A study was carried out in native pinewoods at Abernethy and Glen Tanar to investigate the effect of forest fences on capercaillie and black grouse. Collisions occurred at a rate of 0.25 and 0.03 per km of fence per month for capercaillie and black grouse respectively. Male capercaillie had a higher proportion of severe and fatal collisions than females. Capercaillie tended to collide with those sections of fence where there were moderately sized pines (8.5 m) close (within 4.4 m) to the fence. Collision sites close to trees were most strongly associated with the height of exposed trunk, suggesting that capercaillie fly under the canopy and fail to see the fence. The annual mortality rate of radio-tagged capercaillie caused by collisions was 32 per cent indicating that this was a major mortality factor. Collisions were most frequent between September and November when capercaillie were dispersing. It is recommended that fences are removed where and when they are unnecessary, or are made more visible, or are set back from old woodland.