Ecological succession in the Angora fire:
The role of woodpeckers as keystone species
Final Report to the
South Nevada Public Lands Management Act
Photo Credit: T. Will Richardson, Tahoe Institute for Natural Science
Patricia N. Manley, Ph.D.
Gina Tarbill, M.S.
U.S. Forest Service
Pacific Southwest Research Station
Davis, California
June 2012
1
Executive Summary
Woodpeckers excavate cavities in trees to use as nests for the brooding and rearing of
their young. After fledging, these cavities are abandoned but remain in the environment for use
by other species that are unable to excavate cavities but rely on them for reproduction and cover
(secondary cavity users). When fire destroys cavities, secondary cavity users may be unable to
breed in the burned area until woodpeckers colonize and create new cavities. Therefore,
understanding how woodpeckers utilize burned areas is important as they provide a keystone
function by creating habitat for other organisms.
This research investigated nest site selection in three species of Picoides woodpeckers (P.
arcticus, Black-backed woodpecker; P. villosus, Hairy Woodpecker; and P. albolarvatus, Whiteheaded Woodpecker). Using logistic regression, we determined the factors with the greatest
influence on nest presence and found that they differed among the three species. The density of
small snags (by DBH) was positively associated with the presence of Black-backed Woodpecker
nests. Nest presence of White-headed Woodpeckers was positively associated with tree decay
and negatively associated with tree height and density of small trees. Hairy Woodpecker nests
were negatively associated with small trees. All woodpecker species were more likely to have
nests in more highly scorched trees, underscoring the importance of fire in creating habitat for
these species.
Woodpecker nests were found in 2009 and 2010 and monitored the subsequent year for
use by small mammals and birds, resulting in indices that describe the quantity and quality of
secondary cavity use. White-headed Woodpeckers had highest index of cavity utilization,
although Black-backed and Hairy Woodpeckers were important to some secondary cavity users.
Woodpeckers play an important role in post-fire habitats by rapidly colonizing burned
areas and creating cavities that are used by many other species that rely upon them for nesting,
denning, roosting, and resting. Understanding how wildlife species respond and recover from
fires is critical for conservation and management of forest ecosystems. Meeting management
objectives in burned areas focus on promoting fire safety while maintaining species diversity.
The results from this research indicate that management plans that incorporate habitat for
multiple woodpecker species would maintain the greatest biodiversity. This management goal
may be achieved by leaving large patches of high density small snags away from urbanized areas
and some larger diameter snags in open areas where they pose fewer fire risks.
Introduction
By focusing management efforts on keystone species, managers may monitor a single
population but gain important insight on an entire community (Power et al., 1996). Because
keystone species are organisms that exert a disproportional effect on the structure or function of
their community by virtue of life history traits or interactions with other species (Paine, 1969),
conservation efforts that benefit keystone species will positively influence the community as a
whole. Keystone species may impact the community by modifying, maintaining, and/or creating
habitat and by modulating resource availability to other species through physical changes in
biotic or abiotic materials (Lawton and Jones, 1995). Interestingly, some keystone species have
also been shown to influence ecological succession (Andersen and MacMahon, 1985;
Dangerfield et al., 1998; Rossell et al., 2005), the change in community composition over time.
Keystone species may play an important role in secondary succession, the compositional change
2
of a community after a disturbance (Connell and Slatyer, 1977), by creating or modifying habitat
such that other wildlife may colonize and establish. Facilitative activities of keystone species
result in the creation of habitat in previously unsuitable areas for other organisms, thereby
increasing recruitment (Andersen and MacMahon, 1985; Dangerfield et al., 1998; Rossell et al.,
2005; Kelm, et al. 2008). Focusing conservation efforts on keystone species preserves their large
influence on ecosystem function and has direct and indirect benefits at the community level
(Power et al. 1996).
