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| Bepernent Bird Populations in and

of Agriculture

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Intermountain

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Research Paper
INT-432

| October 1990 Dean E. Medin
Warren P. Clary

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THE AUTHORS

DEAN E. MEDIN is a research wildlife biologist with
the Intermountain Research Station at the Forestry
Sciences Laboratory in Boise, ID. He earned a B.S.
degree in forest management from Colorado State
University in 1957, an M.S. degree in wildlife manage-
ment from Colorado State University in 1959, anda
Ph.D. degree in range ecosystems from Colorado
State University in 1976. His research has included
studies in mule deer ecology, big-game range im-
provement, mule deer population modeling, and
nongame bird and small mammal ecology and habitat
management.

WARREN P. CLARY is project leader of the Inter-
mountain Station’s Riparian-Stream Ecology and
Management research work unit at Boise, ID. He
received a B.S. degree in agriculture from the Univer-
sity of Nebraska and an M.S. degree in range man-
agement and a Ph.D. degree in botany (plant ecology)
from Colorado State University. He joined the Forest
Service in 1960 and has conducted research on
forested and nonforested rangelands in Arizona,
Louisiana, Utah, Idaho, Oregon, and Nevada.

RESEARCH SUMMARY

We compared breeding bird populations and com-
munity organization between a beaver pond habitat
dominated by willows (Salix spp.) and an adjacent
nonwillow riparian habitat on Summit Creek in east-
central Idaho. For the previous 14 years both habitats
had been protected from livestock grazing by a fenced
exclosure (122-ha). Bird populations were determined
by spot-mapping on 9-ha plots in spring 1989. Struc-
tural (physiognomic) differences in vegetation between
the two habitats and the availability of impounded
water on the beaver pond site were reflected in associ-
ated breeding bird populations. Total bird density in
the beaver pond habitat was three times that of the
adjacent riparian habitat. Similarly, our estimates of
total bird biomass, bird species richness, and bird spe-
cies diversity were 3.49, 3.25, and 1.67 times higher,
respectively, in the beaver pond habitat. Further, there
were more foraging and nesting guilds represented on
the beaver pond plot than elsewhere. Our findings
suggest that beaver pond ecosystems can provide
important habitats for nongame breeding birds.

Bird Populations in and
Adjacent to a Beaver Pond
Ecosystem in Idaho

Dean E. Medin
Warren P. Clary

INTRODUCTION

Beavers (Castor canadensis) alter riparian-stream
ecosystems through their woodcutting and dam-
building activities. The resultant habitats can be
beneficial to some forms of wildlife and detrimental
to others (reviewed in Hill 1982; Jenkins and
Busher 1979). But to date, little information is
available on the importance of beaver pond habitats
for nongame birds. This study compared breeding
bird populations and bird community organization
between a willow (Salix spp.)-dominated beaver
pond habitat and an adjacent nonwillow riparian
habitat on Summit Creek in east-central Idaho.

Beavers built both a primary dam and several
secondary dams in a previously unoccupied section
of Summit Creek in the summer and fall of 1979
(Lewis 1989). During the year of study (1989),
about one-third hectare of surface water was im-
pounded by the dams. Both the beaver pond com-
plex and the adjacent site were protected from live-
stock grazing for the previous 14 years by a large
(122-ha) fenced exclosure. The exclosure, con-
structed in late 1975, is on public lands admini-
stered by the Bureau of Land Management, U.S.
Department of the Interior.

STUDY AREA

The study site was 41 km north of Mackay in east-
ern Custer County, ID. Elevation is about 1,975 m.
Summit Creek, a tributary of the Little Lost River,
originates from springs and flows through a broad,
basinlike valley bounded on the east by the Lemhi
Range and on the west by the Lost River Range.
Regional climate is semiarid. Average annual pre-
cipitation at Mackay (elevation 1,797 m) is 247 mm,
with peaks in May and June. The growing season
is short, averaging less than 100 days at Mackay
(USDC NOAA 1982). Microrelief in many parts
of the riparian area is hummocky, with soils high
in total salts (USDA SCS 1987). Except for the
beaver pond complex, the riparian area was seldom
more than 50 to 100 m wide.

