0048463
Estuaries Vol. 19, No. 2A, p. 213-219 June 1996
Dependence of Waterbirdsand Shorebirds on
Shallow-WaterHabitats in the Mid-Atlantic
Coastal Region: An Ecological Profile and
Management Recommendations
R. MICHAEL ERWIN1
National Biological Service
Patuxent EnvironmentalScienceCenter
Laurel, Maryland 20708-4015
ABSTRACT: Waterbirds (waterfowl, colonially nesting wading and seabirds, ospreys [Pandion haliaetus], and bald eagles
[Haliaeetus leucocephalus])and shorebirds (sandpipers, plovers, and relatives) may constitute a large fraction of the toplevel carnivore trophic component in many shallow-water areas of the mid-Atlantic region. The large biomass of many
species (>1 kg body mass for the two raptors and some waterfowl) and enormous populations (e.g., >1 million shorebirds in late May in parts of Delaware Bay) reveal the importance of waterbirds as consumers and as linkages in nutrient
flux in many shallow-water habitats. Salt and brackish marsh shallow-water habitats, including marsh pannes and tidal
pools and creeks as well as constructed impoundments, are used intensively during most months of the year; in fall and
winter, mostly by dabbling ducks, in spring and summer by migrant shorebirds and breeding colonial wading birds and
seabirds. In adjacent estuaries, the intertidal flats and littoral zones of shallow embayments are heavily used by shorebirds,
raptors, and colonial waterbirds in the May to September periods, with use by duck and geese heaviest from October
to March. With the regional degradation of estuarine habitats and population declines of many species of waterbirds in
the past 20 yr, some management recommendations relevant to shallow waters include: better protection, enhancement,
and creation of small bay islands (small and isolated to preclude most mammalian predators) for nesting and brooding
birds, especially colonial species; establishment of sanctuaries from human disturbance (e.g., boating, hunting) both in
open water (waterfowl) and on land; better allocation of sandy dredged materials to augment islands or stabilize eroding
islands; improvement in water management of existing impoundments to ensure good feeding, resting, and nesting
opportunities for all the waterbirds; support for policies to preclude point and nonpoint source runoff of chemicals and
nutrients to enable submerged aquatic vegetation to recover in many coastal bays; and improvement in environmental
education concerning disturbance to wildlife for boaters and recreationists using the coastal zone.
Introduction
Waterbirds (here referring to waterfowl, colonial
wading birds and seabirds, ospreys [Pandion haliaetus], American bald eagles [Haliaeetus leucocephalus]), and shorebirds (sandpipers, plovers, and relatives) are a dominant part of the shallow-water
estuarine region of the mid-Atlantic coastal region.
Waterbirds are top-level consumers in many aquatic systems, feeding largely on fish and macroinvertebrates. They represent important linkages in energy and nutrient transport processes both within
the estuary and among littoral, marsh, and upland
habitats.
The community of waterbirds changes markedly
during the annual cycle. In winter, more than 20
species of waterfowl dominate the shallow waters
of the large coastal embayments and nearshore
marine waters; most of these species migrate north
' Telephone
NBS.GOV.
301/497-5640;
e-mail
C 1996 EstuarineResearch Federation
R-Michael_Erwin@
213
in March and April. During spring (April-May)
and fall (August-October)
migration periods,
enormous numbers of shorebirds (23 species)
briefly visit the mid-Atlantic region, "refueling" between nesting areas in Canada and the arctic and
wintering areas thousands of kilometers to the
south in Latin America. In summer, large numbers
of coastal seabirds, including 13 species of gulls,
terns, double-crested cormorants (Phalacrocoraxauritus), and black skimmers (Rynchopsniger), and 10
species of wading birds (egrets, ibises, and herons),
nest on the bay islands, barrier islands, and along
the wooded tributaries of the coastal embayments.
