With 38 species being part of the assessment, the common bird index for Bulgaria shows a decline in bird populations by 21 per cent between 2005 and 2013. Yet the highest proportion of species show uncertain trends, and the second highest proportion show a moderate decline, with farmland birds being a large group in these categories. The reasons for this are still unknown and require targeted research. However, the designation of Natura 2000 sites resulted in an intensive mapping programme between 2006 and 2013.