Four major vegetation community types were de-
fined on the study site. These were: willow/mesic
herbaceous, sagebrush (Artemisia spp.)/upland, mat
muhly (Muhlenbergia richardsonis)/hummock, and
mesic herbaceous. The upland communities occu-
pied the gentle slopes and terraces that bordered
both the beaver pond complex and the nonponded
area. The willow/mesic herbaceous community type
was found only in the immediate vicinity of the bea-
ver pond. The mat muhly/hummock, and mesic
herbaceous communities were considered elements
of the riparian zone and were found on both the
willow and nonwillow study sites.

Upland vegetation was shrub-steppe. The most
common shrubs were low sagebrush (A. arbuscula),
threetip sagebrush (A. tripartita), and green rabbit-
brush (Chrysothamnus viscidiflorus). The under-
story included Sandberg’s bluegrass (Poa sandber-
gii), bluebunch wheatgrass (Agropyron spicatum),
and long-leaf phlox (Phlox longifolia). The willow/
mesic herbaceous community characterized the
beaver pond complex. Geyer willow (S. geyeriana),
bilberry willow (S. myrtillifolia), Bebb willow (S.
bebbiana), and water birch (Betula occidentalis)
formed the tall-shrub overstory. The understory
included a wide variety of graminoids and forbs.
The stream was closely bordered by clumped mesic
herbaceous communities dominated by beaked
sedge (Carex rostrata), water sedge (C. aquatilis),
Baltic rush (Juncus balticus), and Kentucky blue-
grass (P. pratensis). Hummocky areas, dominated
by mat muhly and thick-spiked wheatgrass (A.
dasystachyum), were generally located in an inter-
mediate position between the streamside communi-
ties and the sagebrush uplands.

METHODS

Two 9-ha plots, one in the upper (westernmost)
section of the exclosure and the other in the beaver
pond (downstream) area, were censused for breed-
ing birds using the Williams’ spot-map method
(International Bird Census Committee 1970). Plot
locations were selected to best represent both the

beaver pond (willow) habitat and the adjacent non-
willow riparian habitat. Nearly half of the plot en-
compassing the beaver pond habitat was made up of
willow/mesic herbaceous communities. The census
plots, 600 by 150 m, were oriented lengthwise along
Summit Creek and included both sides of the creek.
Plots were surveyed and gridded with points flagged
and marked with numbered stakes at 25-m intervals.

Eleven census visits were made to each plot from
May 17 to June 29, 1989. Most of the bird registra-
tions were recorded from sunrise to early afternoon
when birds were most active. To ensure complete
coverage, the plot was censused by walking within
25 m of all points on the grid. Census routes were
varied. Recorded bird observations extended a mini-
mum of 50 m beyond plot boundaries.

At the end of the sampling period, clusters of
observations on species maps were circled to define
breeding bird territories. Fractional parts of bound-
ary territories were included in the results. The recip-
rocal of Simpson’s index (D = 1/Yp;?, where p, is the
proportion of the sample belonging to the ith species)
was used to calculate species diversity (Hill 1973).

Vegetation and other features of the census plots
were measured from July 17 to August 30, 1989. A
50- by 50-cm (0.25-m?) quadrat was located at each of
20 systematically positioned sample locations in each
major vegetation community type on the study plots.
Canopy cover (Daubenmire 1959) was ocularly esti-
mated for the total of each plant life form (graminoid,

forb, shrub) and recorded as the midpoint of one of
eight percentage cover classes (0-1, 1-5, 5-10, 10-25,
25-50, 50-75, 75-95, 95-100 percent). Percentages

of litter, rock, bare ground, and lichen-moss were
similarly estimated. The vegetative height (excluding
flower and seed-head heights) of each graminoid, forb,
and shrub nearest the center of each quadrat was
recorded.

Biomass of graminoids, forbs, and small shrubs was
determined by clipping vegetation from ground level
upward within a vertical projection from the 0.25-m?
quadrats. Clipped materiais were bagged, ovendried,
and weighed. A 3- by 3-m.(9-m?) plot, concentric to
each 0.25-m? quadrat, was used to sample biomass
of large shrubs. Basal diameter, maximum height,
and species were recorded for each shrub stem rooted
within the plot. For willow clumps, average stem
diameter and average stem height were recorded
instead of individual stems. Equations provided by
Brown (1976) were used to estimate biomass of the
large shrubs.

