case (Robinson et al. 2004). Furthermore, survival is reduced during periods of long drought or cold weather when food is likely to be less available (Robinson et al. 2007). However, modelling suggests that climate change may have had a positive impact on the long-term trend for this species, resulting in less negative trends than would have occurred in the absence of climate change (Pearce-Higgins & Crick 2019).

In woodland, drainage of damp ground and the depletion of woodland shrub layers through canopy closure and deer browsing may also be implicated (Fuller et al. 2005). There is also some concern of the impact of overgrazing by deer (e.g. Gill & Beardall 2001) and canopy closure (Mason 2007), due to changes in woodland management (Hopkins & Kirby 2007) on the low woodland layers, although good evidence from the UK is sparse (but there are some experimental studies in America on different species which demonstrate this effect, e.g. McShea & Rappole 2000). A study based on BBS results from 1995 to 2006 found a negative correlation between the abundance of deer and Song Thrush, with the species declining the most where deer population increase had been greatest, although the size of the impact was relatively small with modelling suggesting that deer could have caused a decline of around 4% over this period (Newson et al. 2012). Several papers (e.g. Gosler 1990, Perrins & Overall 2001, Perrins 2003) state that the understorey has declined in Britain, but few data are available to support this on a national scale. However, Amar et al. (2006) found a 27% increase in understorey in the RSPB sites used in the Repeat Woodland Bird Survey.

Robinson et al. (2004) suggested that predation was a possible cause of reduced survival but there is conflicting evidence on the role of predators in Song Thrush decline, and further research is needed. Newson et al. (2010b) found no evidence of effects of avian predators or grey squirrels on Song Thrushes.

Information about conservation actions

Like its close relative the Blackbird , the Song Thrush declined substantially during the 1970s and 1980s. Numbers have subsequently remained low despite recent slight increases. The decline was probably caused by changes to post-fledging and first winter survival and hence actions to increase food availability may benefit Song Thrush. These could include providing supplementary food over winter, managing hedgerows or woodland habitat for wildlife, and providing additional habitat, e.g. wild bird seed or cover mixtures, set-aside or grass buffer strips/margins.

As well as the removal of habitat such as hedgerows and scrub, loss of livestock (along with associated grasslands) and field drainage may also have reduced the amount of good quality foraging habitat, with field drainage being linked to the availability of earthworms (Peach et al. 2004). Peach et al. (2004) recommended that "new policy initiatives should aim to restore nesting cover (scrub and woodland understorey), grazed grasslands in arable dominated areas and damper soils in summer". Kelleher & O'Halloran (2010) also stressed the need to provide good nesting cover as a conservation action for Song Thrush and stated that dense vegetation was important when Song Thrushes were choosing a nest-site. Management actions which could help to increase the undergrowth and ground layers include opening up the canopy and control of deer numbers.

Like Blackbird, Song Thrush is a partial migrant, and local conservation actions to support birds in winter will not necessarily benefit local populations, so changes over a large scale may hence be required in order for actions to have a significant effect on British (and European) populations.