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Cavity-Nesting Bird Requirements and Response to
Snag Cutting In Ponderosa Plne1
Virgil E. Scott and John L. Oldemeyerl
Abstract.--Cavity-nesting bird densities declined 53%
when conifer snags were removed during a timber harvest on
the Apache-Sitgreaves National Forest in Arizona. On an
adjacent cutover area where snags were left standing the
cavity-nesting bird population increased 25%. Cavitynesting bird densitites on an unharvested control plot
increased 32%. Birds that nested in ponderosa pine snags
were affected most by snag removal. Violet-green
swallows2 declined from 41 to 4 birds/100 acres after
snags were removed and pygmy nuthatches declined from 32 to
15/100 acres. Two species responded significantly to the
reduction in basal area: white-breasted nuthatches
decreased and house wrens increased.
For nesting sites, cavity-nesting birds usually
selected ponderosa pine (Pinus ponderosa) snags that were
greater than 18 inches diameter breast high (dbh), dead 6
years and with at least 40% bark cover.
:.tNTRODUCTION
snags (Bull 1975, Gale 1973, McClelland and
Frissel 1975, Thomas et al. 1979) •. Mannan et al.
(1980) reported that the density of hole-nesting
birds was positively correlated with the mean dbh
of snags on their study area in western Oregon.
However, no general statement can be made for ali
birds since size of snags used for nest hole
excavation varies with tree and bird species (Bull
et al. 1980, Lanning and Shiflett 1983, Mannan et
al. 1980, McClelland et al. 1979, and Scott et al.
1980). This report summarizes the characteristics
of snags used by cavity-nesting birds in a forest
dominated by ponderosa pine in Arizona and the
response of birds to snag removal.
Snags serve in a variety of ways as wildlife
habitat. Cavities made by primary cavity nesters
(those birds that excavate their own holes) provide nests and denning sites for secondary cavity
users (those that nest in cavities but are unable
to excavate holes) such as bats, squirrels, and
several birds (Thomas et al. 1979). Snags are
used by raptors, flycatching birds, and bandtailed pigeons (Columba fasciata) for such activities as hunting, feeding, loafing, or roosting
(Scott et al~ 1980). Invertebrates living in the
dead wood and under bark provide food for some of
the woodpeckers.
It has been shown that snags with heartrot
(Fomes _f!f,•) are selected by primary cavity-nesting
birds for excavating nest holes (McClelland and
Frissel 1975, Miller and Miller 1980), and that
larger snags are used more frequently than smaller
STUDY LOCATION AND METHODS
The 124-acre study area was in an oldgrowth
ponderosa pine forest five miles west of Greer,
Arizona, on the Apache-Sitgreaves National
Forest. Ponderosa pine accounted for 83% of the
tree stand which also contained Douglas-fir
(Pseudotsuga menziesii) and limber pine (~
flexilis). Several small aspen (Populus
tremuloides) clones were interpersed throughout
the stand. The study area was divided into three
plots. Plots A and B were selectively harvested;
snags were cut on Plot A but not on Plot B. Plot
C served as an uncut control. Eight bird surveys
were conducted on each plot between 20 May and 30
June for 2 years before harvest and 2 years after
1 Virgil E. Scott and John L. Oldemeyer are
wildlife biologists (research), u.s. Fish and
Wildlife Service, Denver Wildlife Research Center,
stationed at Fort Collins, Colorado.
2 Bird nomenclature follows the Thirtyfourth Supplement to the American Ornithologists'
Union Checklist of North American Birds.
Supplement to the Auk. Vol 99, No. 3, July 1982
and scientific names appear in Table 1.
19
increase in ground nesting birds was related to
reduction in basal area on Plots A and B.
harvest. Bird densities were determined by the
spot map method (Kendeigh 1944) and compared by
analysis of variance.
Density of cavity-nesting birds was 53% lower
on Plot A after timber harvest whereas density
increased on Plots B and c. Some bird species
within the cavity-nesting guild responded independently but the overall response in density was
positively correlated (re0.72) to the number of
snags in the plots.
Basal area of live trees ~five inches dbh was
determined by the plotless method (Grosenbaugh
(1952). Trees were harvested in late summer and
fall 1974 after bird surveys were completed. All
conifer snags on the study area were measured and
examined for use by cavity-nesting birds before
timber harvest occurred.
