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Decay Characteristics of Plleated Woodpecker
Nest Trees 1
Roger D. Harris 2
I analyzed decay and mortality characteristics of 42 pileated woodpecker
(Dryocopus pileatus) nest trees in California. In addition to examining
standing trees and wood chips from excavations, eight felled trees were
sectioned to determine internal decay characteristics. All cavities in
Pinaceae and hardwood trees were associated with decay. Chips excavated
by pileated woodpeckers from undecayed softer wood were comparable in
hardness to chips from harder wood that had decayed. Pileateds selected
a nest cavity site on the basis of the structural quality of the wood
and not just the fact that the tree was dead. Management recommendations
stress the need for reserving hard snags and allowing for recruitment.
Creation of snags by artificial means is not recommended.
INTRODUCTION
trees were available. Sumner and Dixon (1953) alSo
reported pileated woodpecker nest cavities in live
wood in the Sierra Nevada.
To make management decisions consistent with
the retention of nesting habitat for pileated
woodpeckers (Dryocopus pileatus), it is necessary
to know their requisites for mortality and decay of
nest trees. Conflicting results have been reported
over whether pileated woodpeckers require decayed
wood in which to excavate nest cavities. Conner et
al. (1976) cultured wood samples from six pileated
woodpecker nests (five in hardwoods and one in a
conifer) and found evidence of decay fungus in
each. However, Miller et al. (1979) examined wood
chips from nine nests excavated by pileated
woodpeckers in Pinaceae trees and found only four
with signs of decay. They also sectioned and
examined for decay two of the nest snags. Both
cavities were excavated in sound wood, although
limited areas of sapwood decay were present
elsewhere in the trees. McClelland (1977) found
chips from 50S of 22 nest excavations by pileated
woodpeckers in Pinaceae trees with obvious evidence
of heartwood decay.
This study attempts to resolve some of these
conflicting results. Through an analysis of field
data on pileated woodpeckers in California, I show
that neither mortality nor decay necessarily has to
be present as along as the wood is soft enough to
be excavated. However, in all but tree species
with the softest woods, pileated woodpeckers
excavate nest cavities in dead portions that have
been softened by decay.
METHODS
A search for active pileated woodpecker nest
sites was made throughout California as part of a
larger study on the nesti~g ecology of the species
(Harris 1982). I found etghteen nests in 1979 to
1983. Also, over 350 letters were sent in spring
1980 to birders, land managers, forest owners, and
biologists soliciting reports of recent nests. I
accepted reports of 2~ nest sites in which it was
determined that eggs or nestlings had been present;
pileated woodpeckers make false starts on nest
holes and abandon nest cavities before laying (Bull
1975, McClelland 1977).
Furthermore, Conner (1973:36) suggested that
pileated woodpeckers apparently lacked the ability
to make nest cavities in completely live trees.
Yet, Conner et al. (1975) reported that the
pileated often excavated through living hardwood
cambium to reach a decayed heartwood core. Munro
(In Bent 1939:192) observed pileated woodpeckers in
British Columbia "cutting" into green cottonwood
and popular, presumably Populus trichocarpus and!·
tremuloides. Carriger and Wells (1919) likewise
reported "several" pileated woodpecker nests in
live aspens in California, where "plenty" of dead
I examined all trees for decay
characteristics. Hardness of tree was estimated by
making a "standardized" thrust of a knife into the
tree at breast height and atso just below the nest
hole, where bark (if present) had been removed, and
noting penetration (for a 5-cm blade, very hard =
<1 em , hard = ~ 1 em and < 2 • 5 em , soft = ~ 2. 5 em
and< 5 em, rotted = 5 em). The test required
equipment readily available in the field, and
results were replicable. Top condition, limb
condition, and percent of bark were estimated
visually.
1Paper presented at the Snag Habitat
Management Symposium [Northern Arizona University,
Flags~aff, 7-9 June 1980].
Roger D. Harris was a graduate student,
Department of Forestry and Resource Management,
University of California, Berkeley. Presently he
is Director, Flat Rock Brook Center for
Environmental Study, Englewood, N.J.
