Causes of change

There is evidence that changes in survival rates due to lack of food resources, because of agricultural intensification, are the main driver of House Sparrow declines in farmland, although changes in breeding performance may also have played a role. Different processes have affected House Sparrows in towns, where breeding performance could be the most important driver of declines, although the evidence for the ecological causes is less clear.

Further information on causes of change

A temporary drop in first-year survival coincided with the period of steepest decline, but changes in breeding performance, especially reduced nest failure rates at the chick stage, appear to have driven a levelling-off in the long-term population trend (Freeman & Crick 2002). Over the period since 1968, brood size has decreased (see above) but there has also been a major decrease in nest failure rates at the egg and chick stages, so the number of fledglings per breeding attempt has shown a net increase. Further evidence for the role of changing survival in House Sparrow declines has been provided by Hole et al. (2002), who found no evidence of significant differences in most breeding-ecology parameters in declining and stable populations in a farm-scale comparison, while Siriwardena et al. (1999) found that national survival rates were lower during the period of decline in the CBC index. That survival, especially of adult birds, appeared to make the largest contribution to annual population change was also found by Robinson et al. (2014). Crick & Siriwardena (2002), using NRS data, showed that breeding performance per nesting attempt had increased and was positively correlated with population growth rate in the wider countryside (although there was no such correlation in gardens). Analysis of Garden BirdWatch data found higher seasonal peak counts, however, relative to pre-breeding numbers, in the north and west of Britain than in the east and south where population decline is strongest, thus indicating that breeding productivity is influencing population trends (Morrison et al. 2014).

There appear to be different processes affecting urban and agricultural populations. On farmland, changes in farming practices due to intensification of agriculture and the tidying of farmyards have reduced the seed available to farmland populations of House Sparrows during winter, which has resulted in a reduction in survival rates (Siriwardena et al. 1999, Chamberlain et al. 2007, Hole 2001), specifically of first-year birds (Crick et al. 2002). This is supported by a positive effect of supplementary seed in winter on farmland House Sparrow population trends in a landscape-scale experiment in East Anglia (Siriwardena et al. 2007), and a similar positive effect from the provision of areas of seed rich habitat on farms under agri-environment schemes in Northern Ireland (Coulhoun et al. 2017). House Sparrows have probably been deleteriously affected by the decrease in the amount of grain spilt around farm buildings and during the process of harvesting since the 1970s (O'Connor & Shrubb 1986). The move towards autumn-sowing of cereals has meant that cereal stubble has become much rarer, reducing food resources over winter, although Robinson et al. (2001) found no influence of spring-sown cereal on House Sparrow abundance in predominantly pastoral farmland. Conversely, breeding performance is worse where there is more spring cereal (Crick & Siriwardena 2002), although this may reflect geographical associations with areas where spring sowing remains widespread in the UK (the west and north) rather than direct effects of cropping.

Recent declines have been particularly severe in urban areas (Robinson et al. 2005b, Chamberlain et al. 2007). Increased predation by cats and Sparrowhawks, lack of nest sites, loss of food supplies, pollution and disease have all been cited as factors possibly depressing populations in towns (Crick et al. 2002), but supporting evidence for these is mixed. Within urban areas, Shaw et al. (2008) reviewed available evidence and hypothesised that House Sparrows have disappeared from more affluent areas, where changes to habitat structure such as planting of ornamental shrubs and increased demand for off-street parking is likely to reduce the amount of habitat available to House Sparrows and influenced foraging and predation risk. The conversion of private gardens to continuous housing has also had a negative effect on House Sparrow abundan

Vincent (2005) found that annual productivity among suburban and rural human habitation in Leicestershire was lower than that measured on farmland House Sparrows in Oxfordshire, the main cause of the difference being starvation of chicks. Low body masses at fledging, and consequently low post-fledging survival, were also recorded in Leicestershire. Although only a two-year study, Peach et al. (2008) measured reproductive success in a declining House Sparrow population along an urbanisation gradient in Leicester and also found that a year in which reproductive success was too low to sustain the population was characterised by lower chick survival and body mass at fledging (a predictor of post-fledging survival). However, there is no direct evidence that invertebrate food supplies have declined in these areas and variation in survival has not been investigated. Supplying mealworms for garden-nesting House Sparrows in Greater London substantially improved breeding success but did not increase nesting density (Peach et al. 2014, 2015). Supplying unlimited high energy seed throughout the year did not affect overwinter or survival or population size, suggesting that food availability was not currently a limiting factor for suburban sparrows (Peach et al. 2018).

Analysis of cross-colony abundance in Greater London found that numbers were higher in areas with more seed rich habitat and low levels of nitrogen dioxide air pollution, although further evidence about the effects of air pollution are needed to confirm whether it may have contributed to the decline (Peach et al. 2018). There is evidence, however, that avian malaria may have contributed to the decline in London, where infection was found at very high prevalence and survival rates were negatively correlated with infection rates (Dadam et al. 2019).

Negative correlations between indices of Sparrowhawk presence during its post-organochlorine increase and House Sparrow abundance from the Garden Bird Feeding Survey have been interpreted as evidence that increasing predation rates are depressing House Sparrow populations (Bell et al. 2010). However, more sophisticated analyses of large-scale and extensive national monitoring data provide no evidence that House Sparrow population declines were linked to increases in Sparrowhawks (Newson et al. 2010b).

Information about conservation actions

The main driver of the decline in farmland is believed to be a reduction of food resources on farms and in farmyards as a result of agricultural intensification, and therefore conservation actions and agri-environment policies which increase food availability are likely to benefit House Sparrows in rural areas. Research has confirmed positive effects from the provision of supplementary seed in winter (Siriwardena et al. 2007) and from the provision of areas of seed-rich habitat on farms (Colhoun et al. 2017).

The evidence relating to the declines in urban areas is less clear and further research is still needed. It may be that a number of different causes are affecting populations (see Causes of Change section, above). Therefore, a variety of different actions could be needed to reverse the declines, and actions that have increased numbers at some colonies will not necessarily be successful elsewhere. Providing supplementary food during winter may help declining populations (Hole et al. 2002) and actions to increase densities of invertebrates away from major roads may help improve breeding productivity (Peach et al. 2008). Actions to maintain and improve habitat in urban areas could also help, including the planting of native shrubs in gardens (Wilkinson 2010), retaining natural gardens and green spaces rather than paving, and ensuring suitable nesting locations are available (e.g. by providing nest boxes). Other possible causes which could have contributed to the decline include avian malaria (Dadam et al. 2009), for which encouraging improved garden feeder hygiene is important; and air pollution, which may be more difficult to resolve without wider scale policies.