Plant names follow Hitchcock and Cronquist (1973).
Bird nomenclature is from the 1983 AOU checklist
(American Ornithologists’ Union 1983).

RESULTS AND DISCUSSION

Major structural (physiognomic) differences in the
vegetation occurred between the beaver pond complex
and the adjacent riparian habitat (table 1). The most

Table 1—Vegetation and other features of a beaver pond complex (willow) and adjacent
(nonwillow) riparian habitat, Summit Creek, ID, 1989

Beaver pond Adjacent
(willow) (nonwillow)
Item Mean! SD Mean? SD P

Graminoid

Biomass (g/m?) 342 220 268 254 0.07

Canopy cover (pct) 60.8 22.9 61.6 30.5 86

Height (m) 29 15 18 .09 <.01
Forb

Biomass (g/m?) 23:¢ 29.3 24.9 29.8 82

Canopy cover (pct) 12.0 13.4 12.0 13.2 .99

Height (m) .08 .06 .07 .05 58
Shrub

Biomass (g/m?) 183 295 7a 164 <.01

Canopy cover (pct) 12.3 20.7 7.4 14.5 10

Height (m) 64 57 34 21 <.01
Other

Bare ground (pct) 15.7 14.3 20.3 25.0 21

Litter (pct) 13.0 12:33 10.4 11.5 ae

Rock (pct) 46 1.42 64 1.52 48

Lichen-moss (pct) 24 1.05 30 1.15 76

‘n = 80 except for shrub height mean where n = 79.
2n = 60 except for forb and shrub height means where n = 58 and 52, respectively.
3 Probability associated with unpaired ttests. P of 0.10 or less was considered significant.

2

evident difference was in the overstory layer where
shrub biomass, shrub canopy cover, and shrub
height means were higher on the beaver pond site.
Shrub height and shrub biomass values in the
beaver pond habitat were about twice those of the
nonponded area. Differences in the shrub compo-
nent were predominantly due to the abundance

of willows that were found throughout the beaver
pond complex. Photographic evidence shows a pro-
nounced enlargement of the willow stand since the
construction of the beaver dams (Anderson 1989).
Before dam construction, willows formed only a
narrow band along the stream.

Differences between the two areas were also found
in the herbaceous layer where graminoid biomass
and graminoid height means were higher in the
beaver pond habitat (table 1). The differences
seemed to be mostly due to thicker and taller stands
of grasses and sedges, predominantly beaked sedge,
that grew on the moist sites near the beaver pond.
Presumably, those responses were caused by a
higher water table near the beaver impoundments.
Forbs exhibited no differences in canopy cover,
height, or biomass means between study plots.

We recorded 13 breeding bird species on the
Summit Creek study plots (table 2). The most

Table 2—Density (number/ha), diversity, and other attributes of breeding bird populations in a beaver-
pond complex (willow) and adjacent (nonwillow) riparian habitat, Summit Creek, ID, 1989

Foraging
Species guild’

Killdeer GGI
(Charadrius vociferus)

Spotted sandpiper SGI
(Actitis macularia)

Common snipe GPV
(Gallinago gallinago)

Willow flycatcher ASI
(Empidonax traillii)

American robin GGV
(Turdus migratorius)

Yellow warbler LGI
(Dendroica petechia)

Common yellowthroat LGI
(Geothlypis trichas)

Vesper sparrow GFO
(Pooecetes gramineus)

Savannah sparrow GFO
(Passerculus sandwichensis)

Song sparrow LFO
(Melospiza melodia)

Red-winged blackbird GFO
(Agelaius phoeniceus)

Western meadowlark GGI
(Sturnella neglecta)

Brewer’s blackbird GFO

(Euphagus cyanocephalus)

Total number/ha
Biomass (g/ha)*
Species richness (n)

Species diversity°(1/Lp,’)

Density
Nesting Beaver pond Adjacent
guild? (willow) (nonwillow)

GRN 0.22 +9
GRN ae 0.00
GRN .09 .00
BTN 1.04 .00
BTN 07 .00
BTN Ti22 .00
GBN .09 .00
GRN 11 42
GRN 2.16 1.96
GBN oo .00
CRN .20 .60
GRN 27 40

GBN 4.09 +
10.11 3.38

422 121

we 4

4.21 2.52

' After DeGraaf and others (1985). GGI = ground gleaning insectivore, SGI = shoreline gleaning insectivore,
GPV = ground probing vermivore, ASI = aerial-sally feeding insectivore, GGV = ground gleaning vermivore,
LGI = lower-canopy gleaning insectivore, GFO = ground foraging omnivore, LFO = lower-canopy and ground

foraging omnivore.