Woodpeckers are considered keystone species in coniferous forests and may act as
facilitators of succession, largely due to habitat modification and creation with their unique
foraging and nesting activities (Lawton and Jones, 1995; Martin and Eadie, 1999; Martin et al
2004; Bednarz et al., 2004). As primary cavity excavators, woodpeckers create cavities for
nesting and roosting that are later used by secondary cavity users, species dependent on cavities
for reproduction or cover but unable to excavate them. Cavities in snags and live trees provide
nesting, roosting, denning, and resting sites for secondary cavity users (Bull et al., 1997).
Natural (non-excavated) cavities are limited in coniferous habitats and secondary cavity users are
dependent on primary cavity excavators for cavity creation (Aitken and Martin, 2007).
Competition for cavities has been shown to limit population growth of secondary cavity users in
some systems (Holt and Martin, 1997). This creates a guild structure in the community with
secondary cavity users dependent on primary cavity excavators (Martin and Eadie, 1999).
Community linkages may be particularly important after a significant disturbance, such as forest
fire or intensive logging, which can limit habitat availability for secondary cavity users by
directly removing existing cavities from the environment reductions of cavities. Understanding
this community structure will allow managers to predict responses at the community level to
wildlife and forest management practices and preserve habitat that has value to woodpeckers and
the whole cavity-dependent community.
Although interactions between secondary cavity users and woodpeckers have been wellstudied (Martin and Eadie, 1999; Aitken et al., 2002; Martin et al., 2004; Blanc and Walters,
2007 and 2008), there is a lack of knowledge on the impact of fire to these systems (but see
Gentry and Vierling, 2008). Fire is a natural and regular disturbance in mixed conifer forest that
may create snags, alter arthropod communities, and change forest structure (Kotliar et al., 2002).
Woodpeckers are generally early colonizers of burned areas, likely due to abundance of food
(bark and wood-boring beetles) and nest (snag) resources (Hutto, 1995). Their unique foraging
and nesting strategy may allow them to utilize and exploit resources initially unavailable to other
organisms. The cavities woodpeckers leave behind in burned forest create suitable habitat for
secondary cavity users, facilitating their colonization and occupation. Species of woodpecker
may differ in their influence on succession if cavities excavated by one species are preferred by
one or more secondary cavity users. Subsequent use by secondary cavity users may be driven by
both cavity conditions and dimensions and surrounding habitat characteristics (Aitken et al.,
2002; Saab et al. 2004; Gentry and Vierling, 2008; Czeszczewik, et al., 2008). Thus, different
species of woodpeckers may facilitate the colonization of different secondary cavity users.
Managers hoping to increase biodiversity after a fire may selectively manage burned forests to
increase all cavity excavators or only manage for the woodpecker species that has the greatest
impact on the community. There are over 30 species of secondary cavity users in the Sierra
Nevada from several functional groups including seed-dispersing small mammals, insectivorous
birds, and small avian and mammalian carnivores (Raphael and White, 1978, 1984; Verner and
3
Boss, 1980), creating the ideal system for investigating the facilitative effects of woodpeckers on
colonization by secondary cavity user.
The Angora Fire burned approximately 1,255 hectares in South Lake Tahoe, California in
June and July 2007. The fire occurred in an area with a high level of intermixed private and
public land and adjacent to large expanses of undeveloped public land. The primary post-fire
restoration efforts consisted of the removal of dead and dying trees to reduce the risk they could
pose to human life and property, with mulching as an additional measure was used in some areas
to reduce soil erosion.
Sampling Design
The goal of this study was to understand how primary cavity nesters drive ecological
succession following a wildfire and how post-fire restoration activities may affect the recovery
of bird and small mammal communities. Understanding how primary cavity excavators respond
to disturbance is crucial because they drive ecosystem processes and may aid in the recovery of
other wildlife. Disturbances that have positive effects on cavity excavators, such as forest fires
that create nesting and foraging habitat for woodpeckers may also benefit species that depend on
cavities at some point in their life cycle. Identifying keystone species and understanding their
response to disturbance will allow predictions on the community as a whole. These predictions
can guide future research, influence management decisions, and enhance conservation practices.
