This file was created by scanning the printed publication.
Errors identified by the software have been corrected;
however, some errors may remain.
Cavity-Nesting Bird Response to Declining Snags on
a Burned Forest: A Simulation Model 1
Martin G. Raphael
2
Abstract.--! present a simple model to estimate population sizes of primary {PCN) and secondary {SCN)' cavity
nesting birds in relation to a declining snag population
following fire. The model assumes that PCN are limited by
snag suitability, intraspecific territoriality, or snag
density during successive time periods. Two examples
illustrate these patterns, as well as the impact of snag
harvest on subsequent bird populations. A FORTRAN program,
CNBIRDS, performs all computations.
INTRODUCTION
using a generalized poisson distribution function
for each category. The shapes of these decay
curves were adjusted to match empirical data
reported by Raphael and White {in press).
Snags are a dynamic resource: their populations undergo temporal changes in both quality
{snags decay with age) and quantity {new snags
appear and existing snags fall with time). Little
attention has been focused on the implications of
these temporal trends for wildlife. The snag
management model developed by Thomas et al. {1979),
for example, gave estimates of the numbers and
sizes of snags required by woodpeckers, but
failed to take into account attrition of snags with
time. Bull et al. {1980) have, however, presented
a formula for estimating the future density of
snags given their annual falling rate and starting
density. In this paper I extend the latter
approach using a computer simulation model
(CNBIRDS) to track numbers of snags and to
estimate resulting numbers of primary {PCN) and
secondary (SCN) cavity-nesting birds through time
as snags decay and fall. Using a burned Sierra
Nevada pine-fir forest as an example, I examine
the response of birds to reduced densities of
snags with time under both natural and managed
conditions.
At the start of each year PCN are added to
the system, subject to several constraints. First,
the model assumes that each pair of PCN requires a
specified minimum number of snags. Secondly, each
PCN has a maximum population size determined by
intraspecific territoriality. Maximum values used
in this model were based on average densities from
published breeding censuses {Raphael and White 1978)
for each of the 8 species likely to nest in burned
pine fir forest of the Sierra Nevada {Table 1).
Thirdly, I modeled the proportion of each species'
nests expected in each of the 8 snag categories
{Table 1). Finally, each species requires snags
in a particular range of decay-states; the model
incorporates a minimum snag age before snags
become acceptable, under the assumption that age
and decay are directly related.
THE MODEL
To construct the model, I considered 8 combinations of snag dbh and species. Starting values
for the number of snags present in each category
are decreased each year by a proportion calculated
1
Paper presented at the snag habitat management symposium. [Northern Arizona University,
Flagstaff, Arizona, June 7-9, 1983.]
~rtin G. Raphael is Staff Research Associate,
Department of Forestry and Resource Management,
University of California, Berkeley, California
94720.
211
The population of SCN was modeled assuming
that numbers are limited by either the number of
suitable cavities or a fixed maximum, again based
on intraspecific territoriality. The number of
suitable cavities in each snag type resulted from
the following considerations. First, each PCN
excavates from 1-3 cavities per year (see Bull et
al. 1980). The model assumes that all cavities,
except those of Pygmy Nuthatch, are available to
SCN. Next, the current year's cavities are added
to those that exist from the previous year in each
snag type. With the passage of a year, a number of
these cavities are lost as the snags containing
them fall; this number of lost cavities is proportional to.the annual rate of fall of each snag
type. Finally, the number of cavities at the start
of the next year is used to set the potential number of SCN. If this potential exceeds the previously specified maximum density (read in as an
input item), the model sets the current population
Table !.--Proportion of nests expected in each combination of
snag species and diameter class (dbh, em), maximum
density of PCN, and initial snag densities.!
