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

Male American Redstart —
Photo: Garth McElroy
Forest Food Resources
The food available to breeding birds is primarily determined by the
composition and structure of the vegetation present. Availability of
seeds, fruits and flowers, a direct source of food for many species,
is intimately tied to the habitat. Abundance of insects, which serve
as a critical food source during the nestling stage, is also
determined by vegetation structure, and indirectly links birds with
vegetation. Insects provide protein necessary for growth and
development of nestlings and fledglings. Caterpillars, in particular,
are a major component of the diet of many breeding birds, as they
are relatively large and easily captured. Each bird species has a
specific foraging strategy to locate and capture insect prey. Foliage
gleaners pick caterpillars and aphids off the surface of leaves,
whereas bark gleaners pick grubs, spiders, and other insects from
the surface and crevices of bark, twigs, and wood. Aerial foragers
capture moths, beetles, flies, and ballooning spiders and
caterpillars (i.e., insects dangling in the air from strands of silk
attached to trees) from the air, and ground feeders primarily search
for beetles and ants in the leaf litter on the forest floor.
Vegetation structure is very important to foraging birds and can
influence the number of species of birds a habitat supports. The
canopy, mid-canopy, shrub, and understorey layers of a forest
create different foraging microhabitats (see Habitat preferences of
common forest birds pages 22–23). Birds like Cerulean Warblers
and Scarlet Tanagers forage high in the canopy, whereas other
foliage gleaners, like Hooded Warblers and American Redstarts,
will forage low in the shrub layer. Native plant diversity further
influences available food resources. Each plant species supports a
characteristic composition and abundance of insects. Certain bird
species may prefer certain tree species over others because prey is
more abundant or more easily obtained due to their structure. For
instance, foliage gleaners may prefer hardwoods like yellow birch
and oak over sugar maple, because sugar maple has lower
densities of caterpillars and longer leaf stems that make caterpillars
on leaves harder to reach by birds hopping along on branches. The
depth of bark crevices also influences prey abundance, and older
trees with deeply fissured bark are preferred foraging habitat for
species such as the Brown Creeper, which probes the crevices
searching for insect prey. Less common species like the Cerulean
Warbler tend to be highly selective in habitats used for foraging,
while more common species, such as the Red-eyed Vireo, are not
as picky about where they forage.
Ultimately, the diversity of microhabitats within a forest will
strongly influence the bird species that it supports. Maximizing
structural diversity and plant diversity, and retaining specialized
habitat features like fallen dead wood, cavity trees, snags, and
large trees will cascade up the food chain to increase bird diversity.
Male Cerulean Warbler
with caterpillar — Photo: Greg Lavaty
Forest Birds in Ontario 21