Objectives
1) Describe factors influencing presence of primary cavity excavator nests that will later
support secondary cavity user community
2) Understand how woodpecker colonization and modification of burned forest influences
secondary cavity user occupation
3) Predict how management activities may affect primary cavity excavators and
colonization of bird and small mammal communities following fire
Methods
Study Area
The Angora Fire burned approximately 1,255 hectares in South Lake Tahoe, California in
June and July 2007. As a mixed conifer forest, pre-fire dominant tree species included Pinus
jeffreyii, P. contorta, P. lambertiana, Abies concolor, A. magnifica, Populus tremuloides, and
Calocedrus decurrens and dominant shrub genera included Artemesia, Arctostaphylos, and
Ceanothus. The fire occurred in an area with a high level of private lands intermixed and
adjacent to large expanses of undeveloped public land. The severity of the burns varied within
the area, resulting in a mosaic of post-fire conditions (Fig. 1). Portions of the burn area owned
and managed by the California Tahoe Conservancy were treated with a post-fire harvest (PFH)
immediately following the burn in 2007. Additional areas owned and managed by the United
States Forest Service (USFS) were logged in 2008 and 2009. Although additional PFH was
planned for 2009-2010, it was not implemented until fall of 2011 with additional PFH treatments
planned for early spring and summer of 2012.
4
Site selection
The primary objective of this study was to understand the influence of burn severity and
PFH on cavity users. Sites were selected that represented a range of burn severity and PFHs
(Table 1). The USFS established a systematic grid of points spaced 400 meters apart across the
fire area to monitor post-fire vegetation response. For the purpose of site selection, fire severity
was classified into three burn severity intervals (1- 30%, 30-70%, > 70%; Fig. 1). Burn severity
was based on tree mortality ratios (Salvador et al., 2000) obtained from USFS LANDSAT data.
Sites were additionally classified as treated (i.e. trees were removed) or untreated based on PFH
boundaries provided by the USFS and field validation of these sites. We attempted to randomly
select sites from each of the 6 combinations of conditions (burn severity and PFH) in roughly
equivalent proportions, however because much of the fire burned at high severity and much of
the PFH was not completed at time of study, selection was weighted to highly burned and
untreated sites (Table 1). Sites were selected from each category randomly in equal numbers
until particular categories were exhausted. Grid points occurring in each burn severity-PFH
category were selected first to ensure all combinations of PFH conditions were represented.
Thirty-two sites were surveyed both years, with additional sampling at 17 sites in 2009 and 24
sites in 2010 to increase the sample size of nests discovered.
Table 1. All sites sampled for woodpecker nests in 2009 and 2010 in the Angora fire area.
2009 only
Burn severity
Low
Moderate
High
Treated Untreated
0
3
1
4
2
8
2010 only
Treated
0
0
1
5
Untreated
1
8
16
Sampled both
Grand
years
Total
Treated
Untreated
1
2
7
0
10
23
7
9
43
73
Figure 1. Footprint of Angora fire with burn severity and sampling points.
Primary cavity nests – Existing data
Nests were located as part of the SNPLMA funded research project “Biodiversity
response to burn severity and post-fire restoration” (Manley et al., Round 9). Nest searches for
Black-backed, White-headed, and Hairy Woodpeckers were conducted between May and July in
2009 and 2010. Sites were searched for nests on a rotating basis, with each site visited a
minimum of three times, with at least a week between each visit. Trained observers were made
aware of breeding bird behaviors and used cues from adults and nestlings and systematic search
to locate nests (Martin and Geupel, 1993; Appendix A). Birds were observed and followed from
a distance to avoid altering their behavior and nests were found during construction, egg laying,
incubation, or nestling stages.
The search area associated with each sample point had two distance zones: 60 meters
and 100 meters. Observers first searched within 60 meters of each sample site (approximately 1
hectare area) for a minimum of 15 minutes for active cavity nests (Martin and Geupel, 1993) and
foraging cavity nesters (Covert-Bratland et al., 2006). The time allotted corresponded to an
6
effective use of time to locate focal species. Once a focal species was located, no maximum time
was set for following it to determine if it was nesting. If an individual was located while
searching the 60-meter area, they were followed outside this search area to locate their nests.