Fir
2
3
Pine
38-53
23-37
<23
23-37
38-53
>53
<23
Pygmy Nuthatch
0
0.36
0.21
0.14
0
0.07
Hairy Woodpecker
0
0.22
0.30
0
0
Black-backed Woodpecker
0
0.12
0.13
0.12
Red-breasted Sapsucker
0
0.04
0.07
Williamson's Sapsucker
0
0
White-headed Woodpecker
0
Northern Flicker
0.11
0.11
28
57.6
0.13
0.22
0.13
23
12.4
0
0.38
0.25
0
8
1.0
0.56
0
0
0.04
0.29
45
4.0
0.04
0.45
0
0.02
0.08
0.41
49
4.0
0.08
0.25
0.34
0
0.08
0.08
0.17
12
5.7
0
0.11
0.21
0.20
0
0.03
0.17
0.28
65
7.2
Lewis' Woodpecker
0
0
0.16
0.38
0
0
0.22
0.24
37
7.2
4472
5300
1455
402
4413
6684
1987
260
(Piaoides viZtosus)
(Piaoides aratiaus)
(Bphyrapiaus ruber)
(Bphyrapiaus thyroideus)
(CoZaptes auratus)
(Me l.ane:rpes ZeuJis J
~Eairs/100
size
(Si tta pygmaea)
aZbo~atus)
Maximum
density
>53
Bird SEecies
(Piaoides
Sampl~
ha}
(to
Initial density of
snags (N /100 ha~
1
Data from Raphael and White (1978), Raphael and White (in press), and Raphael (unpublished).
2
Includes Abies aoncoZor, A. magnifica.
3
Includes Pinus aontorta, P. jeffreyi, P. ponderosae.
4
Number of nests examined to calculate proportion.
READ INPUT DATA:
INITIAL SNAG DENSITY,
PCN PREFERENCE.
SNAGS PER PAIR,
CAVITIES PER PAl R
to that maximum, and there are surplus (unused)
cavities.
The major features of the model are summarized
in Figure 1. Basically, with the passage of each
year a proportion of snags fall. The remaining
snags are counted in each type and the program
calculates the numbers of PCN and SCN for the
current year. When all snags have fallen (or
when the number of years reaches a preset maximum),
the program stops. This is a simple model, but,
as I hope to show with the following examples, a
potentially useful one.
EXAMPLE ONE:
~
r-----'----YJ
NO
SCN=O
NO TREATMENT
Snag Decline
The first example is designed to examine the
reponse of cavity-nesting birds to the pattern
of snag-fall on a burned forest assuming that no
snags are harvested following the burn. The model
was run for 45 years. I specified that each pair
of PCN required at least 48 snags (from Thomas et
al. 1976) and that each PCN excavated only 1
cavity per year. I also set the maximum density
of SCN at 240 pairs per 100 ha (calculated from
the upper 95% confidence interval of the mean
breeding density of SCN from 63 censuses published
in American Birds [Jackman 1974]). Starting values
for the number of snags in each category (Table 1)
212
Figure 1.--General flow chart for program CNBIRDS.
Data on snag decay rates and use by PCN and
SCN are maintained separately for each of 8
snag size and species combinations (see Table 1).
-
7
patterns. First, density of PCN did not reach a
maximum until year 18. Until then, snags were
abundant but bird numbers were limited by the lack
of sufficiently decayed snags or by their territoriality. The differential responses of selected
PCN species are illustrated in Figure 4. Hairy
Woodpeckers are opportunistic species that rapidly
invade burned forests (Koplin 1969) and are
capable of excavating nests in harder (more recently
killed) snags (Raphael and White, in press).
Northern Flicker and Lewis' Woodpecker require
softer snags for nesting; these species do not
reach maximum densities until year 15 and 19,
respectively (Fig. 4). Other species (not
illustrated) achieved maximum densities at intermediate years.
as
s:.
8
--PINE
0
0
---FIR
"""...
5
~
a.
CD
~
c
as
"
CD
~38cm
:I
0
s:.
3
>
!:
tiJ
z
w
Beyond 18 years, snags were sufficiently
decayed for all species, but snag density had
become too low to support maximum densities of PCN.
The PCN population crashed as snag numbers were
further reduced; none remained by year 30.
2
0
CJ
<
z
tiJ
0
The populations of SCN lagged behind the PCN
for the first 2 years, until the supply of cavities
began to build up. Thereafter, SCN numbers rose
rapidly due to the accelerating rate of cavity
production and the larger inventory of cavities
persisting from previous years. Maximum density
was reached in year 16, 2 years earlier than
achievement of maximum density of PCN. The SCN
maximum persisted until year 22. During this
period, the model assumed that SCN were limited by
territoriality rather than availability of cavities.
Beyond year 22, the accelerating rate of snag fall
caused the number of cavities to decline to the
extent that SCN were again limited by cavity
·availability. It is interesting that the ·sCN persisted at maximum density for 3-4 years beyond the
year when PCN began declining. Apparently, PCN
became limited by numbers of suitable snags while
SCN were still finding surplus cavities present
from previous years. This is an easily tested
prediction resulting from the model.
