In winter, free-living Capercaillies feed almost exclusively upon needles of various conifer species. An important part of the energy and protein requirements of captive Capercaillies, however, is supplied by food rich in starch and proteins. The fecal flora of captive Capercaillies is considerably different from the flora of free-living Capercaillies. Bacterial diseases are frequent in captivity. The aim of the present study is to determine whether the essential oils from conifer needles possess antibacterial properties against several bacteria, some of which are facultatively pathogenic for Capercaillies. Needles of Pinus sylvestris contain few tannins, which could be a reason for the Capercaillies' preference for these needles. The needles of Abies alba contain high amounts of tannins and essential oils, which could explain the refusal of Capercaillies to eat these needles. The antibacterial effect of commercially available essential oils on the bacteria is investigated with a filter paper disc method and in a liquid medium. The essential oils possess a bacteriostatic or a bactericidal effect against some of the tested bacteria. Feeding on natural forage plants may help prevent the colonization of the intestines by facultatively pathogenic bacteria. The authors discuss why Capercaillies, especially young birds that are to be released, should be fed with natural forage plants.