Once the 60-meter radius area was thoroughly canvassed, observers moved out into the area
between 60 and 100 meters from the site (approximately 2 hectare area), and spent
approximately 1 additional hour searching this area for focal species (i.e., a less intensive search
per unit area). Nests that were encountered in the course of moving to and from sites in the
study area were considered in the “matrix.” The distance and azimuth to the nearest sample point
was estimated for all nests. When an active nest was confirmed, the bird species, location of the
nest and stage of nest development was recorded, along with the Universal Transverse Mercator
(UTM) coordinates (North American Datum 83) of the nest site.
Nest site selection
Because much of the PFH had not been completed at time of writing, treatment effects on
nesting woodpeckers were addressed in terms of snag and tree density. These data were collected
as part of the SNPLMA funded research project “Biodiversity response to burn severity and postfire restoration” (Manley et al., Round 9) and a detailed description of vegetation sampling is
provided in Appendix A of that report and in Safford et al. (2009). At each of the nest searching
points, data representing forest conditions were collected. Condition and density of live trees
(hereafter “trees”) and snags and condition and percent cover of shrubs and herbs were
characterized in 1/5ac plots. All trees were identified to species, measured for diameter at breast
height (DBH), and assigned a decay class based on the 5-class system developed by Cline et al.
(1980). Diameter classes were created by separating trees and snags into small (11-24in),
medium (24-36in), and large (>36in) categories by DBH and calculating densities for each class
in stems per hectare. Fine and coarse woody debris, ground cover, and plant species composition
and cover were recorded in a total of twenty 0.5m quadrats evenly spaced along 64m of
transects. Coarse woody debris (CWD) was divided into small (less than 12in) and large (greater
than 12in) diameter classes and percent cover for each site was calculated using methods
described in Waddell (2002). Analysis of Covariance (ANCOVA) was utilized to determine the
effect of PFH treatment on habitat characteristics, with burn severity and degree of urbanization
as covariates. This method allows for differentiation and interaction of effects of treatment,
development, and burn severity on the density of trees and snags, percent cover of coarse woody
debris, and percent cover of shrubs and herbs.
Habitat models for each species of woodpecker were created to determine the habitat
factors most influential for nest site selection and to provide predictions of response to changes
in habitat caused by PFH. We recorded nest tree features, including DBH, height, and decay for
comparison to randomly selected trees from each point representing available habitat.
Additionally, data on habitat around nests were collected at nest sites and available sites. Snag
density, tree density, burn severity, and percent cover of CWD, shrubs, and herbs were collected
in an 11.3m-radius circle (0.04ha area) at nest and available points. Stem counts of trees and
snags were categorized into DBH classes of small (11-24in), medium (24-36in), and large
(>36in); and densities were calculated for each class as number of stems per hectare. Burn
severity was derived from Geographic Information System (GIS) data in ArcGIS 9.3 (ESRI,
Redlands, CA) and calculated as a weighted average in the 11.3m-radius circle around each
point.
7
We used logistic regression models for each species of cavity excavator to determine how
characteristics of snags and live trees, ground cover, CWD, and burn severity affected nest site
selection in burned forests. Models were created at two spatial scales: nest tree and nest site (Fig.
2). Variables entered into nest tree models included burn severity, DBH, height, scorch, and
decay of tree. Density of small, medium, and large snags and trees, percent cover of CWD,
shrub, and herb, and burn severity were included in nest site models.
Nest Tree Scale:
• Height
• Diameter at breast height
• Percent scorch
• Decay class
• Burn severity
Nest Site Scale:
• Percent cover of coarse woody debris
• Percent cover of shrubs
• Percent cover of herbs
• Density of snags by diameter class
• Density of trees by diameter class
• Burn severity
Figure 2. Data collection at woodpecker nests at both spatial scales
Corrected Akaike’s information criterion (AICc) was used to select the most
parsimonious model with the fewest parameters for each woodpecker species (Akaike, 1974).