Recent estimates suggest that a number of waterfowl (Haramis 1991a, b; M. Perry, personal communication), shorebird (Howe et al. 1989), and colonial waterbird (Erwin and Spendelow 1991) populations have either declined over the past 25 yr in
some important parts of their range or their nesting sites have become restricted.
This report reviews some of the major groups of
0048464
214
R. M. Erwin
TABLE 1. Seasonal estimates of selected waterbirds in the mid-Atlantic region.
Season
Species/Group
Waterfowla
Canvasback
Scaup
Canada goose
Snow goose
Shorebirdsb
Wading birdsc
Bald Eaglesd
Winter
Winter
Winter
Winter
Spring (migration)
Summer (nesting)
Summer (nesting)
Area
Estimated Numbers
Chesapeake Bay, Maryland
Chesapeake Bay, Maryland
Chesapeake Bay, Maryland
Chesapeake Bay, Maryland
Delaware Bay
Virginia (barriers and
Chesapeake Bay
Chesapeake Bay
46,700
50,400
260,000
53,000
>500,000
18,000 prs. (1993)
>300 prs. (1993)
a United
States Fish and Wildlife Service, Office of Migratory Bird Management report (1994), unpublished data.
h Myers 1986.
c B. Watts, College of William and Mary, unpublished data.
dM. Byrd, College of William and Mary, personal communication.
waterbirds and shorebirds in the mid-Atlantic
coastal region, some recent population estimates,
some examples of species-specific energetic demands, and some patterns of seasonal use of major
shallow-water habitats. I make suggestions for improved habitat management to ensure long-term
avian conservation in a region of ever-growing human population.
Ecological Importance
NUMERICALABUNDANCE
Large numerical abundance at certain times of
the year, combined with generally large average
body masses, makes the waterbird-shorebird community the dominant top-level consumer in most
shallow embayments in the mid-Atlantic. For example, more than 500,000 shorebirds are estimated in migration in the intertidal zone of the lower
Delaware Bay in spring (Myers 1986). These species range in mass from 10 g to more than 200 g.
During the United States Fish and Wildlife Service's Mid-Winter Inventory, the canvasback (Aythya
valisineria) was estimated at 46,700 individuals in
Chesapeake Bay in January 1994 (Office of Migratory Bird Management, United States Fish and
Wildlife Service unpublished data). Females average about 1.2 kg, with males about 10% larger.
Thus, the total biomass of just one species of wintering waterfowl exceeded 50,000 kg. Even greater
numbers of other large waterfowl occur in the bay
(Table 1).
TROPHICIMPACTS
Waterbirds are often top-level carnivores in estuaries. The local and even regional impact of waterbirds on estuarine macrofauna and forage fishes
may be significant. For example, laboratory studies
of the canvasback indicate that females fed ad libidum may consume 2-3 kg d- of a small bivalve,
Macoma balthica (M. Haramis and D. Jorde, National Biological Survey, unpublished data). Projected
over a 120-d winter period in the bay, this yields a
conservative estimate of 240 kg ingested per female per winter of one clam species. Thus, roughly
50,000 canvasbacks wintering on the bay could potentially consume more than 10 million kg of
clams in a winter season, assuming this was their
sole food source.
Estimates are difficult to obtain for wading birds.
The great blue heron (Ardea herodius),estimated at
2.0-2.5 kg in mass may consume up to 0.88 kg d-1
of energy during the small-chick phase (Butler
1992, 1993). Over the entire 60-d development period of the young, an individual heron may consume ca. 36.7 kg of energy (Butler 1992, 1993).
Some mid-Atlantic colonies consist of more than
3,000 nesting birds, therefore, the energetic demands on the fish communities within foraging
ranges of the colony sites may exceed 100,000 kg
in a nesting season and could have major impacts
on the trophic dynamics of an estuary.