Causes of tree mortality could not be
determined reliably because, for example, fungus
may have infected snags after death, masking the
125
Table 1.--Tree species used for nesting by pileated
woodpeckers.
original agent(s) of mortality. In addition, tree
mortality is usually a result of a combination of
abiotic and biotic stress factors (Bega 1978).
I collected wood chips excavated by
woodpeckers from 22 current nest cavities.
Experienced wood technologists examined breakage
patterns of wood chips to determine soundness.
Presence of decay was noted based on fungal
remains, rot, and discoloration. If wood chips
were judged to be both sound and without
macroscopic evidence of decay, they were further
examined microscopically for fungal tubes, fungal
remains, and rot.
No attempt was made to culture wood chips to
identify fungal species. It cannot be reliably
determined whether the cultured fungal species was
initially present at the time the chips were
excavated or had subsequently infected the wood
while it was on the ground.
Tree 1
Species
N
sp gr2
CR 3
BC 3
QA3
Gs4
WF
WA
pp
RF
AE
D-F
2
1
2
2
13
1
9
2
1
4
1
2
2
0.300
0.315
0.344
0.350
0.365
0.368
0.370
0.372
0.421
0.444
0.520
0.529
0.537
B~
CBO
HAD
~
Snags
~
50
100
0
0
100
100
100
100
100
100
0
0
100
Broken
Tops
50
100
0
0
69
100
100
100
100
100
0
50
100
1
cR =Coast Redwood (Sequoia sempervirens), BC
=Black Cottonwood (Populus trichocarpus), QA =
Quaking Aspen (Populus tremuloides), GS =Giant
Sequoia (Sequoia gigantium), WF =White Fir <!2!!!
concolor), WA =White Alder (Alnus rhombifolia), PP
= Ponderosa Pine (~ ponderosa>, RF = Red Fir
(Abies magnifica), AE =American Elm (Ulmus
americana), D-F =Douglas-fir (Pseudotsuga
menziessii), BLH =Big Leaf Maple (Acer
macrophyllum), CBO =California Black Oak (Quercus
kellossii), MAD= Hadrone (Arbutus menziessii).
2
Hardness of wood is based on specific gravity
(sp gr) of green wood (Cockrell et al. 1971,
Markwardt and Wilson 1935).
I also examined nest trees for internal decay
characteristics and evidence of insect activity.
Four nest trees were felled in late August 1980,
well after the nestlings had abandoned their nests
of the previous spring. Trees were cut
.
horizontally into 2-m sections, nest cavities were
sectioned vertically, and horizontal cuts were made
immediately above and below the nest cavities. An
additional four nest trees fell naturally in
winter storms, and were also examined in the same
manner as above for internal decay characteristics.
Time since death of Pinaceae nest snags was
estimated by the criteria of Cline et al. (1980).
Three age classes were delineated: young snags
(estimated to be recently killed) had either intact
or broken tops, twigs or needles present, >90~
bark, and very hard wood; moderate-aged snags
(estimated to be 4 to 8 years old) had twigs or
branches present, very hard or hard wood, and >501
bark remaining; old snags (estimated to be older
than 8 years) had broken tops, no twigs present,
very hard or hard wood, and <50~ bark remaining.
3Tree species in which pileated woodpeckers
have been known to excavate nest cavities through
green cambium.
4
Tree species in which pileated woodpeckers
excavated a nest cavity in undecayed wood.
All pileated woodpecker nest trees in Pinaceae
were snags. By snag age class, 32~ were young, 461
moderate-aged, and 21~ old. Fifty-six percent of
the ponderosa pines, but none of the white firs
were old.
RESULTS
Pileated woodpeckers demonstrated plasticity
in their choice of nest tree species, representing
a range of green wood specific gravities which are
indicative of wood hardnesses (Table 1). By the
knife test, none of the trees were soft or rotten.
Decay Characteristics of Nest Trees
Wood chips collected from current nest
excavations in one coast redwood, one California
black oak, one madrone, one red fir, seven
ponderosa pines, and nine white firs all revealed
fungal infection and decay. Excavation chips from
a living giant sequoia and the previously described
coast redwood showed no evidence of decay or fungal
infection under microscopic examination.
