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Breeding Bird Use of Flooded Dead Trees In
Rathbun Reservoir, lowa1
Teresa L. Burns 1 and Robert B. Dahlgren 9
Abstract.--Birds using flooded dead trees in a large
flood-control reservoir were compared with those using bottomland timber areas upstream. The flooded dead trees had a lower
avian density and less diversity of species than unflooded
areas, but a greater number of secondary cavity-nesting species.
Nests of three primary cavity-nesting, six secondary cavity-nesting,
and three open-nesting species were found in the flooded trees.
Flooded trees add significantly to the nesting bird population
of the reservoir.
INTRODUCTION
et al. 1978, Lochmiller 1979). The purpose of
this paper is to compare the bird use of dead
trees and of bottomland timber.
With the construction of a large flood-control
reservoir, thousands of acres of riparian habitat
are inundated. Included in this loss is bottomland forest which may contain higher densities and
diversities of birds than do the adjacent uplands
(Odum 1978). It is very costly to remove the
trees, and the current Corps of Engineers' policy
is to remove as little timber from the reservoir
as possible, clearing only for the dam site and
for boating routes.~ The remaining trees soon die
after flooding, providing a unique habitat for
cavity-dependent species of wildlife. It is estimated that many of the dead trees remain
standing for about 25-30 years. 5 Yeager (1955)
found that the number of woodpeckers increased in
bottomland areas along the Mississippi River in
Illinois as the result of flooding and killing of
bottomland trees. There also has been some previous work done on flooded dead trees used by
ducks in shallow reservoirs (Cowardin 1969) and
by woodpeckers in beaver ponds during winter (Hair
STUDY AREA AND METHODS
Rathbun Reservoir, located on the Chariton
River in south-central Iowa, was chosen as the
study site. Flooded dead trees, located only in
the upper reaches of the reservoir, occupy about
10% (420 ha) of the reservoir's 4450 ha of surface
area at the normal pool level of 904 feet above
mean sea level (msl). The water level remained
between 914 and 915 msl for most of the study, but
in July 1982, it rose to the highest level (924
msl) ever recorded. There are two branches in the
upper reaches of the reservoir: the North Fork,
containing 80% of the flooded trees, and the South
Fork, which contains the remaining 20%. Most of
the land surrounding these two branches is managed
for wildlife by the Iowa Conservation Commission.
The reservoir was flooded in 1969, and all trees
below 907 msl were killed by 1971. 6 Because of
the turbidity of the water and the widely fluctuating water levels, there was.no aquatic vegetation
found around any of the flooded trees.
1 Paper presented at the Snag Management Conference. [Arizona State University, Flagstaff,
June 6-9, 1983.] Journal Paper No. J-11068 of the
Iowa Agriculture and Home Economics Experiment.
Station, Ames; Project No. 2399. This study was
funded by the Iowa Conservation Commission.
1
Teresa L. Burns is a graduate student at
Iowa State University, Ames, Iowa.
3 Robert B. Dahlgren is Unit Leader of the
Iowa Cooperative Wildlife Research Unit, Ames,
Iowa. Agencies cooperating in the Unit are U.S.
Fish and Wildlife Service, Iowa Conservation Commission, Iowa State University, and Wildlife
Management Institute.
~Bryan, K.
1983. Personal conversation.
u.s. Army Corps of Engineers, St. Louis, Mo.
5 Tbompson, D. Q.
1983. Personal conversation.
U.S. Fish and Wildlife Service, Ft. Collins, Col.
In addition to the flooded trees, bottomland
timber 10-15 km upstream from the plots in the
flooded trees also was censused. The bottomland
areas chosen were all above the previous reservoir
high of 918 msl and were not subject to the water
level changes in the reservoir. The dominant tree
species on the sample plots include silver maple
6 Brunk, E. L., A. D. Allman, and G. P. Dellinger. 1975. Mortality of trees caused by
flooding during the growing season at two midwest
reservoirs. Unpublished report. Missouri Dept. of
Conservation, Jefferson City, Mo.
99
(Acer saccharinum), American elm (Ulmus americana),
red elm (Ulmus rubra), hackberry (Celtis occidentalis), and green ash (Fraxinus pennsyiVanica).
Other tree species include pin oak (Quercus
palustris), swamp white oak (Quercus bicolor),
bitternut hickory (Carya cordiformis}, shellbark
hickory (Carya lacinosis), mulberry (Moris rubra),
and hawthorn (Crataegus spp.).
by the number of observations for the most abundant species. Relative spatial component (RS) is
the number of different plots on which the bird
was observed divided by the total number of plots.