The relationships between wading birds and
their ecosystems can be highly complex. Bildstein
(1993) reported that of the total nitrogen and
phosphorus input into a coastal marsh-estuary in
South Carolina, the white ibis (Eudocimus albus)
nesting population contributed nearly 10% and
35%, respectively, in 1984, but in 1985, with a population crash, both of these inputs dropped to less
than 5%.
HABITAT ZONATION
The ecological roles and significance of the various species of waterbirds in different shallow-water
habitats of the mid-Atlantic estuaries vary markedly
both spatially and temporally. Some of the major
prey consumed by the different waterbirds in these
shallow estuarine waters are listed in Table 2. Below I outline some of the principal shallow-water
habitats in the mid-Atlantic coastal region and
their associated waterbird guilds:
0048465
WaterbirdDependenceon ShallowWaters
TABLE 2. Important shallow-water resources for waterbirds.
Resource
Species
Waterfowl
Canvasback
Redhead
Black duck
Wood duck
Wading birds
Herons, egrets
Shorebirds
Bald eagle
Osprey
Macoma baltica,wigeongrass, pondweed, etc.
Redheadgrass
Invertebrates, wigeongrass, pondweeds
Mast, invertebrates
Small fishes, grass shrimp
(Paleomanetesspp.)
Small invertebrates, eggs of horseshoe crabs
Fish (catfish, carp), carrion
Fish (menhaden, mullet, catfish)
Marsh Surface
Small pannes, tidal pools, and tidal creeks, all
usually less than 1 m deep, attract large densities
of shorebirds and wading birds especially during
May and August migration periods. Densities may
exceed 500 birds ha-~ at some sites (Master 1992;
Erwin unpublished data). Small forage fishes, polychaetes, mollusks, and insect larvae (especially Tabanidae and Culicidae) serve as primary prey.
Such habitats are particularly important for shorebirds during rising and high tide periods when intertidal flats may become unavailable. In winter,
such habitats may frequently be frozen and unavailable to wintering dabbling ducks (Erwin et al.
1994). During winter, the marsh-dwelling Melampus
snails serve as important prey for American black
ducks (Anas rubripes),a species whose population
declines have caused management concern among
many state and federal wildlife agencies (Rusch et
al. 1989). In many Atlantic coastal areas, artificial
creation of ditches and small ponds for mosquito
control has added habitat for waterbirds; however,
the net ecological effect of these habitat changes
remains controversial (see papers in Whitman and
Meredith 1987; Erwin et al. 1994).
Impoundments
These shallow-water basins (usually <2 m deep)
were constructed mostly during the 1930-1970 period largely for wintering waterfowl and may attract
more than 1,000 birds ha-1 (Burger 1988). The
species include dabbling ducks (7-8 species), Canada geese (Branta canadensis), snow geese (Chen ca-
erulescens), 10 species of wading birds, large numbers of roosting gulls and terns, as well as a dozen
or more species of small shorebirds (see Whitman
and Meredith 1987). In general, diving ducks use
these areas mostly during migration, while use by
dabbling ducks is heaviest during breeding and in
late summer (Burger et al. 1984). Some of the dab-
215
bling ducks are present year round, such as American black ducks. Shorebirds used the areas mostly
during spring and fall migration periods, wading
birds during the summer breeding season, but
gulls are present year round in large numbers in
most of the mid-Atlantic (Burger 1988). For wintering waterfowl and migrant shorebirds, these areas can be highly valuable during extended periods, serving as roost sites, refuges from hunting
and other disturbances, and feeding habitats (Chabreck 1988).
The ecological costs versus benefits of impounding high marsh has a long and controversial history (see references in Whitman and Meridith
1987). The benefits to a number of bird species
must be weighed against the habitat loss for marshdependent estuarine fisheries and the overall net
loss of natural coastal marsh, which has already
been substantial along the Atlantic (Tiner 1984).