All Pinaceae nest snags had broken tops,
except for four white firs with intact tops. (See
table 1 for scientific names of trees.) Half of the
coast redwoods, neither of the giant sequoias, and
601 of the hardwood nest trees had broken tops.
Tree Mortality ,
Chips excavated by pileated woodpeckers from
undecayed softer woods were comparable in hardness
to chips from harder woods that had undergone decay
(Table 1). Specific gravity of wood chips from
dead ponderosa pine and western larch (Larix
occidentalis) trees used by pileated woodpeckers
for nest cavities was tested by Bull (1975). She
calculated a mean specific gravity of 0.34 for dead
In seven of the eight live trees, nests were
excavated in dead portions of the bole. In the
eighth tree, an entirely living coast redwood, the
cavity appeared to have been excavated through
green cambium into sound heartwood. The nest hole
was in an area of clear wood, not the site of a
broken limb.
126
and decayed ponderosa pine and western larch,
comparable to specific gravities of 0.30, 0.32, and
0.34 for green coast redwood, black cottonwood, and
quaking aspen respectively (Markwardt and Wilson
1935).
configuration of decay in the heartwood. Three of
the eight cavities showed evidence of insect
galleries around entrance holes and corridors.
Sapwood surrounding the nest cavities of all eight
felled trees was sound by the knife test and visual
inspection. Of the nest trees that had fallen by
natural causes, none had broken at the level of the
nest cavity, attesting to the relative strength of
the sapwood surrounding the cavity.
Decay patterns were patchy on nest trees used
by pileated woodpeckers (Fig. 1). Decay
characteristics around the nest cavity were not the
same as those at breast height, nor was decayed
wood distributed in continuous columns for the five
white firs and three ponderosa pines examined.
DISCUSSION
Apparently sound and even green wood of softer
tree species was used before decay had set in. In
tree species with wood harder than coast redwood,
black cottonwood, or quaking aspen, nest cavities
were excavated by pileated woodpeckers after the
heartwood had been softened by decay.
Cavity chambers were excavated through decayed
heartwood. Chamber shapes followed the
Broken tops, even on live trees, may serve as
entry courts for heart rot fungus (Bega 1978). The
high incidence of pileated woodpecker nest trees
with broken tops suggests the birds may select for
broken topped ones because of their decay
characteristics.
22.4
20.4
Sound sapwood around the cavity chambers
provides structural support for the nest and may
make it less susceptible to predators (Kilham 1971,
Conner 1977). Regardless of tree species, no nest
snag was soft or rotten from advanced decay. Such
trees apparently cannot support the large nest
cavities of pileated woodpeckers. Mean and
standard error for 7 nest chambers were 24.0+1.1 om
by 40.3~1.9 om (Harris 1982). Old white fir-snags
were not used by pileated woodpeckers, as these
less resinous and, therefore, less decay resistant
tree species (Cline et al. 1980) either became too
soft to be usable or fell down and became
unavailable.
18.4
16.4
14.4
12.4
10.4
Insect galleries around some nest hole
entrances and corridors suggest pileated
woodpeckers may first use the future nest snag as a
foraging site or that insect activity through the
otherwise sound sapwood facilitates excavation by
the woodpeckers. Neither hypothesis is mutually
exclusive.
Areas of fungal decay on nest trees_~ere
patchily distributed. Beca~se most Pinaceae snags
used by pileated woodpeckers in this study were
young or moderate-aged, fungal decay may not have
had sufficient time to spread more continuously.
In contrast, Conner (1978) in the southeast found
decay spread throughout the heartwood in nest snags
used by most woodpecker species. Miller et al.
(1979) in the northwest reported extensive decay
columns in most woodpecker nest trees. In both
these studies, trees with extensive decay were used
by woodpecker species less well adapted for
excavating hard wood than the pileated (Burt 1930).
