The Ecological Basis for the Conservation of
Migratory Birds in the Mississippi Alluvial
Valley
Cynthia R. Brown
Charles Baxter
David N. Pashley
Abstract—In 1994, leaders in bird conservation in the Mississippi
Alluvial Valley (MAV) formed the MAV Migratory Bird Planning
Group to devise a conservation plan that integrates habitat objectives for three bird species groups: Wintering waterfowl, migratory
shorebirds, and breeding and wintering migratory forest birds. The
intent was to better coordinate the activities of three separate
conservation efforts—the North American Waterfowl Management
Plan, Partners in Flight, and the Western Hemisphere Shorebird
Reserve Network—in a geographic area of great importance to
birds. The goal of the Planning Group was to develop habitat
objectives, based on population goals for each species group, which
can be met simultaneously through integrated and complementary
landscape management activities.
The MAV once was defined and driven by hydrofluvial processes.
At present, land clearing and hydrologic manipulations have dramatically altered the natural structure and processes of this vast
wetland ecosystem. Setting management objectives at the sitespecific level of wildlife management area, national refuge, or
privately owned land, has therefore necessitated an analysis of how
regional changes in ecosystem structure and processes have affected the ability of the MAV to function as wintering habitat for
mid-continent waterfowl, migratory habitat for shorebirds, and
breeding and wintering habitat for forest birds.
changes in ecosystem structure and process have affected
the ability of the MAV to function as bird habitat.
This planning process is an explicit effort to better coordinate the activities of the North American Waterfowl Management Plan (NAWMP), Partners in Flight (PIF), and the
Western Hemisphere Shorebird Reserve Network. The Lower
Mississippi Valley Joint Venture staff in Vicksburg spearheaded this effort with strong support from elsewhere in the
U. S. Fish and Wildlife Service, and from wildlife agencies
from the seven states of the MAV (fig. 1), academia, private
landowners, and nongovernment organizations such as The
Nature Conservancy, the American Bird Conservancy, and
the Tennessee Conservation League.
In 1994, the Mississippi Alluvial Valley (MAV) Migratory
Bird Planning Group began to devise a conservation plan
that integrates habitat objectives for three groups of bird
species—wintering waterfowl, migratory shorebirds, and
breeding and wintering forest birds. The goal of the plan was
to develop habitat conservation objectives based on population goals for each species group—objectives that could be
met simultaneously through integrated and complementary
management activities. The objectives were to be set within
a large ecosystem that originally was heavily forested and
defined and driven by hydrofluvial processes, but which now
is highly altered and functionally impaired. The planning
process that resulted required a regional analysis of how
In: Bonney, Rick; Pashley, David N.; Cooper, Robert J.; Niles, Larry,
eds. 2000. Strategies for bird conservation: The Partners in Flight planning process; Proceedings of the 3rd Partners in Flight Workshop; 1995
October 1-5; Cape May, NJ. Proceedings RMRS-P-16. Ogden, UT: U.S.
Department of Agriculture, Forest Service, Rocky Mountain Research
Station.
Cynthia R. Brown, The Nature Conservancy, P.O. Box 4125, Baton Rouge,
LA 70821. Charles Baxter, U. S. Fish and Wildlife Service, 2524 South
Frontage Road, Suite B, Vicksburg, MS 39180. David N. Pashley, American
Bird Conservancy, P.O. Box 249, The Plains, VA 20198.
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Figure 1—Lower Mississippi River Basin.
USDA Forest Service Proceedings RMRS-P-16. 2000
Ecosystem Description __________
Historic Structure and Processes of the
Mississippi Alluvial Valley Landscape
The Mississippi Alluvial Valley (MAV) once was a 10
million ha forested wetland system—the largest in North
America—created and maintained by regular backwater
and headwater flooding, and by localized ponding on poorly
drained soils. The headwater or mainstem flooding resulted
from rainstorms and snow melt in the watersheds of Mississippi River tributaries and produced great spring floods that
characterize the MAV. Backwater flooding is a phenomenon
in which high water stages on the Mississippi create a
damming effect, preventing tributary drainage into the
mainstem and at times reversing tributary flow upstream.