IntertidalFlats
These shallow (mostly <2.5 m deep) areas include both unvegetated mudflats and in some
regions extensive beds of submerged aquatic vegetation (SAV). Large mudflat expanses are especially critical to migrant shorebirds in May and
August-October, while SAV beds are important to
waterfowl both as summer brood-rearing habitat
and winter foraging habitat. The primary SAV species for waterfowl in the mid-Atlantic region include wigeongrass (Ruppia maritima), wild celery
(Valisneria americana), redhead grass (Potamogeton
perfoliatus), and several species of pondweeds (Potomagetonspp.) (Haramis 1991a, b). Expansion of
the exotic Hydrilla verticillatain the Potomac River
south of Washington, D.C., has been an ecological
benefit both from water quality and waterfowl food
source perspectives.
The most valuable areas for large SAV beds are
on the backsides of large barrier islands in the New
Jersey to North Carolina barrier region, and along
the major subwatersheds of the Chesapeake Bay
such as the Susquehanna Flats, Eastern Bay in
Maryland, the Potomac, Patuxent, and Choptank
rivers, and Mobjack Bay in Virginia. Because most
shorebirds concentrate on the outer coastline in
spring, the most important intertidal flat areas are
in the major inlets along the eastern shore of Virginia and southern New Jersey and along both
shores of lower Delaware Bay. This latter area is
one of international significance to migrant shorebirds and is one of only a few sites of hemispheric
importance within the Western Hemisphere Shorebird Reserve Network (Myers et al. 1987). An estimated 0.5-1.5 million birds may congregate there
in late May (Myers 1986). Dabbling ducks and wading birds also use these intertidal areas most inten-
0048466
216
R. M. Erwin
sively where eelgrass or other beds are found and
where water depths are 0.1 m to 0.5 m (Custer and
Osborn 1978; Helmers 1992). Longer-legged
shorebirds use the flat areas (to depths of 10-15
cm) as waters recede during ebb tide, while all
shorebird species invade the newly-exposed mudflats (Helmers 1992).
Littoral Zone
The deeper part of the shallow-water zone (2-5
m deep) is used by the fewest species of waterbirds,
generally diving ducks and tundra swans (Cygnus
columbianus) in winter, mute swans (Cygnus olor)
and the threatened American bald eagle year
round, and the osprey during the breeding season
from April to August. The latter two raptor species
feed on medium-size fish such as catfish and menhaden (Brevoortiatyrannus) in the littoral zone, and
eagles also feed on carrion (various fish, birds, and
mammals) in the littoral zone and along undeveloped beaches. As mentioned above, one of the major wintering divers, the canvasback, along with
scaup (Aythya spp.) are attracted to the littoral
zone with dense Macoma balthica beds; these are
mostly on the eastern shore of the Chesapeake Bay
(Jorde et al. 1992). Swans and geese formerly fed
in SAV beds in these shallow littoral areas but, with
the decline of SAV in Chesapeake Bay and other
mid-Atlantic bays, have shifted to feeding in agricultural fields (M. Perry personal communication).
Management Concerns
Because of the declines in some species in the
mid-Atlantic region (Howe et al. 1989; Rusch et al.
1989; Haramis 1991a, b), and the degradation of
many shallow-water habitats, I discuss below a number of major management issues of concern to waterbird conservation. These have been elaborated
upon in earlier publications (see papers in Funderburk et al. 1991; Erwin et al. 1993; Erwin et al.
1995).
PROTECTION, ENHANCEMENT, CREATION OF
SMALL BAY ISLANDS
Many small bay islands are eroding at a rapid
pace in the Chesapeake Bay region and elsewhere
(Erwin et al. 1993). These small islands are critical
as nesting sites for a number of waterbirds including American black ducks, bald eagles, ospreys,
and all of the wading birds nesting in mixed-species colonies. Islands that were formed by dredged
material deposition have been especially valuable
to colonial waterbirds from Long Island, New York,
south to Florida and west to Texas (Table 3). In
many areas, the islands are eroding rapidly, and
dredged material is being diverted to beach nour-
TABLE 3. Percent of colonial waterbirds nesting on coastal
dredged material islands, 1977a.
