Causes of change

There is convincing evidence that nest failure rates rose during the principal period of population decline and this represents the most likely demographic mechanism driving the observed decreases in abundance. The most likely ecological driver of this pattern is habitat impoverishment due to agricultural intensification.

Further information on causes of change

Siriwardena et al. (1999, 2000b) provide convincing evidence that nest failure rates at the egg stage rose during the principal period of population decline and this represents the most likely demographic mechanism driving the observed decrease in abundance. They found an obvious change in the egg-stage failure rate of Linnet nests after 1975 and this was detectable in the total fledglings produced, suggesting that the deterioration in breeding performance had an important role in driving the species' concurrent decline in abundance (Siriwardena et al. 2000b). Moorcroft & Wilson (2000) concur that the severe decline during the 1970s and 1980s occurred via a reduction in breeding success, attributing this to a reduction in the availability of breeding-season food supplies on arable farmland caused by agricultural intensification. However, they state that the precise demographic mechanism involved is unclear: instead of breeding performance per attempt, they suggest reductions in the number of nesting attempts being made by individual females or a reduction in immediate post-fledging survival due to resource limitations as more likely, although these hypotheses were not tested. BTO monitoring data do not permit analysis of these parameters but it is plausible that such effects occurred in parallel with the breeding success effects indicated by NRS results. Nevertheless, all these patterns are consistent with the results of Siriwardena et al. (1999), who reported that index change was not significantly correlated with adult and first-year survival. They found no significant trend-specific difference in survival, and survival rates in periods of decline were higher than those in periods of increase.

After 1986, egg-stage nest survival increased and this led to a slight increase in breeding performance, although, as with the earlier decline, greater numbers of breeding attempts or increased post-fledging survival may also have contributed to the ending of population decline (Siriwardena et al. 2000b, Wilson et al. 1996, Moorcroft et al. 1997). Increases in the crop area of oilseed rape are thought to have improved Linnet breeding success by compensating for the herbicide-mediated decline in many farmland weeds that were traditionally important in this species' summer diet (Moorcroft et al. 1997). Both the number of breeding attempts possible in a season and post-fledging survival could have increased in response to this improvement in food supplies, as could chick survival. Oddly, Siriwardena et al. (2001b) identified a significant negative effect of rape on breeding performance through the egg-stage daily nest failure rate and no positive effect on success through the nestling stage in a further analysis of nest record data. This is clearly inconsistent with the results of intensive work on Linnets (Wilson et al. 1996, Moorcroft et al. 1997), perhaps reflecting the different geographical biases affecting nest records and this particular intensive study. Nevertheless, it suggests that environmental effects on Linnet breeding success show complex spatial variation and that the knock-on effects on trends in abundance could also be difficult to characterise. 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).

The current long-term pattern, spanning the Linnet's periods of decrease and relative stability, is of linear increase in nest failure rates and linear decline in the number of fledglings per breeding attempt.

Information about conservation actions

The decline of this species has been linked to decreased breeding success as a result of agricultural intensification. However, the exact mechanism behind the decline remains unclear and hence no specific conservation actions have been recommended relating to this species.

However, actions and policies aimed at helping other farmland species are also likely to benefit Linnets, in particular those aimed at improving breeding habitat and food availability during the breeding season (e.g. management of hedgerows, reducing pesticide and herbicide use and set-aside). One study suggested that increases in the crop area of oilseed rape may have improved Linnet breeding success (Moorcroft et al. 1997) but this was not supported by another study (Siriwrdena et al. 2001b, see Causes of Change section).

Although the decline in the 1970s and 1980s is believed to have been caused by problems during the breeding season, actions aimed at increasing food availability over winter could also benefit Linnet if they increase overwinter survival (e.g. overwinter stubbles, wild bird seed or cover mixtures, set-aside, buffer strips or uncultivated margins). One study in central England found that Linnets were found more frequently on barley stubbles than on wheat stubbles (Moorcroft et al. 2002), which is likely to be due to greater spillage losses of grain during harvesting of barley crops (e.g. Stacey et al. 2006).