20
YEARS SINCE BURN
Figure 2.--Simulated decline of snag density following fire, by species and dbh class. Snags
less than 23 em dbh are not included.
were determined from field data (Raphael and White,
in press).
Fifty percent of the snags fell by the first
10 years (Fig. 2); the falling rate slowed thereafter. Larger snags fell at a slower rate than
smaller ones, and fir snags fell slower than pine
(Fig. 2). By year 20 nearly all of the pine snags
had fallen; by year 25 nearly all the fir snags
had fallen. By year 35, no snags were left.
I used the periodic breeding census from a
burned study plot at Sagehen Creek to test this
prediction. Ten censuses are available, covering
the period from 6 to 22 years following fire (Bock
and Lynch (1970, Bock et al. 1978, Harris and
Raphael 1982, Yoder-Williams, in press). Unfor-
Bird Response
The simulated response of cavity-nesting
birds (Fig. 3) revealed several interesting
G 15
s:.
-; 200
0
z:
.0
0
0
.-150
"""10
.....
z
......
z
>
...
ii
100
c
50
z
w
>
1-
5
tiJ
z
w
0
30
0
YEARS SINCE BURN
0
10
20
Figure 4.--Response of selected PCN to changing snag
density following fire. Species are Hairy
Woodpecker (HAWO), Northern Flicker (NOFL),
and Lewis' Woodpecker (LEWO)
YEARS SINCE BURN
Figure 3.--Simulated response of cavity-nesting
birds to changing snag density following fire.
213
tunately, the census data do not support the model
prediction (Fig. 5). Both PCN and SCN declined at
nearly the same rate over this 15 year period.
Perhaps the model is wrong, or perhaps the small
size of the study grid (8.5 ha) results in such
exaggerated year-to-year variation that subtle
differences are masked. In addition, comparison of
figures 3 and 5 shows that modeled population sizes
of PCN and especially SCN are higher than those
actually estimated from the censuses. Further
monitoring of this plot, and comparison with other
long-term censuses on burned plots will be necessary to validate model predictions.
8
0
0
o.
~
)(
U)
a:
<
w
One obvious result of this exercise is an
explicit demonstration of the futility of using
ratios of the number of snags with cavities to the
number without cavities as an estimate of the snag
requirements of PCN, as has been suggested by
Thomas et al. (1979). Reference to Figu·re 3 shows
that this ratio will change constantly with the
history of the plot. In year 1, for example, only
a few snags will have cavitieis. In year 25, however, probably 100% of the remaining snags will
contain at least one cavity. Thus, conclusions
resting on such ratios must be tempered with
knowledge of the age of the snags on the study plots.
EXAMPLE TWO:
EFFECTS OF SNAG HARVEST
Program CNBIRDS can also be used to explore
impacts of snag treatments on cavity-nesting birds.
In this section I examine the response of birds to
reduced initial densities of snags, simulating
various snag harvest levels immediately following'
fire. My measure of bird response was derived from
the accumulated total number of birds (both PCN and
as 100
s:;
0
0
~
....... 75
25
0
15
30
SNAG DENSITY (N/ha)
Figure 6.--Simulated response of cavity-nesting
birds to changing initial density of snags
greater than 38 em dbh. The upper curve (max)
was computed assuming that each pair of PCN
requires 15 snags and excavates 3 cavities
per year; the lower curve (min) assumes 48
snags per pair and 1 cavity.
Total bird-years rose dramatically from the 0
to 10% treatment level, corresponding to 0 to 4.5
snags >38 em per ha (Fig. 6). The maximum number
of bird-years for any particular snag density
occurred where I assumed a snag requirement of only
15 per pair and a cavity excavation rate of 3 per
pair. The lowest response occurred under assumptions of 48 snags and 1 cavity per pair. Because
all other combinations yielded intermediate values,
I display only these extremes. Beyond the 15 snags
per ha (30%) level, bird response rose relatively
slowly. The model predicted, then, that optimum
snag densities under the present constraints should
be between 7 and 15 snags per ha. I find it
significant that tripling the assumed snag requirements and cavity production resulted in about only
a 17% decrease in bird use at any snag density. It
would appear that birds are much more sensitive to
changes in snag density than variation of model
parameters within the range I tested.
