Pine marten Martes martes and red fox Vulpes vulpes sign indices in Scottish forests: population change and reliability of field identification of scats

The range and abundance of the pine marten Martes martes and red fox Vulpes vulpes are considered to have increased in the Scottish Highlands in recent decades. In 2009, we resurveyed 11 forests surveyed for predators in 1995, together with an additional five forests. Our survey techniques followed those used in 1995. In all, we used DNA analysis to identify 414 mammal scats, representing 30% of the scats collected and identified in the field as being from either fox or marten. Of these, 77% of scats had been identified correctly by two field observers, with no difference between the individual observers. However, of the scats incorrectly identified, there was a bias towards identifying marten scats as fox scats. We used DNA confirmation to obtain correction formulae for scats identified in the field as fox or martens. The corrected marten sign index had increased 3.9-fold since 1995 and the fox sign index had increased 2.2-fold in the 11 forests surveyed in both years.