Ecology of Red Maple Swamps in the Glaciated Northeast: A ...

territorial species) and all bird registrations (i.e.,
songs, calls, and visual observations). Among the
most significant factors influencing the avian community
in these studies were wetland size, vegetation
structure, and water regime.
Wetland Size
Breeding bird species richness is correlated
with the size of red maple swamps (Merrow 1990).
In Merrow's study, species richness ranged from 3
to 15 species per site for singing birds, and from 7
to 24 species per site for all bird registrations.
Sites 4 ha or smaller had significantly lower species
richness than sites ranging from 6 to 19 ha. In
larger (30-45 ha) swamps in Massachusetts, Swift
(1980) found richness to range from 18 to 26 species.
By combining data from Swift, Merrow
(1990), and pertinent breeding bird censuses, a
more comprehensive picture of the species-area
relationship can be developed (Fig. 7.4). Although
factors other than wetland size also affect avian
species richness, size clearly is a key determinant.
Whether swamp size has any effect on breeding
bird density or relative abundance is unclear.
Breeding bird censuses have shown that avian
density may vary widely, from as few as 4.3 to as
many as 11.0 males per ha (Table 7.3), even among
areas of swamp that are comparable in size (5-
10 ha). In Rhode Island red maple swamps less
than 20 ha in size, avian relative abundance
ranged from 0.6 to 2.0 singing males per census per
0.28-ha plot, and there was no significant relation
between relative abundance and wetland size
(Merrow 1990). Relative abundance values were
higher (mean 2.8 singingmales per census per plot;
range 0.8-4.5) in the larger swamps censused by
Swift et al. (1984). Unfortunately, direct comparisons
among studies may be misleading because of
differences in census methods. Additional research
is needed Lo clarify the relation between swamp
size and avian abundance.
Vegetation Structure
The influence of vegetation structure on breeding
bird communities has been well documented
(Beecher 1942; MacArthw 1964; Tramer 1969; Anderson
and Shugart 1974; James and Warner 1982).
Tramer, for example, showed that species richness
and diversity of breeding birds are higher in forest
habitats that contain several vegetation layers
than in simpler communities dominated by herbs
or shrubs. Avian richness and diversity in northeastern
red maple swamps are comparable to those
of upland deciduous and upland coniferous forests,
but lower than in floodplain forests (Fig. 7.5).
The study areas selected by Swift et al. (1984)
represented a wide range of vegetation structure;
they included five mature red maple forested
swamps, as well as three wetlands containing areas
of both forested swamp and shrub swamp. Avian
abundance was significantly higher in the structurally
diverse forested-shrub wetlands (mean
3.7 males per plot per census) than in the mature
forests (mean 2.2 males per plot per census), based
on our calculations from data in Swift (1980). Species
richness, however, was similar for the two
types. Species present only in forested-shrub wetlands
included the yellow warbler (Dendroica petechia),
warbling vireo (Vireogilvus), swamp sparrow
7
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z 20 1 t o Fig. 7.4. Avian breedmg species richness
K
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0
0 as a function of wetland size in north-
o 5 i ~ . A 1 eastern Bird Census red maple (BBC) data swamps. are &om Breedmg Slack
6,
et al. (1975), Meyers et al. (1981), Tayn
; 10 lor (19&i), Black and Seeley (1953),
t_
and Seeley (1954, 1955, 1956, 1957,
o BSR = 97 + (025 x IogAREA) 1966). Results of the latter six cen-
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(D
suses rtre plotted RZ a &year mean
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