~
50
a:
m
I used 19 snag treatment levels. These varied
from 1% to 10% of the pretreatment snag density
(in 1% increments) and from 20% to 100% (in 10%
increments). I also varied 2 of the major model
assumptions: the number of cavities excavated per
pair could be 1, 2, or 3 and the number of snags
required per pair could be either 15 or 48. Thus,
I ran the model a total of 114 times to include all
possible combinations of snag density, snag requirements, and cavities excavated.
,..,.
...
02
SCN) over all years. For each run, this amounted
to an integration of the curves illustrated in
Figure 2. If, for example, 5 pairs existed in each
of 4 years, the total bird-years would equal 20.
125
z
>
-en
z
w
c
>I
............ seN
PCN
YEARS SINCE BURN
Figure 5.--Censused density of cavity-nesting birds
following fire on a Sierra Nevada forest.
214
CONCLUSIONS
Bull, E.L., A. D. Twombly, and T.M. Quigley, 1980.
Perpetuating snags in managed mixed conifer
forests of the Blue Mountains, Oregon. p.
325-326. In Management of western forests
and grasslands for nongame birds. USDA
Forest Service General Technical Report INT-86,
535 p. Intermt. For. and Range Exp. Stn.,
Ogden, Utah.
Harris, R.D., and M.G. Raphael. 1982. Breedingbird census: burned pine-fir forest,
California. American Birds 36:88.
Jackman, S. M. 1974. Woodpeckers of the Pacific
Northwest: their characteristics and their
role in the forest. 147 p. M.S. thesis,
Oregon State University, Corvallis, Oregon.
Koplin, J. R. 1969. The numerical response of
woodpeckers to insect prey in a subalpine
forest in Colorado. Condor 71:436-438.
Raphael, M. G., and M. White. 1978. Snag
management guidelines for habitat types on
the western slope of the Sierra Nevada.
69 p. Unpublished report to USDA Forest
Service, Pacific Southwest Forest and Range
Experiment Station, Fresno, California.
Raphael, M. G., and M. White •.. In press. Use
of snags by cavity-nesting birds in the
Sierra Nevada Mountains. Wildlife
Monographs.
Thomas, J. W., R. J. Miller, H. Black, J.E. Rodiek,
and c. Maser. 1976. Guidelines for maintaining and enhancing wildlife habitat in
forest management in the Blue Mountains of
Oregon and Washington. p. 452-476.
Program CNBIRDS seems to be a useful model to
examine relationships between numbers of snags and
numbers of cavity nesting birds and changes in
both that occur through time when snags are not
replaced. The model can be used to develop a
number of testable hypotheses regarding the relative numbers of PCN and SCN and their respective
rates of increase and decrease with time. Even if
the model proves unrealistic, it can be useful as
a classroom tool to demonstrate a simple but
thought-provoking example of the coupling between
wildlife populations and their habitats. Finally,
the program can be used to examine the potential
consequences of harvesting any proportion of any
combination of snag diameter, and species.
A listing of program CNBIRDS, which is written
in FORTRAN IV for CDC machines and consists of a
main program and 8 subroutines, is available from
the author in request.
ACKNCULEDGMENTS
Field studies were funded by Susan Raphael
and the USDA Forest Service, Pacific Southwest
Region. Computing time was provided by the
University of California, Department of Forestry
and Resource Management. I thank John Menke for
programming advice.
Thomas, J. W., R. G. Anderson, C. Maser, and E.
L. Bull. 1979. Snags. p. 6G-77. In
Wildlife habitats in managed forests--the
Blue Mountains of Oregon and Washington.
USDA Forest Service Agriculture Handbook
No. 553, 512 p. U.S. Govt. Print. Office,
Washington, D.C.
Yoder-Williams, M. P. In press. Breeding-bird
census: burned pine-fir forest, California.
American Birds.
LITERATURE CITED
Bock, C.E., and J.F. Lynch. 1970. Breeding bird
populations of burned and unburned conifer
forest in the Sierra Nevada. Condor 72:
182-189.
Bock, C.E., M. Raphael, and J.H. Bock. 1978.
Changing avian community structure during
early post-fire succession in the Sierra
Nevada. Wilson Bulletin 89:119-123.
215