Some birds occurred in numbers too small or in
too few plots and no inferences could be made
about the effect of timber harvest on their density (i.e., American kestrels, yellow-bellied
sapsuckers, downy woodpeckers, hairy woodpeckers,
three-toed woodpeckers, brown creepers, and mountain bluebirds). We found nests of American
kestrels, three-toed woodpeckers, and violet-green
swallows only in ponderosa pine snags; whereas,
nests of Williamson's sapsuckers and hairy and
downy woodpeckers were found only in aspen. Scott
et al. (1980) reported violet-green swallows
nesting in both aspen and conifer snags. We also
found northern flickers, mountain chickadees, and
white-breasted nuthatches nesting in aspen and
ponderosa pine snags. One pygmy nuthatch was
observed nesting in an aspen snag on Plot A after
snag removal.
RESULTS AND DISCUSSION
Basal area of live trees ~five inches dbh
before timber removal was 110, 107, and 102
feet2/acre on Plots A, B, and C respectively
(Scott 1979). Post harvest basal area on A and B
was 51 and 64 feet2/acre respectively. Basal
area in Plot C was unchanged.
There were 4.6 conifer snags/acre 2:_1 inches
dbh in Plot A before harvest and all were cut
during timber harvest and most were left on the
ground. After timber harvest 14 aspen snags ~5
inches dbh (0.3/ac) remained. On .Plot B there
were 6.1 conifer snags/acre before harvest. Some
snags were accidentally knocked down during timber
harvest and some were blown over; 4.1 conifer
snags/acre remained. After timber harvest 29
aspen snags ~5 inches dbh (0.8/ac) remained on
Plot B. Plot C had 4.0 conifer snags/acre in 1973
and 3.6 in 1976; no aspen snags ~5 inches dbh
occurred in Plot c.
Densities of three species did not respond
with timber harvest: Williamson's sapsuckers,
mountain chickadees, and western bluebirds (table
1). Greatest densities of Williamson's sapsuckers
were found in the control plot both before and
after harvest, and densities were somewhat lower
in all plots after harvest. Mountain chickadees
responded in a similar manner, whereas western
bluebirds increased slightly, but not significantly so, after harvest in all plots. The
remaining aspen (trees and snags) probably
provided nesting habitat for these species. We
observed one pair each of pygmy nuthatch,
Williamson's sapsucker, and western bluebird nesting in one aspen snag on Plot A after timber harvest. Western bluebirds usually nest near openings or in more open woodlands and the timber
harvest created this type of habitat.
Birds usually selected ponderosa pine snags
greater than 18 inches dbh, those had been dead 2:_6
years, and those with at least 40% bark cover for
nest sites and 71% of the snags meeting these
criteria had holes made by cavity nesters. Twelve
percent of the snags ~ 8 inches dbh and dead ~ 5
years had holes whereas 62% of those dead ~ 6
years had been used by cavity-nesting birds.
Nearly all of the cavity nests found in ponderosa
pine were in snags. Those cavity nests found in
living trees were in dead portions of the tree
(ie. dead tops or a dead strip resulting from
lightening strikes). Larger snags tended to have
more holes and were used more frequently than
smaller snags. Tall snags (>45 feet) were used at
about the same frequency as shorter ones (<45
feet) but more holes were present in the taller
snags. There were 2.6 snags/acre with cavities
and 75% of the snags containing cavities were ~19
inches dbh. Holes were present in 28% of
snags <19 inches dbh and 54% of those2:.19 inches.
Two species responded significantly to the
reduction of basal area (table 2). White-breasted
nuthatches decreased significantly in Plots A and
B, where post-harvest density averaged 20% of the
pre-harvest density. Density of western
flycatchers followed a similar, though nonsignificant, pattern to white-breasted
nuthatches. Although white-breasted nuthatches
are cavity nesters, their decrease was probably
due to the decrease in standing timber. House
wrens, on the other hand, increased in Plots A and
B after harvest where its post-harvest density
averaged 222% greater than pre-harvest density.
This increase was probably due to the increase in
slash piles or brush found in the cutover areas.
Densities of all birds before harvest ranged
from 225/100 acres on Plot C to 303/100 acres on
Plot B (table 1) and were not different (P~0.05)
among plots. Within treatments, the posttreament
bird densities were not different from pretreatment. Density of foliage-nesting birds did not
change after harvest. Ground-nesting birds
increased on all plots but the percentage increase
was least on the Plot c. Most of the numerical
Three species decreased significantly on Plot
A with the reduction in snag density (table 2);
northern flicker, violet-green swallow, and pygmy
20
Table I.--Number of cavity-nesting birds/100 acres in a
ponderosa pine study area before and after timber
harvest. Densities are means of surveys conducted
2 years before and 2 years after timber harvest.
Comparisons were made by ANOVA (NE a not examined,
NS = non-significant, S a significant P ~ 0.05).