8.4
6.4
4.4
2.4
0.4
TREE HT
Cm)
Managing for Pileated Woodpeckers
Figure 1.--Illustration of decay patterns (cross
hatching) on three horizontally crosssectioned ponderosa pine snags and a white fir
[third from left], Eldorado County, Calif.,
used by pileated woodpeckers for nesting.
Solid black arrows indicate nest hole height.
(Diameters are not drawn to scale.)
Pileated woodpeckers appear to choose a nest
cavity site on the basis of the structural quality
of the wood, particularly hardness, and not merely
the fact that the tree is dead. Creating snags by
topping, girdling, fungal inoculation, or silvicide
treatment has been suggested to enhance cavity
nesting bird habitat (Bull et al. 1980). I would
127
Bent, Arthur Cleveland. 1939. Life histories of
North American woodpeckers. U.S. National
Museum Bulletin 174, 33q p. Washington, D.C.
Bull, Evelyn Louise. 1975. Habitat utilization of
the pileated woodpecker, Blue Mountains,
Oregon. Masters thesis. 58 p. Oregon State
University, Corvallis.
Bull, Evelyn Louise, A. D. Twombly, and T. M.
Quigley. 1980. Perpetuating snags in managed
mixed conifer forests of the Blue Mountains,
Oregon. p. 325-336. In DeGraaf, R. M. and
N. G. Tilghman (editorS>, Workshop proceed.:
management of western forests and grasslands
for nongame birds. USDA Forest Service
General Technical Report INT-86, 535 p.
Intermountain Forest and Range Experiment
Station, Ogden, Utah.
Burt, W. H. 1930. Adaptive modifications in the
woodpecker. University of California
Publication in Zoology 32:q55-52q.
Carriger, H. W. and G. Wells. 1919. Nesting of
the northern pileated woodpecker. Condor
21:153-156.
Cline, S. P., A. B. Berg, and H. M. Wight. 1980.
Snag characteristics and dynamics in Douglasfir forests, western Oregon. Journal of
Wildlife Management qq:773-786.
Cockrell, Robert A., R. M. Knudson, and A. C.
Stangenberger. 1971. Mechanical properties
of southern Sierra old- and second-growth
giant sequoia. Agricultural Experiment
Station Bulletin 85q, 9 p. Berkeley, Calif.
Conner, Richard N. 1973. Woodpecker utilization
of cut and uncut woodlands. Masters thesis.
82 p. Virginia Polytechnical Institute,
Blacksburg.
Conner, Richard N. 1977. The effect of tree
hardness on woodpecker nest entrance
orientation. Auk 9q:369-370.
Conner, Richard N. 1978. Snag management for
cavity nesting birds. p. 120-128. In
DeGraaf, R. M. (editor), Management of
southern forests for nongame birds. USDA
Forest Service General Technical Report SE-1q,
175 p. Southeast Forest Experiment Station,
Asheville, N.C.
Conner, Richard N., J. G. Dickson, and B. A. Locke.
1981. Herbicide-killed trees infected by
fungi: potential cavity sites for woodpeckers.
Wildlife Society Bulletin 9:308-310.
Conner, Richard N., R. G. Hooper, H. S. Crawford,
and H. s. Mosley. 1975. Woodpeckers nesting
habitat in cut and uncut woodlands in
Virginia. Journal of Wildlife Management
39:1qq_150.
Conner, Richard N., 0. K. Miller, Jr., and c. s.
Adkisson. 1976. Woodpecker dependence on
trees infected by fungal heart rots. Wilson
Bulletin 88:575-581.
Evans, K. E. and R. N. Conner. 1979. Snag
management. p. 21q-225. In DeGraaf, R. M.
and K. E. Evans (editors),lManagement of
northcentral and northeastern forests for
nongame birds. Workshop Proceedings, USDA
Forest Service General Technical Report NC-51,
268 p. North Central Forest Experimental
Station, St. Paul, Minn.
Harris, Roger D. 1982. Nesting ecology of the
pileated woodpecker in California. Masters
thesis. 97 p. University of California,
Berkeley.
not recommend creation of snags by artificial means
because the methods may not usually achieve the
requisite decay conditions for pileated woodpecker
nest trees.
