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

Male Northern Cardinal feeding recently
fledged young — Photo: Marie Read
Ovenbird nest and
surrounding habitat
— Photo: Ken Elliott
Close-up of
Ovenbird nest —
Photo: Ken Elliott
This in turn governs the range of habitats available to different
organisms. Generally, forests that have many layers from the
ground to the canopy will support a greater array of plant and
animal species compared to forests in which the vegetation is
concentrated in a single layer. Lack of vegetation structure will
have negative effects on species that rely on specific layers of
vegetation for food or cover. Territories of different species can
overlap horizontally and vertically within the forest.
Suites of species that have similar habitat requirements are
often grouped according to a nesting or foraging guild. Some birds
prefer open, shrubby habitat found in young forests (i.e., early
successional guild), whereas others prefer closed canopy forests
with large trees and belong to the mature forest. For an individual
bird, selecting suitable habitat can have the greatest influence on
its ability to survive and reproduce. Faced with a multitude of
possible choices in the landscape, how do birds choose the most
appropriate habitat? They do this by making a series of choices,
first at very large or coarse scales (e.g., all of southern Ontario),
then at increasingly finer, smaller scales. For example, an
Ovenbird (a small ground nesting bird with bold breast spots,
that returns to breed in the woodlots of southern Ontario in the
spring), first chooses a forest patch. This may be based on the
size of the patch, the surrounding landscape features, and
adjacent land uses. Next it will have to choose a territory within
that patch. This will be based on the structure of the forest and
the availability of food within it. Finally, the Ovenbird will choose
a nest site. Ovenbirds nest on the ground and will look for
particular vegetation features and structures that may hide the
nest from predators and cowbirds.
The Reproductive Challenge
Although each bird attempts to raise as many young as possible
over its lifetime, to maintain a population, births must equal deaths.
Each adult, within its lifespan, must replace itself with an offspring
of reproductive age. For most songbirds this means a pair must
produce one young that survives to the next breeding season. This
may not seem difficult over an average life span of two to three
years, given that most birds lay three to four eggs per nest, and
many can produce more than one successful nest per year (double
brooding). However, once you consider rates of nest failure (50–80
percent) and losses due to parasitism (15–50 percent of nests are
parasitized and fledge half as many young), the probability of
those eggs successfully hatching and the young surviving the
winter to return and breed is greatly reduced. As well, once a
nestling has left the nest, its survival is often very poor within the
first week of life, never mind the additional challenges of
migration and over-winter survival!
20
Forest Birds in Ontario