After Harrison (1979). GRN = ground nester, BTN = bush and small tree nester, GBN = ground and bush
nester, CRN = cattail, rush, sedge, reed, grass, and bush nester.

3+ indicates the bird was observed infrequently.
‘Species weights from Dunning (1984).

*After Hill (1973). Here, p,is the proportional abundance of the ith species in the sample.

common species were the savannah sparrow (Pas-
serculus sandwichensis), Brewer’s blackbird (Eu-
phagus cyanocephalus), yellow warbler (Dendroica
petechia), willow flycatcher (Empidonax traillii),
and red-winged blackbird (Agelaius phoeniceus).
Collectively, those five species accounted for over
80 percent of the breeding bird community. Other
common species included the western meadowlark
(Sturnella neglecta), vesper sparrow (Pooecetes
gramineus), and song sparrow (Melospiza melodia).
Wide-ranging raptorial birds, although commonly
seen, were not included in our analysis. Transient
species were also excluded.

Breeding bird densities were markedly different
on the two study plots (table 2). Total bird density
in the beaver pond habitat (10.11 birds/ha) was
three times that of the adjacent riparian habitat
(3.38 birds/ha). Similarly, our estimate of total
breeding bird biomass was appreciably higher
(422 g/ha versus 121 g/ha) in the willow-dominated
beaver pond complex. Further, bird species rich-
ness and bird species diversity values were 3.25 and
1.67 times higher, respectively, in the beaver pond
habitat.

Total breeding bird density recorded in the beaver
pond habitat at Summit Creek exceeds densities
found in most North American rangeland habitats
(Wiens and Dyer 1975) and compares favorably with
those of coniferous forests throughout North Amer-
ica (Wiens 1978). But Johnson and others (1980)
list breeding bird densities in riparian wetlands,
especially cottonwood-willow habitats in the south-
western United States, that are substantially higher
than those we found at Summit Creek.

There were important organizational differences
between the two breeding bird communities of the
beaver pond complex and the adjacent riparian
habitat. Numerically, the dominant species on
the beaver pond area were Brewer’s blackbirds,

savannah sparrows, yellow warblers, and willow
flycatchers (table 2). Those four species made up

84 percent of the breeding bird community in the
beaver pond habitat. Shorebirds—killdeer (Charad-
rius vociferus), spotted sandpiper (Actitis macu-
laria), and common snipe (Gallinago gallinago)—
were present in low densities. They were found as
breeding birds only near the beaver pond; none bred
in the nonponded riparian habitat. The American
robin (Turdus migratorius) and common yellow-
throat (Geothlypis trichas) also bred only in the
beaver pond habitat. Of the four species breeding
in the nonwillow riparian.habitat, all were included
among the much larger number of species that bred
in the beaver pond habitat. Only the savannah
sparrow showed a clear pattern of numerical domi-
nance within the nonwillow habitat.

Birds may be placed in categories, or guilds,
based on similarities in their use of environmental
resources (Root 1967). The breeding bird species
listed in table 2 were organized into feeding and
nesting guilds based on their foraging behavior and
the substrates used for nesting (table 3). Birds that
forage or probe for food on the ground formed the
largest single category. That guild accounted for
66 percent of the breeding bird population in the
beaver pond habitat and 88 percent in the non-
willow riparian habitat. These, along with the
lower-canopy feeders, made up the largest fraction
of all feeding guilds in the beaver pond habitat.
Aerial-sally feeders were represented by a single
species—the willow flycatcher. Only ground feeders
were found in the nonwillow habitat.

