Causes of change

Agricultural intensification is the ultimate cause of population declines. However, the mechanisms underlying the decline remain unclear.

Further information on causes of change

Changes in agricultural practices have been proposed as the main reason for declines via their impact on the quality of foraging and breeding habitats. The magnitude of Yellow Wagtail decline appears to vary between habitats, being strongest in wet grassland and marginal upland areas (Henderson et al. 2004, Wilson & Vickery 2005). Chamberlain & Fuller (2000, 2001) found that there were greater range contractions in regions dominated by pastoral agriculture. The decline in pastoral habitats has been proposed to be due to agricultural intensification, specifically farmland drainage, the conversion of pasture to arable land, changes in grazing and cutting regimes, the loss of insects associated with cattle and changes to grassland ecosystems in marginal upland areas (Gibbons et al. 1993, Chamberlain & Fuller 2000, 2001, Flyckt 1999, Vickery et al. 2001, Nelson et al. 2003, Bradbury & Bradter 2004, Henderson et al. 2004). Such changes are likely to have reduced the quality of grasslands as a nesting and foraging habitat.

Data from eastern England suggest a strong avoidance of grassland and preference for spring-sown crops (Mason & Macdonald 2000), though breeding can also be successful in landscapes dominated by winter cereals (Kirby et al. 2012). A detailed autecological study by Gilroy et al. (2008) provides good evidence that, on arable land, soil penetrability had a significant influence on the abundance of Yellow Wagtails, together with crop type and soil type, as these influenced invertebrate capture rates. There was a strong relationship between Yellow Wagtails and soil penetrability, suggesting a potential causative link between soil degradation and population decline (Gilroy et al. 2008). Breeding-season length may also be limited in cereal-dominated areas, as Yellow Wagtails avoid autumn-sown cereals late in the season (Gilroy et al. 2009, 2010). Predation was also considered and it was found that predation rate was closer nearer to tramlines and field-edges (Morris & Gilroy 2008). It is uncertain how important nest predation in tramlines is as a limiting factor for Yellow Wagtail populations but no studies have reported predation as a major driver of population decline for this species. Work carried out by Benton et al. (2002) showed that, in Scotland, arthropod abundance was significantly related to agricultural change and that this was also linked to measures of farmland bird density. Although Yellow Wagtail does not breed on Scottish farmland, it is an obligate insectivore, so this evidence adds support to the hypothesis that reduced food availability due to agricultural change may have contributed to the declines in this species.

Yellow Wagtails are long-distance migrants, moving to wintering grounds in western Africa south of the Sahara. Factors relating to conditions on the wintering grounds may also play a role (Bradbury & Bradter 2004, Heldbjerg & Fox 2008, Stevens et al. 2010) but evidence for this is lacking.

Information about conservation actions

The decline of the Yellow Wagtail since the 1980s is believed to be driven by agricultural intensification and resultant habitat changes, although the exact mechanism behind the decline is unclear and therefore specific evidence-based conservation actions to reverse the decline are limited. However, there is good knowledge about breeding habitat preferences.

The research suggests that changes to cattle farming and associated grassland may have reduced quality and food availability; hence actions which enable low intensity pastoral farming may benefit Yellow Wagtails, including a reduction in the use of fertilisers, herbicides and pesticides to provide more diverse semi-natural grasslands. A detailed study of Yellow Wagtail breeding ecology by Bradbury and Bradter (2004) provided good evidence of the species' breeding requirements on grassland. Territories were associated with a greater proportion of bare earth in the sward, the presence of shallow-edged ponds or wet ditches in the field, and a greater probability of a prolonged winter/spring flood, although the relative importance of these and how they impact on demographic processes was indecipherable.

Where Yellow Wagtails nest in arable fields, providing spring sown crops may help improve breeding productivity by extending the breeding season (Mason & McDonald 2000). Alternatively, providing a mosaic of crops may enable Yellow Wagtails to raise early broods in autumn sown cereal fields but switch to other crops (e.g. potatoes) for later broods (Gilroy et al. 2010).