Topping may create entry courts for fungus.
Fungal inoculation along with topping may lead to
the requisite decay characteristics. Girdling and
silvicide treatments might not lead to heart rot
(Evans and Conner 1979, Miller and Miller 1980),
which typically initiates infection of the tree
when alive (Bega 1978). Most importantly, Conner
et al. (1981) suggested that trees killed by
silvicide fall within 3 to q years, resulting in a
net loss of snags several years after treatment,
compared to an unmanipulated situation.
Forest rotation periods suitable for the
maintenance of pileated woodpecker nesting habitat
should be calculated on the basis of how long it
takes to grow trees with characteristics needed by
the species for nesting under local edaphic and
climatic conditions. Conner (1978) recommended
that stands of largely hardwoods used by pileated
woodpeckers should have minimal 150 year rotations
to allow time for decay to spread in potential nest
trees. In California, however, pileated
woodpeckers will use Pinaceae trees less than 75
years old with isolated patches of decay (Harris
1982).
Because pileated woodpeckers do not use soft
or rotted snags, it is necessary to reserve
sufficient numbers of "hard" snags and to provide
for future recruitment of such snags. Most snags
fall in winter storms, which vary in strength and
frequency. What appears to be a sufficient supply
of snags to maintain pileated woodpeckers in a
period of "normal" winters may prove insufficient
after a particularly severe winter with exceptional
wind-fall of snags.
ACKNOWLEDGEMENTS
D. Airola and M. G. Raphael were instrumental
in all phases of the study. I also thank D. R.
McCullough and M. L. Morrison for reviewing the
manuscript. M. Sundove was an able field
assistant. I am indebted to correspondents from
across the state who provided information on nest
sites. R. c. Heald and R. H. Barrett provided
research facilities and logistical support at the
University of California's Blodgett Forest Research
Station. I thank F. W. Cobb, R. A. Cockrell, s.
Holmen, R. R. Parmeter, and W. W. Wilcox for advice
on fungus and analysis of wood chips. Assistance
on data analysis and computer operations were
generously provided by A. G. Stangenberger, along
with M. F. Dedon, D. Pitcher, W. D. Spencer, and W.
J. Zielinski. Partial funding was provided by the
Wollenberg Foundation, the University Foundation
Wildlife Fund, and the Department of Forestry and
Resource Management, University of California,
Berkeley.
LITERATURE CITED
Bega, Robert V. 1978. Diseases of Pacific coast
conifers. Agricultural Handbook 521, 206 p.
USDA Forest Service, Washington, D. c.
128
Kilham, Lawrence 1971. Reproductive behavior of
yellow-bellied sapsuckers: I. Preference for
nesting in Fames-infected aspens and nest hole
interrelations-with flying squirrels,
raccoons, and other animals. Wilson Bulletin
83:159-171.
Markwardt, L. J. and T. R. C. Wilson. 1935.
Strength and related properties of woods grown
in the U.S. U.S. Department of Agriculture
Technical Bulletin ~79, 99 p. Washington,
D.C.
McClelland, B. Riley. 1977. Relationships between
hole-nesting birds, forest snags, and decay in
western larch-Douglas-fir forests of the
northern Rocky Mountains. Ph.D. dissertation.
~95 p.
University of Montana, Missoula.
Miller, E. and D. R. Miller. 1980. Snag use by
birds. p. 357-368 In DeGraaf, R. M. and N. G.
Tilghman (editors),-workshop proceedings:
management of western forests and grasslands
for nongame birds. USDA Forest Service
General Technical Report, 535 p.
Intermountain Forest and Range Experiment
Station, Ogden, Utah.
Miller, E., A. T. Partridge, E. L. Bull. 1979.
The relationship of primary cavity nesters and
decay. Transactions of the Northeast Section,
The Wildlife Society 36:60-68.
Sumner, L. and J. s. Dixon. 1953. Birds and
mammals of the Sierra Nevada. ~8~ p.
University of California Press, Berkeley.
129