Clear-felling systems can be improved for forest birds by increasing tree
species diversity, and by leaving a proportion of live and dead trees unharvested.
These may be individual trees or groups of trees. However, suitable breeding sites
for some important early successional species, such as Woodlark, can only be cre-
ated by clear felling moderate sized areas (Bowden 1990). Species-richness of
birds in conifer plantations can be increased by incorporating a proportion of
broad-leaved trees. This can be done by planting, allowing natural regeneration,
or retaining existing broad-leaved trees in areas too wet, steep, rocky, and other-
wise inaccessible for harvesting conifers. There is evidence that species-richness
of birds on conifer plantations is greater if, for a fixed area of broad-leaved trees,
these are dispersed throughout the conifers, rather than concentrated in a few
large blocks (Bibby et al. 1989). Retention of trees during harvesting is common
practice in much of the world, but its long-term effects on birds are yet to be fully
evaluated. However, in the first few years of regrowth, areas of forest where patches
of trees have been retained support higher densities of breeding birds typical of
more mature forest, particularly ground and tree-nesting and forest canopy-gap
species, than areas of forest which have been clear-felled (e.g. Annand and
Thompson 1997; Merrill et al. 1998). Retained trees also provide hunting
perches for raptors, which might otherwise not be there.
14.7.5 Thinning and creating gaps
The avifauna of structurally simple forest can be diversified by increasing the
structural complexity of the forest. Structural complexity increases without
intervention, as trees out-compete one-another (self-thinning), create gaps and
allow patchy regeneration, but this natural process can be accelerated by felling
individual, or groups of, trees. Felling a proportion of trees throughout dense,
closed canopy forest (thinning) will encourage regeneration of the shrub layer,
allow trees to attain greater size and encourage suppressed broadleaved trees in
conifer plantations to mature. Thinning can be carried out selectively to modify
tree species composition, while the thinning of dense, multi-stemmed, aban-
doned coppice (Section 14.7.4) to produce single-stemmed trees (singling). This
is thought to increase shrub cover and overall densities of birds, particularly
warblers, and probably hole-nesting species as singled trees mature (Fuller
and Green 1998). Glades and gaps in the canopy which occur where trees are
felled should not always be restored with new trees, as they can be important,
albeit often short-lived, habitats for birds and other forest-associated flora
and fauna.
Thinning and group felling produce substantial benefits to the woodland
avifauna only if done on a large-scale. In practice, this is usually possible only
350 |Habitat management