Important value (IV) is equal to RN + RS.
Birds were censused in each of the areas by
using a circular-plot method. All birds seen or
heard within a 50-m radius in 10 minutes were
recorded. Flying birds such as swallows that were
foraging over the plot were included in the census,
but aquatic birds resting on the water were not.
Counts were begun in late May 1982 and continued
through early July until all plots were censused
four times. Forty plots were established, 20 each
in the bottomland timber and flooded trees, with
10 plots on the South Fork and 30 on the North
Fork of the river. Plots were censused in early
morning and were systematically rotated as to the
time when counts were made. Birds were censused
from a canoe in the flooded trees. The flooded
trees also were searched for evidence of nests.
Nests that could not be viewed from the canoe were
verified by observing at least three feedings of
nestlings or by hearing nestlings in the nest.
Species diversity and species richness were
greater in the bottomland timber than in the flooded
trees (table 1). A total of 15 species were
observed in the flooded trees during censusing as
compared with 28 in the bottomland timber. Species
diversity usually is associated with increasing ·
complexity of the vegetation structure in a habitat
(MacArthur 1964); however, dead trees (Balda 1975)
and water (Karr 1968) may also increase diversity.
RESULTS AND DISCUSSION
Table 1. Mean bird species diversity (H') and
species richness (S) for census plots in
flooded dead trees and bottomland timber
areas during summer 1982 at Rathbun Wildlife
Unit, Iowa. (Means were different;
! ~ 0.0001)
s
H'
X
Species diversity was calculated by using the
Shannon formula ( ~ - p log p) • Species richness
is the average number of sp~cies per plot. A
t-test was used to test for differences between
means.
Relative numerical component (RN) was calculated as the number of bird observations divided
SE
X
SE
Bottomland timber
(!! = 20)
1.97
0.05
10.25
0.56
Flooded timber
(!! = 20)
1.32
0.04
5.45
0.31
Table 2. Relative numbers, relative spatial components, and importance values by habitat for cavity-nesting
birds during summer 1982 at Rathbun Wildlife Unit, Iowa. The mean numbers of birds per plot per census
are given in parentheses.
Flooded dead trees
Species
European starling (3.78)
(Sturnus vulgaris)
Tree swallow(2.41)
(Iridoprocne bicolor)
House sparrow(1.90)
(Passer domesticus)
Red-headed woodpecker(0.69)
(Melanerpes erithrocephalus)
Northern flicker(0.24)
(Colaptes auratus)
Common grackle(0.13)
(Quiscalus quiscula}
Wood duck(0.04)
(Aix sponsa)
House wren(0.01)
<
(Troglodytes aedon)
1
Red-bellied woodpecker (0)
(Melanerpes carolinus)
Great crested flycatcher 1 (0)
(Miiarchus crinitus)
1
Bottomland timber
RN
RS
IV
1.00
1.00
2.00
0.64
1.00
1.64
0.50
0.90
1.40
0.18
0.85
1.03
0.06
0.55
0.61
0.03
0.40
0.43
0.01
0.05
0.06
0.01
0.05
0.05
0
0
0
0
0
0
Species
RN
Red-headed woodpecker(0.88)
1.00
(Melanerpes erythrocephalus)
House wren(0.75)
0.86
(Troglodytes aedon)
Black-capped chickadee(0.24)
0.27
(Parus atricapillus)
Great crested flycatcher(0.16) 0.18
(Myiarchus crinitus)
Northern flicker(0.10)
0.11
(Colaptes auratus)
Barred owl(0.03)
0.03
(Strix varia)
White-breasted nuth~tch(0.03) 0.03
(Sitta carolinensis)
Red-bellied woodpecker(0.03)
0.03
(Melanerpes carolinus)
Hairy woodpecker(0.01)
0.01
(Picoides villosus)
Downy woodpecker(0.01)
0.01
(Picoides pubescens)
RS
IV
0.95
1.95
1.00
1.86
0.65
0.92
0.40
0.58
0.30
0.41
0.10
0.13
0.10
0.13
0.10
0.13
0.05
0.06
0.05
0.06
Species was not observed during censusing but their nests were found in flooded dead trees.
100
Lack of foliage probably is the major reason for
less species diversity in the flooded trees.
smaller colony with 6 nests located in live bottomland trees near the gradient between the reservoir
and river. Two of the open-nesting species,
red-tailed hawk and great blue heron, used snags
with limbs to support their nests, whereas eastern
kingbird and mallard nests were all on stumps.
