Neotropical Migrant Birds:
Threats to Populations Breeding
in the Midwest
The life cycles of migrant birds are very complex.
To understand their population dynamics, one must consider their requirements
and habits on their breeding grounds in North America, their wintering
areas in Latin America, and during migration to and from these points.
Global climate change, loss of stopover habitat,
loss of winter habitat, acid rain, disruption of food supplies due to environmental
contaminants, and the spread of another species of cowbird, the Shiny Cowbird,
are all threats to Neotropical migrant birds.
During a 1995 symposium on Neotropical migrant
birds held at the Midwest Fish and Wildlife Conference in Detroit, Scott
Robinson of the Illinois Natural History Survey summarized the threats
facing Neotropical migrant birds breeding in the Midwest. This is
an adaptation of his findings:
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Habitat Fragmentation -- Disruption in the
continuity of habitat
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Area sensitivity:Some
species require areas of forest to be a certain size, and will not be found
in woodlots below their required size even if suitable habitat is present.
-
Cowbird parasitism:Cowbirds
lay their eggs in the nests of other birds, leaving the hosts to raise
cowbirds, most often to the exclusion of their own young. Cowbirds feed
in pastures, row crops, lawns, etc. and will not venture into deep forests
to search for nests to parasitize. Fragmented forests provide more
habitat for cowbirds to penetrate to locate hosts.
-
Nest predation:
Frequent
nest predators
such as raccoons,
squirrels, crows, jays, grackles, cats, and snakes are much more common
in fragmented woodlots, particularly those in urban areas.
-
Edge effects: Nest
predation and cowbird parasitism are higher near edges, and reproductive
success of birds tends to be lower. Four small woodlots of 10 acres
each offers twice the edge perimeter than one forest of 40 acres.
Depending on the dynamic involved (i.e., how deeply into the forest one
must go before the edge effect in question disappears), entire small woodlots
may be considered edge habitat.
-
Habitat heterogeneity:There
is evidence that forest tracts managed for diversity may adversely impact
migratory birds that rely on large areas of homogenous habitat. Areas
managed for heterogeneity may support more habitat generalists, or promote
populations of game animals that may have detrimental effects on habitat
specialists.
-
Metapopulation dynamics:
Breeding areas can act as population "sources" (where reproductive success
creates a surplus of individuals that emigrate to other areas), or "sinks"
(where the rate of reproduction does even reach replacement level).
Fragmented areas often serve as population sinks.
-
Habitat Loss -- Many Midwestern habitat types
are threatened.
-
Floodplain forest:
There have been severe losses of this type of habitat in the Midwest. In
addition to species of birds such as Swainson's Warbler which rely on floodplain
to nest in, floodplain and riparian forests are thought to be important
migratory corridors.
-
Central hardwoods:
Threatened
by agricultural development and logging practices.
-
Savanna/burned forests
and barrens: Reduced by fire control measures.
-
Grasslands:
Of all Midwestern habitat types, grasslands have undergone the severest
reduction.
In addition, "substitute" grasslands such as hayfields are now managed
so that successful nesting is nearly impossible.
-
Northern hardwood/conifer
forests: Primary threat is from logging practices.
-
Agricultural landscapes:
Now managed so they are of little value to birds, and pose threats via
pesticides and other contaminants.
-
Changes in habitat structure and composition
-
Loss of oaks:
Loss of oaks (due to fire control) may have a negative impact on certain
species of birds such as Cerulean Warblers. Also, oaks are superior
providers of insect resources crucial to spring migrants.
-
Loss of other tree
species
-
Tree monocultures:
Have unpredictable effects on breeding bird communities. One concern
is that some monocultures, especially non-native communities, may act as
ecological "traps," for nesting birds that would normally breed further
north or south.
-
Loss of understory:
Deer browse can substantially reduce understory growth, which is necessary
for species that nest in the dense shrub layer.
-
Floodplain disturbance:
Natural
meandering rivers are being replaced by restrained systems, eliminating
natural floods that create important habitat.
-
Shrubland/edge species:
Some species rely on early successional habitats, which are disappearing
as farmland reverts to forest.
-
Exotic plant invasion:
Effects of replacement of native ecosystems with exotic species (often
homogenous) is unknown.
-
Snag loss:
Snags are important nesting sites for birds, although few Neotropical bird
species
use them. Management techniques must also consider that cowbirds
often use snags are perches to locate host nests.
-
Mid-successional
dynamics: The effects of how bird communities change through
mid-succession stages is unclear.
Rouge River Bird Observatory,
Environmental Interpretive Center., University
of Michigan-Dearborn,
Dearborn, MI 48128
Created 25 July
97