Seabirds
Area
Percent
Texas
Florida
North Carolina
Great Lakes
Pacific Northwest
60
59
75
25
10
Numberb
203,000
311,000
86,000
272,000
17,200
Wading birds
Percent
Numberb
62
52
99
0.2
0
54,000
241,000
15,300
750
750
From Soots and Landin (1978).
b Number of breeding adults estimated.
a
ishment programs (Erwin et al. 1995). In some areas such as North Carolina and Maryland, federal
and state wildlife agencies, working with the United States Army Corps of Engineers, have enhanced
existing islands with a combination of sandy materials, riprap, and grass plantings (J. Gill, United
States Fish and Wildlife Service, personal communication). In some regions (e.g., the Poplar Island
group in the upper Chesapeake Bay), old barges
may become useful both to stabilize shorelines in
lieu of bulkheading or as potential nesting sites for
least terns (Sterna antillarum).
SANCTUARY
ESTABLISHMENT
With an increasing human population along the
Atlantic Coast and a commensurate increase in
boating activities, fewer wildlife refugia remain, especially during the warm seasons of the year. Although state wildlife personnel post many colony
sites of waterbirds during the nesting season, little
protection exists for nesting areas of waterfowl or
even bald eagles. A number of publications recommend "setback" or buffer distances for various
species (see papers in Funderburk et al. 1991). At
one extreme, nesting bald eagles may require as
much as an 800 m distance (Fraser et al. 1991)
whereas ospreys may not be disturbed when boats
are within 50 m. Figure 1 indicates ranges recommended for a number of species or guilds.
Outside of the nesting season, birds still require
undisturbed habitats. Resting or feeding flocks of
canvasbacks may need a zone of 1,000 m from
commercial clamming or fishing vessels (Erwin et
al. 1993). In migration, roosting shorebirds congregate on the tips of barrier islands during July
and August when recreational activities are often
intense; they may require as much as 100 m of
buffer from pedestrians on beaches. Such traditional high-use areas, whether they be on land or
in the estuary, should be protected using signs,
marker buoys, etc. Effective enforcement is problematic in many areas, but citizen volunteers or student interns have been used effectively in many
areas.
0048467
WaterbirdDependenceon ShallowWaters
Piping plover(N)
Osprey(N)
system (see Batuik et al. 1992, and citations therein). Protecting and restoring these beds should receive the highest priority not only for water quality
and sediment stability but because of their value as
I
IBaldEagle (R)
Bald Eagle(N)
800
I
Wadingbirds
(N.
Canvasback(R)
I
I
| ea birds(N)
P.
1000
cormorants
Pelicans,
(N)
Shorebirds(R)
l
_habitat
5|Ir__(R)
l
I
I
I
I
I
I
I
100
200
300
400
500
600
BufferDistances
(iRoostin
217
g
Fig. 1. Recommended buffer distances (in m) to reduce human disturbance at activity sites of selected waterbird and shorebird species. Sources: piping plover - nesting (S. Haig, National
Biological Survey, unpublished data); bald eagle - roosting and
nesting (Fraser et al. 1991); osprey - nesting (M. Byrd, College
of William and Mary, unpublished data); wading birds - nesting
(Erwin 1989); Canvasback - roosting (Haramis 1991a; Erwin et
al. 1993); seabirds - nesting (Erwin 1989); shorebirds - roosting
(Helmers 1992); pelicans and cormorants - nesting (Anderson
1988).