This situation results in long-duration flooding, accompanied by sediment and nutrient deposition, throughout tributary watersheds.
Concomitant with these flooding mechanisms were the
hydrofluvial processes associated with meandering river
systems. The high energy inherent in the Mississippi River
and its tributaries originally sculpted the MAV landscape,
producing a surface geomorphology consisting of natural
levees, meander scar lakes, point bars, and ridges and swales.
These hydrofluvial processes created primary successional
substrate for the establishment of pioneer communities that
eventually were replaced by later seral communities distributed according to varying flood regimes. Site conditions
within the MAV ranged from permanently flooded areas
supporting only emergent or floating aquatic vegetation to
high elevation sites supporting complex hardwood forests.
The historic dominant natural communities of the MAV
were various types of bottomland hardwood forests. The
distribution of types and stages of bottomland hardwood
communities is in part determined by the timing, frequency,
and duration of flooding. Elevational differences of only a
few inches result in great differences in soil saturation
characteristics and thus plant distribution. As a result,
components of a bottomland hardwood ecosystem range
from bald cypress/tupelo swamp communities in saturated
or inundated situations, to a cherrybark oak/pecan community where inundation is infrequent and temporary. Between these distinct types are transitional overcup oak/
water hickory, elm/ash/hackberry, and sweetgum/red oak
communities.
With time and sediment deposition, succession in the
MAV proceeds from pioneer communities dominated by
black willow or cottonwood (depending on soil drainage
characteristics), to a red oak- and finally white oak-dominated climax community (Hodges 1994). Disturbances, both
nature- and human-induced—including windfall, beaver
activity, hydrologic alteration, and silvicultural practices—
greatly influence the rate and direction of succession. There
is emerging thought that the dynamic nature of this waterand sediment-driven system, coupled with frequent disturbance, generally precludes the development or long-term
viability of a closed canopy of senescent trees commonly
thought of as old-growth (Meadows 1994). The 24 million
acres of presettlement forests in the MAV likely consisted of
a shifting mosaic of small, even-aged patches, themselves of
USDA Forest Service Proceedings RMRS-P-16. 2000
differing ages, further defined by minute differences in
elevation.
Historic Habitat Function of the
Mississippi Alluvial Valley
The diversity of landforms and community types on the
historical landscape provided extraordinary habitat for a
range of species, including more than 200 species of fish, 50
species of mammals, 45 species of reptiles and amphibians,
37 species of mussels, and approximately 60 percent of all
bird species in the conterminous United States (Fremling
and others 1989; Sparks 1992; USACE 1988).
Migratory dabbling ducks historically have made significant use of the natural wooded wetland/slough complex in
the winter, feeding primarily on acorns, seeds, and invertebrates. The bottomland hardwoods of the MAV provided the
primary overwintering habitat for Mallards (Anas
platyrhynchos) in North America. Diving ducks benefited
from food sources characteristic of permanently flooded
wetlands. These wintering habitats were critical not only to
sustain ducks through the winter, but also to provide for
nutritional demands that influence breeding success in the
following spring (Yaich 1990).
The bottomland hardwood forests of the MAV also provided diverse habitat for a variety of forest birds. The
continuous nature of the historical forest, along with its
complex vertical structure and multiple seral stages, provided a haven for forest-interior species and niche specialists. Migratory forest birds used the bottomland hardwood
forest in various ways, with many showing habitat preferences based on vertical structure, flooding regimes, and
microhabitat features such as vine tangles, canebrakes,
Spanish moss, and scour channels.
Use by migratory shorebirds was relatively low in the
predominantly forested and shrub-scrub wetland types of
the historical MAV. However, late summer evaporation
from sloughs and backwater areas created some habitat.
