Abstract from BTO Research Report No. 459:

John Calladine, Liz Humphreys & Fiona McPhie (2007)

The Effects of Thinning in Commercial Conifer Plantations on Breeding Bird Abundance and Diversity in the North of Scotland
ISBN: 1-904870-95-3

Executive Summary

1. It is predicted that the thinning of dense woodland could influence breeding bird populations especially if that restructuring leads to changes ground vegetation cover and types. This study assesses the influence of thinning practices, which are currently typical within commercial conifer plantations in northern Scotland, on the species richness and abundance of breeding birds.
2. Breeding bird populations were assessed using timed point counts within ten pairs of study sites in the north of Scotland. One of each pair was a conifer plantation that had undergone thinning as part of its ongoing management and was paired by tree species, plantation age, geographical location, aspect and altitude with a reference plantation in which no thinning had been undertaken. Bird abundance, occurrence rates and species richness were compared to assess any effect of thinning.
3. The mean tree density within the thinned sites (11 stems within 5 m radius of survey points, or equivalent to 1408 trees per ha) was 31% less than in the reference sites (16 stems within 5 m radius of survey points, or equivalent to 2055 trees per ha). There were no significant differences in measures of ground cover vegetation or in the occurrence of shrubs within the plantations between treatments, however.
4. Forty-four bird species in total were recorded within study sites: 37 in the thinned plantations and 36 in the unthinned reference sites suggesting no difference in species richness. Initially, the only statistically significant difference in species abundance between treatments was for Bullfinches to be recorded 3.5 times more frequently in the unthinned reference sites. Pheasants were recorded at 3.8 times more count points in the thinned sites but there was no statistically significant difference in their abundance. Both these differences were only marginally significant (P = 0.05 & 0.04 respectively) and both became non-significant after standard Bonferroni correction. Although questionable, we suggest these initial apparent differences should not be totally disregarded.
5. For the more abundant species (Chaffinch, Coal Tit, Goldcrest, Robin, Siskin, Song Thrush and Wren), generalised linear modelling found no significant effect of tree density on their abundance within the range found within the 20 study sites. Tree species and study site both had a greater influence on the abundance of those species than thinning treatment.
6. We conclude that thinning, as currently practiced in commercial conifer plantations in the north of Scotland has minimal effect on breeding bird populations. However, greater degrees of plantation restructuring may benefit some of the species that were encountered rarely during our survey and potentially also others that were not recorded at all.

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