In comparing bird species found in flooded
trees with those in bottomland timber, birds were
divided into two groups based upon their nesting
habit, primary or secondary cavity-nesting. The
number of secondary cavity-nesting species was
greater (P < 0.0001) in the flooded trees ( i e
32.0) than in the bottomland timber (K e 4.8).
Starlings, tree swallows, and house sparrows had
the highest importance values in the flooded trees
(table 2), making up 88% of birds observed there.
None of these species was observed in the bottomland timber. The secondary cavity-nesting species
that are found in both areas are the great crested
flycatcher and house wren; however, both had much
higher importance values in the bottomland timber
(table 2).
CONCLUSIONS
Although there were more secondary cavitynesting species in the flooded trees, the number of
primary cavity-nesting birds (K e 3.6 for flooded
trees, X e 4.0 for bottomland timber) did not differ between the areas (P e 0.66), though there were
more birds seen in the bottomland timber. The redheaded woodpecker had the highest importance value
for primary cavity nesters in both areas (table 2),
followed by the northern flicker and red-bellied
woodpecker. Yeager (1955) found red-headed woodpeckers to be the most numerous woodpecker 1 to 8
years after flooding at a dam built near the confluence of the Illinois and Mississippi rivers.
Lochmiller (1979) also found that the red-headed
woodpecker was the most common of seven woodpeckers
using flooded dead trees in beaver ponds in Georgia
during winter. Downy and hairy woodpeckers were
not observed in the flooded trees at all and only
rarely in the bottomland timber at Rathbun.
Nests were found of all species observed during the censusing in the flooded trees except those
of chimney swifts (Chaetura pelagica), barn swallows (Hirundo rustica), American crows (Corvus
brachyrhynchos), blue jays (Cyanocitta c~a),
and red-winged blackbirds (Agelaius phoeniceus).
These species used the flooded trees mainly as
perch sites and for aerial foraging for insects.
In contrast, all the birds that nested in the
flooded trees were seen flying into adjacent
upland woodlands, row crops, and pastures to forage. Such foraging must be necessary to support
the large numbers of cavity-nesting birds in the
flooded trees.
Nests of three open-nesting species were
found in the flooded trees including those of
red-tailed hawk (Buteo jamaicensis) and eastern
kingbird (Tyrannus tyrannus) in 1982 and, in spring
of 1983, a mallard (Anas platyrhynchos). There is
a great blue heron (Ardea herodias) colony with
about 20 nests in the flooded trees as well as a
101
Although this study was conducted in the
summer, flooded and downed trees were used by birds
throughout the year. Migrating ducks often used
fallen trees as resting areas, and raptors used
dead trees as perch sites. Although flooded trees
cannot mitigate the loss of the diversity and
richness of bird species in bottomland timber,
their presence can increase the number of birds
nesting in the area for about 30 years. The low
species diversity value in the flooded trees is
greater than the value that would have occurred if
the trees had been cleared.
LITERATURE CITED
Balda, R. P. 1975. Vegetation structure and
breeding bird diversity. p. 55-88. In
D. R. Smith, Tech. Coord. Symposium on management of forest and range habitat for non··
game birds. U.S. Dep. Agric., For. Serv.
Gen. Tech. Rep. W0-1.
Cowardin, L. M. 1969. Use of flooded timber by
waterfowl at the Montezuma National Wildlif£
Refuge. J. Wildl. Manage. 33(4):829-842.
Hair, J. D., G. T. Hepp, L. M. Luckett, K. P.
Reese, and D. K. Woodward. 1978. Beaver
pond ecosystems and their relationships to
multi-use natural resource management.
p. 80-92. In R. R. Johnson and J. F. McCormick, tech.-coords. Strategies for protection and management of floodplain wetlands
and other riparian ecosystems. u.s. Dep.
Agric., For. Serv. Gen. Tech. Rep. W0-12.
Karr, J. R. 1968. Habitat and avian diversity
on stripmined land in east-central Illinois.
Condor 70(4): 348-357.
Lochmiller, R. L. 1979. Use of beaver ponds by
southeastern woodpeckers in winter. J.
Wildl. Manage. 43(1):263-266.
MacArthur, R. H. 1964. Environmental factors
affecting bird species diversity. Am. Nat.
98(903):387-397.
Odum, E. P. 1978. Ecological importance of the
riparian zone. p. 2-4. In R. R. Johnson
and J. F. McCormick, tech:-coords. Strategies for protection and management of flood
plain wetlands and other riparian ecosystems.
U.S. Dep. Agric., For. Serv. Gen. Tech. Rep.
W0-12.
Yeager, L. E. 1955. Two woodpecker populations
in relation to environmental change.
Condor 57:148-153.