Also, sandbars and mudflats of the major river systems—
including the Mississippi and Arkansas rivers—probably
served an important function for many species of in-transit
shorebirds, as have localized prairie habitats of eastern
Arkansas and north central Louisiana.
Ecosystem Alteration ____________
The last two centuries have witnessed dramatic changes
in the structure and processes of the bottomland hardwood
system of the MAV, which have affected its ability to function as habitat for waterfowl, shorebirds, and migratory
forest birds. A concerted flood control effort began in 1879
with the establishment of the Mississippi River Commission. Its flood control functions were assumed by the U. S.
Army Corps of Engineers after the great flood of 1927 and
the passage of the 1928 Flood Control Act (MacDonald and
others 1979). Since that time, one of the world’s most
comprehensive flood control systems has been developed
along the Mississippi River and its tributaries, consisting of
some 6,880 km of levees (Interagency Floodplain Management Review Committee1994). As a result, mainstem flooding
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has been virtually eliminated, and tributary flooding has
been reduced by approximately 90% (Galloway 1980). In
addition, channels have been cut and rivers straightened to
improve drainage of the hydric soils that are characteristic
of the vast majority of the landscape. These activities have
reduced localized ponding due to rain by about 90% (Charles
Baxter, personal communication). Continued rechannelization of streams that flow through agricultural lands, and
general stream dredging and/or straightening still are carried out on most tributaries. This drainage has resulted in a
gradual drying of the landscape. Flooding and ponding on
poorly drained soils have decreased in both duration and
frequency. Even in more flood susceptible areas where
drainage has been less achievable, winter flooding is often
less extensive and more ephemeral.
By the late 1930s, the elaborate system of levees and
drainage projects had created increased opportunities for
agricultural production. As a result, the bottomland hardwood forest has been reduced to only 1.8 million ha, or about
20 percent of its former extent. The remaining forest exists
as fragmented patches of varying size and habitat quality.
Recent satellite data show more than 35,000 discrete forest
blocks of 1 ha in size or larger (Mueller and others this
proceedings). Much of this remaining habitat is found in the
wettest backswamp systems of the Yazoo River in Mississippi, the Tensas River in Louisiana, and the Cache/Bayou
DeView/White River in Arkansas. Forests on drier ridges
and higher terraces were cleared early in the history of
human settlement in the MAV, as these better-drained soils
provided optimal conditions for growing commodity crops.
Altered flow regimes along the Mississippi and its tributaries have threatened the structure and function of the
remaining forested wetland ecosystem. Most forested wetlands outside the mainstem levees of the Mississippi River
no longer are connected to floodplain processes that once
shaped community age, composition, and distribution. Timing, duration, depth, and velocity of flood events have been
severely altered, with the overall effect of eliminating the
natural, annual process of overbank and backwater flooding
in much of the historic floodplain, thus preventing the
delivery of nutrient-rich sediments and water. Natural
hydrofluvial processes, critical for establishment of early
successional conditions, have been virtually eliminated from
the floodplains. Only in a few places, such as the Lower
Arkansas River, are processes such as bank erosion still
relatively active.
Effects on Avian Populations ______
Habitat loss and fragmentation and hydrologic alteration
in the MAV have resulted in population declines in both
overwintering waterfowl and migratory forest birds.
Populations of dabbling ducks have decreased in the past
several decades. Little evidence suggests that factors such
as disease, environmental contaminants, or predators are
responsible for this decline. Rather, availability of foraging
habitat has had the greatest influence on the abundance,
distribution, and body condition of waterfowl in the MAV
(Loesch and others 1994). The NAWMP suggests that the
foraging requirements for wintering waterfowl in the MAV
include more than 2.8 million ha of naturally flooded
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bottomland hardwood forest. With only 1.8 million ha of this
forest remaining, and with much of it hydrologically impaired, land managers have augmented habitat by mimicking seasonal ponding through winter water management on
agricultural land. Unfortunately, the increased reliance on
managed habitats has resulted in a shift in waterfowl diet
from natural food sources to waste grain and weed seeds.
