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

Wood Thrush nestlings — Photo: OMNR
they have a full clutch (three to five eggs for most species, but up
to eight eggs for some cavity nesters). For many, the eggs will
hatch in approximately 12–14 days, and the young will stay in
the nest 10–14 days before they are big and strong enough to
leave (fledge). Eggs and young birds (nestlings) are popular food
for many nest predators. On average, a cup nest has less than a
50 percent chance of seeing any young leave the nest, but
primary cavity nesters, like woodpeckers, tend to be more
successful. Nest predation accounts for most nest failures
(80 percent), though some nests may fail because of bad weather,
or young may die of starvation. The suite of nest predators
includes: small mammals (mice, shrews, and chipmunks), larger
mammals, (squirrels, raccoons, cats, opossums, and weasels),
avian predators (American Crow or Blue Jay), and snakes. If a
nest does get destroyed, adults will attempt to re-nest until they
are successful or they run out of time and need to migrate south.
Although nest predation is a natural part of evolutionary history,
predation levels can be greatly increased by human activities such
that these bird populations are no longer sustainable.
In addition to the direct loss of eggs and nestlings to
predation, many unsuspecting birds lose out without even
knowing it — because of cowbirds. The Brown-headed Cowbird
is a brood parasite, common to southern Ontario’s highly
fragmented landscape. Brood parasites never build a nest or raise
their own young. Instead, they lay their eggs in other birds’ nests,
often removing one or two of the host eggs in the process. Female
cowbirds are prolific breeders and can lay up to 40 eggs each
breeding season. Host parents raise the cowbird young alongside
their own; often at the expense of the host’s young (see photo
below). Cowbird nestlings usually hatch earlier, are faster
growing, and more aggressive than host young, outcompeting
them for food, resources, and space. As a result, fewer, often less
healthy host young, or sometimes no host young fledge from a
parasitized nest. This can substantially lower the number of
young produced by each pair, especially when some nests may
contain two or three cowbird eggs.
Cowbirds are dependent on open fields, crops, and pasture
for foraging on seeds and insects. Prior to European settlement,
cowbirds were restricted to the buffalo-grazed and firemaintained
grasslands of the Great Plains of central North
America. However, with the clearing of eastern forests for
agriculture and settlement in the 1800s, cowbirds were able to
expand their range eastward and in the process exposed many
new bird species to nest parasitism. Unlike species that
developed with cowbirds and learned to reduce their impact,
(like removing cowbird eggs from the nest or abandoning
parasitized nests), species with no long term exposure to
cowbirds have no defence and can be particularly vulnerable. For
example, nearly half of all Wood Thrush nests are parasitized in
some parts of southern Ontario. This can significantly reduce
Wood Thrush productivity to half that of unparasitized nests.
Together nest predation and nest parasitism have the potential
to reduce the nesting success for many forest songbirds. This can
be especially true in highly fragmented landscapes, where
additional food in the form of crops elevates populations of some
nest predators (like racoons) and where grass exists for cowbirds
Blue-gray Gnatcatcher host parent feeding Brown-headed Cowbird
nestling — Photo: G.K. Peck
Squirrel eating nestling bird —
Photo: Bob McBroom
16
Forest Birds in Ontario