Effects of forest management and grazing on breeding bird communities in plantations of broadleaved and coniferous trees in western England

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Management options in commercial forestry include choice of conifers or broadleaves, rotation length, stand size and grazing regime. Each factor potentially affects the conservation value of woodland for birds. Relationships between these factors and the structure and composition of breeding bird communities were examined in 69 stands distributed across a range of plantations composed of predominantly native broadleaved and non-native coniferous trees in the Forest of Dean, western England, in 1992 and 1993. Each stand was classified as one of three high forest types: broadleaved, coniferous or mixed broadleaves and conifers. Stand size had no effect on bird communities. Species richness and overall bird abundance increased with forest age when all forest types were combined. Within stands of similar ages, there were no consistent differences in species richness or overall bird abundance between the three forest types or between grazed and ungrazed stands. However, bird communities in mixed stands were intermediate in their overall species composition to those in broadleaved and coniferous stands. Regression and gradient analyses (CCA and PCA) revealed that major gradients in the species composition of the bird communities were associated with stand age and with tree species composition. The proportion of individuals contributed by hole-nesting species was higher in broadleaved than coniferous stands and increased with stand age. The proportion of individuals contributed by migrants was higher in especially the early years, but also in the late years of the rotation. The proportion of migrants was higher in ungrazed than in grazed stands and increased with openness of the canopy and development of low vegetation. The diversity of stands in terms of tree sizes and tree species was positively correlated with both number of bird species and overall bird abundance. The relevance of these findings is discussed in relation to the integration of bird conservation into coniferous forestry, focusing particularly on the value of broadleaved stands and the effects on bird communities of stand structure and grazing pressure.