A land manager's guide to conserving habitat for forest birds in ...

Elderberry — Photo: Robert McCaw
like the Wood Thrush will respond positively to the structural
changes associated with low intensity selection system harvesting
and have higher nesting success in these sites. However, this
benefit does not occur until at least five years after harvest, but
can persist for a number of years (5–10 years).
Predation and parasitism are not the only changes associated
with harvesting that can affect avian productivity. Reductions in
food availability can strongly influence a parent’s ability to feed
its nestlings. This can result in fewer young or less healthy young
produced by the pair of birds. Research shows both Northern
Cardinals and Rose-breasted Grosbeaks fledge fewer young per
nest in diameter-limit sites than uncut sites. This may be due to
lower food availability, increased parasitism (as cowbirds typically
remove a host egg), or both. Virtually all songbirds eat insects
and other invertebrates during the breeding season. When you
open up the canopy by harvesting, a forest becomes sunnier,
hotter, and drier. This can reduce the biomass of insects that are
vulnerable to desiccation. The changes in micro-climate can be
extreme as the intensity of harvest increases, and translate into
larger changes in forest structure and insect communities.
Reductions in the density of mature trees can result in less bark
and canopy dwelling insects available for canopy and bark
feeders. In selection system harvests where you retain most of the
mature canopy and the abiotic environment changes little, insect
communities will be more similar to uncut forests. Flying insects,
on the other hand, can respond positively to abiotic changes
created by canopy gaps, and thus group cuts can create ideal
foraging habitat for aerial insectivores, such as flycatchers. As
insects are not the only source of food, changes in forest structure
and composition may alter availability of fruits and seeds.
Growth and regeneration of mid-tolerant tree species like red
and white oak, bitternut hickory and black cherry through
shelterwood or group selection can increase the availability of
tree seeds (also known as mast). Similarly, gaps will often
stimulate shrub growth, creating new sources of food, like
raspberry or elderberry. Thus harvesting can have positive or
negative effects on food availability depending on species-specific
foraging preferences and the system employed.
Survival of juveniles after they leave or fledge from the nest is
another key component influencing avian productivity. During
the post-breeding period, adults and juveniles of mature forest
bird species start to make use of early successional habitats,
where an open canopy creates new growth in the form of a dense
shrub layer. These habitats offer greater protection from
predators, and for some species, high food sources. For species
like Hooded Warbler, heavy diameter-limit cut sites may actually
have higher juvenile survival rates than single tree selection or
unharvested sites. While for other species like Rose-breasted
Grosbeaks, harvest treatment has little impact on fledgling
survival parameters. Thus, early successional habitats can provide
valuable habitat to juvenile birds of some species but do very
little for another.
In summary, the intensity of the harvest strongly influences
the extent to which harvesting affects bird communities. Evenaged
management creates very different breeding conditions and
different bird communities, than uneven-aged management. In a
continuously forested landscape (like the boreal forest region),
even-aged management minimizes many of the dramatic changes
associated with harvesting, since the bird species are adapted to
infrequent large-scale disturbances. Loss of canopy cover and
Rose-breasted Grosbeak fledglings — Photo: Brad Woodworth
Harvesting Effects on Birds 55