Causes of change

Declines in annual survival have been proposed as the demographic mechanism for decline, due to winter resource limitation, although ring-recovery data are sparse and so most evidence for this is indirect.

Further information on causes of change

Yellowhammer is unique among farmland birds in that its population was stable until the mid 1980s, followed by a decline, suggesting that it alone was affected by some change that occurred in the 1980s (Siriwardena et al. 1998a). There is some evidence that survival rates decreased during the initial period of decline (Siriwardena et al. 1998b, 2000a, Kyrkos 1997), and that breeding performance tended to improve (Siriwardena et al. 2000b). Long-term demographic trends presented here (see above) show that nest failure rate at the egg stage decreased during the decline and the breeding improvement consequently improved. Cornulier et al. (2009) confirmed that change in breeding frequency was not a cause of decline, as the number of breeding attempts was increasing. As mean laying dates are calculated across all broods, the increase in the number of breeding attempts is also likely to explain why, unlike most other species, the mean date is now later than it was in the 1960s.

Best estimates of the variation in adult and first-year Yellowhammer survival (from ring recoveries) suggest that it has been sufficient to explain the species' decline (Kyrkos 1997). Reductions in winter seed availability as a result of agricultural intensification (for example, the loss of winter stubbles and a reduction in weed densities) are widely believed to have contributed to the population decline, presumably through impacts on survival rates. Siriwardena et al. (2007), found that Yellowhammer declines were less steep in areas where the species received more overwinter provisioning, providing experimental evidence for winter resource limitation. Food availability (and therefore, as a conservation measure, supplementary feeding) in late winter appears to be particularly important because demand for seed food is greatest at this time and this is also when the food supply resulting from agri-environment conservation measures is at its lowest (Siriwardena et al. 2007). Further evidence comes from Gillings et al. (2005), who used two complementary extensive bird surveys undertaken at the same localities in summer and winter to show that the areas of extensive stubble in winter were correlated with better population performance, presumably because overwinter survival is relatively high. This is supported by another study, in Oxfordshire (Wilson et al. 1996), which found that the only habitat type for which a clear preference was displayed in winter was stubble. In Northern Ireland, Colhoun et al. (2017) observed an increase in abundance over five years on farms participating in established agri-environment schemes, but found no direct positive association with the provision of seed rich habitat or any other specific management options. In Wales, experimental provision of ryegrass plots appeared to fill the late winter 'hunger gap' and Yellowhammer body condition was positively related to the amount of ryegrass eaten; however breeding numbers did not change (Johnstone et al. 2019).

In terms of changes to habitat, Kyrkos et al. (1998) found that Yellowhammer breeding density decreased with increasing proportion of farmland under grassland. It may be that modern improved grassland has neither the weed density required by adult Yellowhammers nor sufficient invertebrate prey for birds feeding nestlings. The dense sward structure of highly fertilised leys may also reduce access to invertebrate prey (Perkins et al. 2000). This is supported by the results of Douglas et al. (2010a) who found that foraging in grass margins was increased by experimental mowing, showing that access to prey in dense vegetation limits feeding activity. Siriwardena et al. (2000b, 2000c) provide further evidence that grazing supported the lowest breeding performance, although the best breeding performance was associated with mixed farmland, suggesting that loss of heterogeneity in the landscape may be a factor in the decline, although they state that this is unlikely to be the main mechanism behind the declines. Bradbury & Stoate (2000) further suggest that loss or degradation of hedges and field margins, loss of stubbles and intensification of grassland management may have reduced nest-site and food availability for farmland Yellowhammers.

Increased use of pesticides may have also played a role in decreasing breeding success. Boatman et al. (2004) used an experimental set-up to look at the effect of pesticides on breeding performance, and further evidence was provided by Morris et al. (2005), who showed that increased use of pesticides results in reduced invertebrate abundance, lower brood production and fewer chicks fledging. Hart et al. (2006) also demonstrated how insecticide applications can depress Yellowhammer breeding productivity. Whittingham et al. (2005) found that the local availability of rotational set-aside was a good predictor of sites chosen for breeding territories, which could reflect the benefits of both sparse vegetation (access to bare ground for foraging) and lack of pesticide use. Similarly, McHugh et al. (2016b) found that territories were preferentially located close to enhanced field margins, and suggested that the more open sward structure in these margins may increase prey availability.

Information about conservation actions

The decline from the 1980s is believed to be linked to changes to farming practices resulting from agricultural intensification, with decreased survival believed to be the main cause. Therefore, conservation actions which help improve survival by ensuring additional food is available during winter are most likely to be able to reverse the decline. Food availability in late winter (the 'hunger gap') appears to be a problem for Yellowhammer (Siriwardena et al. 2007) and winter stubble (Gillings et al. 2005; Anderson 2014) and set-aside (Whittingham et al. 2005) are important. As well as the retention of stubble and the provision of set-aside areas, other options which could increase the amount of seed available over winter include the direct provision of supplementary food, the planting of wild bird seed or game cover, and the provision of semi-natural habitats, e.g. through the provision of buffer strips and conservation headlands, and through less intensive farmland management practices. Provision of ryegrass plots helped to fill the 'hunger gap' in Wales (Johnstone et al. 2019).

Although breeding productivity is not believed to be the main driver of decline, some potential issues during the breeding season have been noted and conservation actions which improve breeding performance may also help the recovery. These could include: reducing the usage of pesticides (Boatman et al. 2004; Morris et al. 2005; Hart et al. 2006); and the provision of 'enhanced margins' which provide invertebrate rich herbaceous vegetation adjacent to hedges, such as wild flower mixes, agricultural legumes or nectar plots (Stoate & Szczur 2010; Burgess et al. 2015; McHugh et al. 2016b). Where enhanced margins have not been provided and grass margins are heavily fertilised, mowing of margins may help increase accessibility to invertebrates in dense swards (Perkins et al. 2000; Douglas et al. 2010a).

Note, however, that Dunn et al. (2016) warn that, while land management can promote high densities, breeding success can be reduced by density-dependent effects on provisioning rates, thereby creating an ecological trap. Hence, agri-environment policies should try to ensure that breeding conservation actions are widely applied in order to make them effective.