Ground and ground-bush nesters occurred in simi-
lar densities in the beaver pond habitat (table 3).
Numerically, those species accounted for 75 percent
of the breeding bird population in the beaver pond
habitat. Bush or small tree nesters were repre-
sented by the willow flycatcher, yellow warbler,

Table 3—Guild structure of breeding birds in a beaver pond complex (willow) and adjacent
(nonwillow) riparian habitat, Summit Creek, ID, 1989

Beaver pond Adjacent
Guild (willow) (nonwillow)
Number/ha Percent Number/ha Percent

Foraging

Ground-shoreline gleaning 0.78 8 0.40 12

Ground foraging-probing 6.65 66 2.98 88

Aerial-sally feeding 1.04 10 0. 0)

Lower canopy gleaning-foraging 1.64 16 0 0
Nesting

Ground nesting 3.07 30 2.78 82

Ground-bush nesting 4.51 45 0 0

Bush-small tree nesting 2.33 23 0 0

Cattail, rush, sedge nesting .20 2 .60 18

and American robin. Most of the birds in the non-
willow habitat nested on the ground. The only ex-
ception was the red-winged blackbird, which we
found nesting in tall, thick sedge communities near
the stream.

Of the 13 species breeding in the willow-
dominated beaver pond habitat, six (willow fly-
catcher, American robin, yellow warbler, song spar-
row, red-winged blackbird, and Brewer’s blackbird)
nest in shrubs and feed on the ground, in bushes, or
in the air. Each of those species is oriented to de-
ciduous riparian plant communities for reproductive
and feeding activities (Thomas 1979). The killdeer,
spotted sandpiper, and common snipe nest on the
ground, usually but not necessarily near water
(Harrison 1979). At Summit Creek, we observed
them most often near the beaver pond in mesic her-
baceous communities dominated by grasses, sedges,
and rushes. Of the four species breeding in the
adjacent nonwillow habitat, three (vesper sparrow,
savannah sparrow, and western meadowlark)
nested and fed on the ground. Each is oriented
to dry meadow plant communities for reproduction
and feeding (Thomas 1979). In general, our obser-
vations of the habitats used by the birds breeding
on the Summit Creek site generally support those
reported by Douglas and Ratti (1984) for the
Centennial Mountains of eastern Idaho.

Among the species found on the two study plots,
only the American robin and Brewer’s blackbird
were assigned medium or high versatility ratings
with respect to the number of plant communities
used for reproductive or feeding activities; all others
were given low versatility ratings by Thomas (1979).
Five of the species found in the beaver pond complex
at Summit Creek—the common snipe, willow fly-
catcher, yellow warbler, common yellowthroat, and
song sparrow—appear ecologically dependent upon
riparian vegetation for breeding habitat (Knopf and
Samson 1988).

CONCLUSIONS

We found sharply defined structural (physiog-
nomic) differences in vegetation between a willow-
dominated beaver pond complex and an adjacent
nonwillow riparian habitat. Differences were most
pronounced in the overstory layer where average
values of shrub biomass, shrub height, and shrub
canopy cover were significantly higher on the beaver
pond site. Those differences, plus the availability of
impounded water, were reflected in the composition
and density of associated breeding bird populations.
Our estimates of total breeding bird density, bird
species richness and diversity, and total breeding
bird biomass were markedly higher in the beaver
pond habitat. Further, more foraging and nesting

guilds were represented in the beaver pond complex
than in the adjacent riparian habitat. Our findings
suggest that beaver pond ecosystems can provide
important habitats for nongame breeding birds in
the Western United States.

ACKNOWLEDGMENTS

We gratefully acknowledge the field and office
assistance of John W. Kinney, Sherri A. Brown,
Justine L. Wirch, Kevin Tom, and Patrick V.
Turner. Lyle A. Lewis, Caryl Elzinga, Loren D.
Anderson, and Gloria Romero provided unpublished
information about the study area and grazing
history.

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Medin, Dean E.; Clary, Warren P. 1990. Bird populations in and adjacent to a beaver pond
ecosystem in Idaho. Res. Pap. INT-432. Ogden, UT: U.S. Department of Agriculture,
Forest Service, Intermountain Research Station. 6 p.

Breeding bird populations and community organizations were compared between a
beaver pond habitat dominated by willows and an adjacent nonwillow riparian habitat.
Total bird density in the ponded habitat was three times that of the nonponded area. Bird
biomass, bird species richness, and bird species diversity were 3.49, 3.25, and 1.67 times

higher, respectively, in the beaver pond habitat.

KEYWORDS: breeding birds, density, diversity, standing crop biomass, guilds