A Land Manager's Guide to Improving Habitat for Forest Thrushes

10
Forest Structure
Effects
When all the landscape level characteristics,
such as forest size, degree of isolation, and landscape
context are acceptable, forest structure and age become
the key characteristics that determine a habitat’s
suitability for a particular bird species. A healthy forest
aging under the natural forces of ecological succession
typically has the most diverse vegetation structure. An
important aspect of forest structure is the amount of
vertical layering. Early and mid-successional forests
often have two layers—an understory of shrubs and
young trees and an overstory (canopy) of interlocking
branches created by mature trees. Late-successional
(mature) forests usually provide the greatest amount
of vertical layering. It’s possible to have several layers,
including herbaceous ground cover, an understory of
shrubs and saplings, an intermediate layer of small
trees, a lower canopy of medium sized trees, and a
primary canopy of mature trees (Figure 8). The diverse
habitat associated with this vertical layering supports
a wide variety of birds. For example, you may have
Wood and Hermit thrushes in the understory, Red-eyed
Vireos and Rose-breasted Grosbeaks in the intermedi-
Figure 8. Late-successional forests often provide a high
degree of vertical layering, including a dense shrubby
understory important for thrushes and other birds.
Photo by Kenneth V. Rosenberg
ate layers, and Scarlet Tanagers and Black-throated
Green Warblers in the canopy. Many forest birds,
including the thrushes, depend on the cover, food, and
nest sites provided by a diverse and well-developed
understory.
Management Tips
• Creating a Broken Canopy: Permitting sunlight to
reach the forest floor by creating gaps in the forest
canopy will help to stimulate a poorly developed
understory in mid-successional and mature forests.
When understory vegetation is very deficient or
absent, planting appropriate native, shade-tolerant
shrubs, in addition to maintaining a broken canopy,
may be required to establish an understory layer.
• Employing Silviculture: Silvicultural practices that
create uneven-aged forests with well-developed, but
broken, canopies usually provide the best habitat for
forest birds. Avoid loss or change in tree-species diversity
and fitness by minimizing “high-grading” (the
removal of only the most valuable species and the
most structurally superior trees). Where possible, preserve
existing old-growth stands and provide for the
development of future old growth by leaving areas
undisturbed or unharvested for 150 years or more.
• Controlling Ungulates: Given the tremendous
increase in white-tailed deer densities during the
past three decades, it may be necessary to control
deer numbers or forest access to preserve a well-developed
understory (Figure 9). Where appropriate,
hunting can help to control deer numbers. When
hunting isn’t feasible, deer exclosures (fences) can
help to protect understory vegetation. In addition,
grazing by livestock should be monitored carefully
to detect signs of damage to the understory layer.
Figure 9. Heavy browsing by white-tailed deer sometimes
results in complete loss of understory shrubs and
saplings, reducing the suitability of forests to thrushes
and many other birds. Photo by Kenneth V. Rosenberg.