Models with ∆AICc values less than 2 are considered strongly supported. Akaike weights (wi)
were calculated to determine the parameters with the greatest influence on nest site selection and
the coefficients of parameters (β) indicate whether the relationship between the predictor and
nest presence was positive or negative. Thresholds for the most influential parameters on nest
site selection were determined by plotting the predicted probability of nest presence and
determining the value for habitat characteristics when for probability of 0.75.
Secondary Cavity Use
Nest cavities of focal species were monitored after they were abandoned to determine the
influence of each species of woodpecker on secondary cavity use in a burned forest. Remotetriggered digital cameras (Leaf River Outdoor Products, Taylorsville, MS) were set twice per
season to determine use during breeding (March-August) and non-breeding seasons (SeptemberFebruary). Cameras monitored cavities for 7-day sessions. The use of cameras allowed for
detection of elusive, diurnal, and nocturnal organisms. Cameras were set on an adjacent tree
facing the cavity at the appropriate height and angle to maximize detections. Cameras were
tested to ensure functionality and loaded with fresh batteries and empty camera cards. After
seven days of monitoring, cameras were collected and photos from memory cards were
downloaded. All organisms detected were identified to species whenever possible and assigned
to either “breeding” or “non-breeding” categories based on whether eggs or evidence of nests or
8
young were observed. Additionally, we used a Treetop Peeper (Sandpiper Technologies;
Manteca, CA) to observe the interior of cavities twice during each season to check for active
nests or dens, nesting material, and other evidence of use. Once a cavity was identified as
occupied, any subsequent detection of the same species was considered a detection of the same
individual and not “double” counted, although the cavity was still monitored to determine if it
was used by other species subsequently.
Mean species richness of secondary cavity users detected at nests was compared between
the three woodpecker species using the Kruskal-Wallis Test (SAS 9.2) for nonparametric data.
Species richness was calculated as the number of different species detected over the season at
each cavity, then averaged over all cavities excavated by each species of woodpecker. We
modeled the relationship between excavators and secondary cavity users with nest webs for each
woodpecker species following Martin and Eadie (1999). Nest webs reflect the relative
dependence of secondary cavity users on individual species of woodpecker by comparing
differential use of cavities by secondary cavity users. Nest webs have been used to study direct
and indirect effects of woodpeckers on secondary cavity users and the community at large
(Martin and Eadie, 1999; Aitken et al., 2002; Martin et al., 2004; Blanc and Walters, 2007;
Gentry and Vierling, 2008) and are analogous to food webs, with trees/snags as the fundamental
“producers”, primary cavity excavator as the “manufacturers”, and secondary cavity user as the
“consumers” of cavities (Fig. 3; Martin and Eadie, 1999).
Nest webs can also be modeled at the species level to determine the relative ecological
importance of specific excavators on secondary cavity users. Predictions may be made on
community responses to changes in the system, such as decreases in the number of snags or
decreases in a particular primary cavity excavator population. Proportional use was calculated as
the number of cavities used by a species of secondary cavity user for each species of woodpecker
divided by the number of cavities used by the species of secondary cavity user for all
woodpeckers. Secondary cavity users were grouped as birds or small mammals and divided into
“open” or “closed” canopy species, based on known habitat preferences (Verner and Boss,
1980). Fisher’s Exact Tests were used to analyze preferences of each group for cavities
excavated by the different species of woodpecker.
SCU1
SCU2
SCU3
SCU4
SCU5
PCE1
PCE2
PCE3
Tree species
1
Tree species
2
Tree species
3
Proportion of use
< 0.10
0.10- 0.49
> 0.50
Figure 3. Nest web illustrating relative influence of primary cavity excavators on secondary cavity users.
PCE=Primary Cavity Excavator, SCU= Secondary Cavity User.