Although agricultural grains and natural wetland foods are
largely interchangeable as sources of energy, grains do not
provide the same protein and nutrients. Furthermore, in
early winter when the landscape is naturally dry, and in all
seasons during years when precipitation is normal to below
normal, there is a scarcity of foraging habitat, managed or
natural, to meet waterfowl requirements (Loesch and others
1994).
Populations of diving ducks, because of their dependence
on permanently flooded wetlands that still are abundant in
the MAV, and because of the establishment of catfish and
crayfish ponds in the southern reaches of the floodplain,
appear to be faring well.
Agricultural expansion has resulted in an increase in
cleared habitat that potentially could be used by migratory
shorebirds if it were not impaired by hydrologic alteration.
Migratory shorebirds use the MAV predominantly in the
early fall and spring, when they exploit flooded farm fields
and moist soil units as foraging habitat at all latitudes.
Spring habitat requirements for shorebirds are largely met
in the MAV as a result of natural flooding and ponding.
Winter waterfowl management can be a relatively passive
process of damming runoff from typically abundant rainfall;
this form of management generally is compatible with agricultural practices (Loesch and others 1994). However, active
water management is required to provide the summer/early
fall shorebird habitat and early winter habitat for waterfowl. Late summer/early fall is the period of lowest rainfall,
and water delivery during this time will require greater
expense.
The decline in the extent, form, and composition of the
bottomland hardwood forest has been mirrored by a decline
in many species of migratory forest birds in the MAV. Fewer
than 1% of the remaining forest patches are large enough to
support source populations of the more area-sensitive species such as Cerulean Warblers (Dendroica cerulea),
Swainson’s Warblers (Lymnothlypis swainsonii), and Swallow-tailed Kites (Elanoides forficatus) (Mueller and others
this volume). Forest fragmentation has resulted in the
creation of edge habitat along forest patch margins, which
encourages predation and brood parasitism. This and other
factors associated with small patch size have had a negative
impact on many bird populations in all but the largest
remnant blocks of forest. Some species have been affected by
changes in habitat quality separate from issues of forest loss
and fragmentation. The demise of Bachman’s Warbler
(Vermivora bachmanii), for example, may have been caused
by the eradication of the formerly abundant canebrake
habitat (Remsen 1986), a community typically found on
better drained soils that now support agricultural crops.
Cerulean Warblers may be reduced in number because the
tall trees that they favor, such as cottonwoods in early
successional communities, have been largely eliminated
from the landscape.
USDA Forest Service Proceedings RMRS-P-16. 2000
Summary ______________________
Ecosystem management requires “protecting or restoring
the function, structure, and species composition of an ecosystem while providing for its sustainable socioeconomic
use”(U.S. Fish and Wildlife Service 1994a). Partners in
Flight Bird Conservation Plans should follow this general
guideline, but must be more specific regarding the status
and conservation needs of birds. A first and necessary step
toward this end is to develop a clear understanding of the
ecosystem: The way it functioned prior to human alteration,
the history of change, and structure and function as they
currently exist.
USDA Forest Service Proceedings RMRS-P-16. 2000
Alterations to the ecological processes and structure characterizing the floodplain of the Mississippi River system
have been widespread and thorough. Over 80% of the bottomland hardwood forest has been converted to non-forest
conditions, and the natural hydrologic and geomorphologic
processes that created and maintained this vast wetland
system have been virtually eliminated. It is clear that one
result of these changes is a reduced capability to support
populations of birds. In this paper, we have identified the
nature of the problem. The remaining papers in this section
trace the thought processes that have been aimed toward
ameliorating, if not actually solving, the problems that bird
populations face in this degraded ecosystem.
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