IMPOUNDMENT MANAGEMENT
With sea-level rise along the coasts, the availability of shallow-water habitats may become limited to
many species of waterbirds in the next several decades (Hayden et al. 1991). This could accentuate
the use of impoundments by many species; thus,
the proper maintenance and management of impoundments could become paramount (Whitman
and Cole 1987). Relative to adjacent natural intertidal or high marsh areas, impoundments in the
mid-Atlantic may provide superior feeding, nesting, and roosting opportunities for many species
of waterbirds during the entire year (Chabreck
1988; Erwin et al. 1994; Weber 1994). Proper water
control at critical time periods, avoidance of salinization, control of exotics such as purple loosestrife (Lythrum salicaria; Thompson et al. 1987),
and allowance for ingress of marine fishes are
some concerns at many state and federally managed areas (see papers in Whitman and Meredith
1987; Helmers 1992). One chronic conflict has
been the maintenance of shallow water during the
late July-August period when shorebirds migrate
through the mid-Atlantic region (Helmers 1992).
Traditionally, managers have kept most of their impoundments relatively dry during these periods to
permit the growth and germination of annual
plants as waterfowl foods for the fall and winter
(Chabreck 1988).
SUBMERGED
AQUATICVEGETATION(SAV)
Many volumes have been written about the significance of SAV to the health of the estuarine eco-
wildlife
habitat
(Haramis
1991a,
b; Erwin et al.
1993). The demise of these extensive beds of pondweeds, eelgrass, wigeongrass, and redheadgrass especially has resulted in either major changes in
use (e.g., shifting to upland feeding by
swans and geese) or distributional changes (e.g.,
canvasbacks shifting from Chesapeake Bay to
North Carolina [Haramis 1991a] or redheads [Aythya americana] avoiding their traditional ChesaR
neae
tl.
wiAnterincr
.llllJO
1991b]).VVIL11~!1116
Arei
ea.!
nii
nrt t
entirfxlv
t
IY
r--rm-ic
L _l11l
II
1991b]).
Recommendations
To protect and enhance shallow-water habitats in
the mid-Atlantic region for a large array of waterbird species, I recommend the following:
Encourage collaborative efforts for controlling
both point and nonpoint runoff of chemicals and
nutrients to encourage the reestablishment of submerged aquatic vegetation. Focus on sewage treatment facilities and buffers along all bay shorelines
and tributaries to mitigate agricultural influences.
Protect and augment small bay islands, especially
those with known use by nesting and feeding waterbirds. Focus protection during the early breeding season (late February for great blue herons)
through the chick-rearing period, generally April
to July for most species.
Promote the use of dredged material (especially
sandy materials) for augmentation of estuarine islands to reduce the effects of erosion and sea-level
rise. Focus on islands used by nesting colonial species or American black ducks. Dredging should be
performed during periods of nonuse by waterbirds
(from late August through February usually).
Encourage establishment of buffer zones (generally 100 m or greater) for nesting colonial waterbirds and roosting flocks of shorebirds by seasonal posting of signs. In the mid-Atlantic region,
signs should be posted in late April and left until
either late August (nesting colonies) or late September (shorebirds). For wintering flocks of canvasbacks, scaup (spp.), and redheads, traditional
littoral feeding areas should be posted from December to March using marked buoys similar to
designations used for crabbing grounds. Promote
seasonal closure (late February to July) of critical
areas such as shoreline areas near eagle nests.
Strongly encourage stronger enforcement by marine police and natural resource officers.
Strengthen environmental education concerning disturbance effects on wildlife. Encourage de-
0048468
218
R. M. Erwin
velopment and wide dissemination of brochures at
marinas, parks, sporting-goods stores, and public
ramps.
Promote more in-depth training of wetland managers in proper management of existing impoundments and restoration of dysfunctional impoundments and control structures. Emphasize management not only for waterbirds but for the entire array of native flora and fauna in the region.
ACKNOWLEDGMENTS
I thank G. M. Haramis, D. Krementz, and M. C. Perry for
discussions and ideas on this subject. B. King assisted with the
preparation of the manuscript. The editor, G. Gaston, G. M.
Haramis, and two anonymous reviewers provided useful comments on earlier drafts of the manuscript.
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Receivedfor consideration,October13, 1994
Acceptedfor publication,August 1, 1995