Causes of change

The mechanisms underlying the decline in this species are largely unknown, although demographic trends suggest that factors operating during the breeding season are not the main driver.

Further information on causes of change

The number of fledglings per breeding attempt has improved substantially as population sizes have decreased (see above), suggesting that decreases in productivity were not responsible for the decline. This has been driven by declines in daily failure rate at both the nest and chick stages and increases in clutch and brood sizes. It is thus more likely that survival has been the critical demographic measure, although ring-recovery analyses have produced equivocal results, perhaps because of small sample sizes (Siriwardena et al. 1998, 2000b).

Components of agricultural intensification, such as reductions in winter stubble, have been implicated in the decline, although direct evidence supporting such ideas is largely incidental. Tree Sparrows aggregate in areas where seed food is available during the winter and they have declined at the same time as other farmland seed-eaters (Siriwardena et al. 1998), providing circumstantial evidence for shortage of food. In winter in Scotland (Hancock & Wilson 2003), the highest densities of Tree Sparrows were recorded in cereal stubble fields (undersown with grass) and weedy brassica fodder crops. These habitats remain relatively seed-rich but have declined in area in the UK (Fuller 2000, Hancock & Wilson 2003). Field & Anderson (2004) also state that anecdotal evidence suggests that many Tree Sparrow colonies are strongly associated with winter seed food sources, and provision of new seed sources is frequently associated with the establishment of new breeding colonies. Although Siriwardena et al. (2007) did not find a significant positive relationship between winter food supply and breeding population trajectory in areas provisioned by RSPB Bird Aid, this may be due to the fact that the BBS trends for this species are increasing; therefore winter food may not currently be limiting, as the remaining populations are in small remnants of suitable habitat and many are subject to active conservation action (e.g. provision of nest boxes). In Northern Ireland, Colhoun et al. (2017) observed an increase in abundance over five years on farms participating in agri-environment schemes, but found no direct positive association with the provision of seed rich habitat or any other specific management options.

During the breeding season, Field & Anderson (2004) found that wetland-edge habitats played a key role in providing invertebrate prey to allow successful chick rearing throughout the long breeding season and suggest that it is possible that large areas of UK farmland that were formerly occupied no longer provide these invertebrate resources, due to the effects of intensification in the late 20th century. In a study in Wiltshire, McHugh et al. (2016a) examined faecal sacs from nestlings and found a higher proportion of seed in their diet in areas with wild bird seed cover planted to provide seed resources in winter. They surmised that this indicated a shortage of insects, which are a more suitable nestling food. In this study, colony size increased but breeding success decreased in areas with wild bird seed cover (McHugh et al. 2017)

Information about conservation actions

The main drivers of change for the Tree Sparrow are uncertain, but it is believed most likely that declines have been caused by changes in survival. Therefore actions and agri-environment policies to increase seed food availability over winter could potentially benefit this species. However, Siriwadena et al. (2007) did not find a significant positive relationship between winter food supply and trends in a UK-wide study, possibly indicating that winter food may not currently be limiting as the remaining populations are in small remnants of suitable habitat and many are already subject to active conservation action (e.g. provision of nest boxes).

Providing overwinter food using wild bird seed cover may also have unintended negative impacts on breeding performance, with nestlings being fed on larger quantities of seed and experiencing reduced fledging success (McHugh et al. 2016a, 2017). This is likely to reflect a shortage of invertebrates and therefore actions to address this shortage may be at least as important in some areas as providing overwinter food. These actions could include using existing agri-environment options to provide habitats which support key invertebrate prey items used by Tree Sparrows during the breeding season (Field et al. 2010). Wetland-edges are a key habitat which help provide invertebrates and hence may help improve Tree Sparrow productivity (Field & Anderson 2004).

Further actions which encourage Tree Sparrows to return to sites from which they have been lost due to agricultural intensification are also likely to be required in order to enable a full recovery to take place. As well as providing nest boxes at sites close to existing populations (e.g. von Post et al. 2015), local conservation actions and agri-environment options will be needed to ensure that invertebrate prey are available at these sites during the breeding season and that seeds are available over winter.