Results
of ANOVA
Plot A
Timber Harvested
Snags Cut
Before
After
Plot B
Timber Harvested
Snag Left
Before
After
Plot C
Timber
Not Harvested
Before
After
<1
3
5
0
0
2
<1
3
3
0
0
NS
9
4
8
5
10
7
NE
0
2
1
0
1
<1
NS
5
3
<1
4
2
7
NE
3
<1
3
3
2
<I
s
10
5
9
12
11
15
Western flycatcher
(Empidonax difficilis)
s
7
2
11
8
10
10
Violet-green swallow
(T.achycineta thalassina)
s
41
4
50
77
29
46
Mountain chickadee
(!!.!:!!!. gambeli)
White-breasted nuthatch
(Sitta carolinensis)
NS
11
9
14
11
15
10
s
7
2
8
<1
10
7
s
32
15
37
45
27
40
Brown creeper
(Certhia americana)
NE
2
1
5
<1
0
7
House wren
(Troglodytes aedon)
s
8
11
1
18
0
<1
Western bluebird
(Sialia mexicana)
NS
2
5
5
7
2
5
Mountain bluebird
<!• currucoides)
Cavity nesters
Foliage nesters
Ground nesters
Total Birds
NE
<1
2
9
11
0
3
s
139
105
31
275
66
83
51
200
167
98
38
303
209
129
67
405
119
81
25
225
157
79
48
284
American kestrel
(Falco sparverius)
NE
Yellow-bellied sapsucker
(Sphyrapicus varius)
NE
Williamson's sapsucker
<!• thyroideus)
Downy woodpecker
(Picoides pubescens)
Hairy woodpecker
(!_• villosus)
Three-toed woodpecker
(!• tridactylus)
Northern flicker
(Oolaptes auratus)
Pygmy nuthatch
(!. pygmaea) ·
NS
s
.S
21
Table 2.--Density of birds (number/100 ac) related to
basal area and to snag density in a ponderosa pine
forest in Arizona. Contrasts based on Scheffes'
procedure.
Densitz: related to basal area
Northern flicker
Western flycatcher
Violet-green swallow
Mountain chickadee
White-breasted nuthatch
Pygmy nuthatch
House wren
Western bluebird
Cavity nesters
Foliage nesters
Ground nesters
Densitz: related to snag density
Plots A, B, and C
before harvest and
C after harvest
(hish basal area)
Plots A and B
after harvest
(low basal area)
Plot A after
harvest
(snags removal)
11.3
9.5
41.5
12.5
8.o
34.0
2.5
3.5
145.5
90.8
35.5
8.5
5.0
40.5
10.0
1.5 *
30.0
14.5 *
6.0
137.5
106.0
59.0
5.0
2.0
4.0
9.0
2.0
15.0
11.0
5.0
66.0
83.0
51.0
Plots B and C
after harvest
(snags not removed)
13.5
9.0
61.5
10.5
4.0
42.5
9.5
6.0
183.0
104.0
57.5
*
*
*
*
*Probability of observing differences in means of this magnitude were~0.05%.
nuthatch. Violet-green swallows and pygmy
nuthatches depended heavily on ponderosa snags for
nest sites. No other species followed an increasing or decreasing trend that seemed related to
removal of snags; however, the cavity nesting
guild decreased significantly when the density in
Plot A (with snag removal) was compared to the
post-harvest densities of Plots B and c. Cavitynesting birds were affected by removals of conifer
snags on Plot A although there were 0.3 aspen
snags/acre ~5 inches after timber harvest. Those
birds that nested in ponderosa pine snags before
harvest (violet-green swallows, 'pygmy nuthatches,
and three-toed woodpeckers) were most affected by
snag removal.
than 23 inches dbh. We found that 75% of the
snags with holes were ~19 inches dbh. Boles were
present in 28% of the snags ~18 inches dbh
whereas 54% of those snags ~19 inches dbh had
holes. For those snags that had been used by
birds, the larger snags also had more holes/snag
(3.8) than the smaller snags (2.8). Larger snags
also remain standing longer than smaller snags
(Cline 1977). If two to three snags/acre are
accepted as a management goal in the ponderosa
pine type, then all should be ~19 inches dbh in
order.to fulfill the cavity-nesting bird's
requirements. Trees with dead tops or dead portions in the trunk provide nesting sites for
cavity nesters and if retained in the forest, can
provide cavity nesting opportunities over a long
period of time.
MANAGEMENT SUGGESTIONS
Cavity-nesting bird nest site requirements
vary with the species since the larger birds
physically require larger snags for hole excavation than do the smaller birds. Bull and Meslow
(1977) suggested that snags ~20 inches dbh should
be retained for pileated woodpeckers (Dryocopus
pileatus). Smaller birds, such as chickadees
(Parus sp.), can use snags as small as six inches
dbh (Scott et al. 1980).
