The influence of a native tree species mix component on bird communities in non-native coniferous plantations in Ireland

Capsule: Norway Spruce plantations with Scots Pine as a secondary tree species had higher bird densities than pure Norway Spruce. Shrub cover was the most important structural variable, influencing bird density, species richness and Simpson's diversity. Aims: To investigate whether incorporating a native tree component into non-native coniferous plantations had any effect on bird communities or vegetation structure. Methods: Birds were surveyed in plantations of Norway Spruce mixed with Oak and Scots Pine, each paired with a plantation of pure Norway Spruce. DISTANCE was used to generate bird densities. Bird density, species richness and Simpson's diversity were compared between each mix type and pure Norway Spruce. GLM s were used to investigate relationships between structural components of plantations and bird data. Results: Bird communities of mixed plantations differed only slightly in their composition from pure Norway Spruce. Bird density was significantly higher in Scots Pine mixes than in Oak mixes or pure Norway Spruce. Neither species richness nor Simpson's diversity differed significantly between the plantation types. Some vegetation components differed between the plantations and shrub cover was positively associated with bird density, species richness and Simpson's diversity. The presence of rides also increased bird density. Conclusions: There is a positive effect on bird communities of including a native tree species in non- native coniferous plantations, but the magnitude of the effect is small. The influence of shrub cover on birds suggests that forest management may play an important role in determining the utility of plantations for birds. We recommend the establishment of mixed tree species plantations where possible, although, in the case of Oak mixes, the Norway Spruce appeared to suppress growth of the Oak and thus may be restricting its effect on birds. Changes in management, such as planting Oaks in clumps or heavier thinning of the coniferous component, could address this problem.