1. Agri-environment schemes (AES) are the main European policy response to biodiversity loss caused by agricultural intensification. Maximizing their effectiveness is a key policy challenge. Monitoring is essential to inform adaptation and improvement of schemes over time, and to understand how measures may need to vary across a species’ range.
2. We measured changes in breeding abundance of a severely declining bird, the corn bunting Emberiza calandra, in response to AES in Scotland over 7 years and 71 farms. Two AES were monitored, one with general management for farmland birds, and one with targeted, adaptive management for corn buntings. We use these data to estimate the proportion of the population that AES must influence to halt the overall decline.
3. Corn buntings increased by 5·6% per annum on farms in the targeted AES, showed no significant change on farms in the general AES, and declined by 14·5% per annum on farms outside AES.
4. In arable-dominated areas, AES management that increased food availability reversed population declines. However, where a high proportion of corn buntings nested in grasslands, an additional AES option that delayed mowing was essential to achieving population increase.
5. Results suggest that approximately 72% of the corn bunting population in mainland Scotland must receive targeted AES management to halt the current decline. In 2009, only 24% was targeted in this way.
6. Synthesis and applications. AES measures are capable of reversing corn bunting declines in Scotland, and the same measures are likely to benefit a wide range of other taxa too, but require geographical targeting and flexibility to adapt and improve management options, backed by expert advice. Targeted AES provision to the required level for corn buntings will cost approximately £120 000 per annum, with 500–600 ha under appropriate management. This is 0·02% of annual subsidies paid to Scottish farmers, and 0·5% of land in the remaining mainland range of the corn bunting. These outcomes illustrate the value of AES monitoring studies to assess scheme effectiveness, identify improvements, and determine the scale of implementation required for reversing species declines.