Literature Cited
Balda, Russell P. 1975. The relationship of
secondary cavf.ty-nesters to snag densities in
western coniferous forests. USDA. For. Serv.,
Southwest Reg. Wild!. Habitat Tech. Bull. 1,
Albuquerque, NM, 37 P•
Bull, Evelyn Louise. 1975. Habitat utilization
of the pileated woodpecker, Blue Mountains,
Oregon. M.s. Thesis, Ore. State Univ.,
Corvallis, 58 p.
Bull, Evelyn L., Asa D. Twombly, and Thomas M.
Quigley. 1980. Perpetuating snags in managed
mixed conifer forests of the Blue Mountains,
Oregon. Pp 325-336 in R.M. DeGraff and N.G.
Tilgham, eds. Management of western forests
and grasslands for nongame birds. USDA For.
Serv. Gen. Tech. Report, Intermt. For. and
Range Exp. Stn. Ogden, Utah. 535 P•
Balda (1975) suggested that 2.7 snags/acre are
necessary to maintain maximum densities and
natural species diversity of secondary cavity
nesters in the ponderosa pine forest. Cunningham
et al. (1980) found that 2.1 snags/acre might be
sufficient for secondary cavity nesters at natural
levels. We found evidence of cavity nesting or
roosting in 2.6 snags/acre in this uncut ponderosa
pine forest which should have been at natural
levels. Cunningham et al. (1980) reported that
75% of the nests located were in snags greater
22
McClelland, B. Riley, and Sidney s. Frissell .
1975. Identifying forest snags useful for
hole-nesting birds . J . For. 73(1):414-417.
McClelland, B. Riley, Sidney s. Frissell, William
c. Fisher and Curtis H. Halvorson . 1979 .
Habitat management for hole- nesting birds in
forests of western larch and Douglas-fir . J ,
For. 77(8):480-483.
Miller, Eileen, a nd Donald R. Miller . 1980. Snag
use by birds. Pp . 337-356 i n R.M. DeGraff and
N.G. Tilgham, ed s. Management of western
forests and grasslands for nongame birds .
USDA For . Serv. Gen. Tech . Repo r t. Intermt.
For. and Range Exp. Stn . Ogden, UT. 535 p.
Scott, Virgil E. 1979. Bird response to snag
removal in ponderosa pine. J. For.
77(1) : 26- 28.
Scott, Vi r gil E., Jill A. Whelan, a nd Peggy L.
Svoboda . 1980 . Cavity- nesting birds and
fores t management. Pp 311- 324 in R.H.
DeGraff, and N. G. Tilgham, eds.--Management of
western forests and grasslands for nongame
birds . USDA For . Serv . Gen. Tech. Report .
Intermt . For . and Range Exp . Stn., Ogden ,
Utah. 535 p .
Thomas, Jack Ward, Ralph G. Anderson , Chris Maser,
and Evelyn L. Bull . 1979 . Snags. Pp 60-77
in J . W. Thomas, ed . Wildlife habitats in
managed forests--the Blue Mountains of Oregon
and Washington . USDA For. Serv . , Agric .
Handb . No . 553 . US Print. Off., Wash. , D.c .
512 P •
Bull, Evelyn L. and E. Charles Meslow. 1977.
Habitat requireme nts of the pileated
woodpecker in northeastern Oregon. J.
Fo r es try 77(6):335- 337.
Cline, Steven Paul . 1977. The characteristics
and dynamics of snags in the Douglas-fir
forests of the Oregon coast range. M. S.
thesis. Oregon State Univ . , Corvallis, 106 p.
Cunningham, James B., Russell P. Balda, and
William s. Gaud. 1980. Selection and use of
snag by secondary cavity- nesting birds of the
po nde r osa pine fo re s t. USDA. For . Serv . Res.
Pap. RM- 222. Rocky Mountain For. and Range
Exp . Stn., Fort Collins , 15 p .
Gale, Robe rt M. 1973. Snags, chainsaws and
wildlife: one aspect of habitat management.
1973 Cal- Neva Wildl. Trans . :97-111.
Grosenbaugh, L. R. 1952. Pl otless timber
estimates - new, fast, easy. J. For .
50:32-3 7.
Kendeigh , s. Charles. 1944. Measurements of bird
populations. Ecol . Monog. 14:67-106.
Lanning, Dirk v., and James T. Shiflett. 1983.
Nesting Ecology of thick- billed parrot s .
Condor 85 : 66- 73.
Mannan, R. William, E. Charles Meslow , and Howard
M. Wright . 1980 . Use of snags by birds in
Douglas-fir forests, wester n Oregon. J,
Wild! . Manage . 44(4):787-797 .
23