9
The contribution of each species of woodpecker to bird and small mammal colonization
was represented by the Utilization Index (UI) based on the richness and diversity of secondary
cavity users associated with cavities of each species of woodpecker (Eq. 1),
Equation 1. Utilization Index
UI
(2Sb Snb T 1)
Where Sb = the number of breeding species detected, Snb= the number of non-breeding species,
and T=the number of taxonomic families detected. Breeding species were weighted by a factor of
two to highlight the importance of providing reproductive habitat (Brawn and Balda, 1988;
Steele, 1993). Taxonomic diversity (T) was included to highlight the importance of providing
habitat to multiple families, which included bluebirds (Turdidae), nuthatches (Sittadae),
chickadees (Paridae), wrens (Troglodytidae), woodpeckers (Picidae), and squirrels and
chipmunks (Sciuridae).
This index was averaged over all nests for each species of woodpecker and multiplied by the
proportion of nests with detections to obtain the Cavity Utilization Index (CUI, Eq. 2).
Equation 2. Cavity Utilization Index
CUI UI * nd / n
Where nd= number of nests with detections and n=total number of nests monitored.
Results
Nest Searching
A total of 158 nests were found for the three focal woodpecker species in 2009 and 2010
combined (Fig. 4). In 2009, 15 Black-backed Woodpecker nests, 37 Hairy Woodpecker nests,
and 15 White-headed Woodpecker nests were located. In 2010, 24 Black-backed Woodpecker
nests, 41 Hairy Woodpecker nests, and 26 White-headed Woodpecker nests were located.
10
Figure 4. Woodpecker nests located in 2009-2010 in the Angora Fire area.
Nest site selection within burned forests
Results from the analysis of covariance indicate that burn severity, PFH, development,
and time since fire greatly affected the structure and composition of the forest (Fig. 5 and 6).
Density of trees was negatively impacted by increasing burn severity and time since fire, with a
stronger response in 2010 than earlier years, especially in low and highly burned sites. This may
be attributed to the death of damaged trees over time. Density of trees was also impacted by
development, with higher mean tree density in urban sites than wild sites. Snag density was
impacted by several environmental variables. Predictably, snags increased with increasing burn
severity, and this relationship varied in intensity with PFH. PFH negatively impacted density of
snags, and dampened the response of snag density to burn severity, reducing the recruitment of
snags as burn severity increased. Development also impacted the density of snags, with higher
mean density found in wild sites than in urban sites. Ground cover was also impacted by the
environmental variables (Fig. 6). Coarse woody debris (CWD) was not significantly influenced
by burn severity or time since fire, suggesting that the amount of debris created is roughly
equivalent to the amount destroyed within the first years after fire. In fact, only wild and urban
sites differed in percent cover of CWD, with higher mean cover in wild sites.
11
All sites
Untreated
Treated
300
250
200
150
100
50
0
0%
20%
40%
60%
80%
b.
Untreated
Treated
500
Snag density (stems/ha)
Density (stems/ha)
a. 350
400
300
200
100
0
100%
0%
Burn severity (%)
50%
Burn severity (%)
100%
Figure 5. Density of trees and snags by PFH and burn severity. a. Tree density in all sites was
negatively influenced by burn severity, with no significant effect of PFH. b. Snag density was
positively influenced by burn severity, although PFH reduced the intensity of the response by
removing snags.
Herbaceous cover was also not significantly impacted by burn severity. Percent cover of herbs
did differ by development; with higher mean cover in urban sites. Herbaceous cover also varied
by year, with significantly higher mean cover in the third year after fire than the first.
Herbaceous cover was not impacted by any interaction effect. Percent cover of shrubs was
positively influenced by burn severity. While we did not find significant post-hoc results to
determine how each year differed, mean shrub cover differed significantly by year, generally
increasing from the first year after fire.
Herb
Shrub
Percent cover (%)
100%
80%
60%
40%
20%
0%
0%
50%
100%
b.
12%
Percent cover (%)
a.
Coarse woody
debris
10%
8%
6%
4%
2%
0%
0%
Burn severity (%)
20%
40% 60% 80%
Burn severity (%)
100%
Figure 6. Ground cover response to burn severity. a. Shrub cover was positively influenced by
burn severity, but herb cover was not significantly impacted by burn severity. b. Percent cover of
coarse woody debris was not significantly impacted by burn severity.
Mean values of habitat parameters at nests of each species of woodpecker and available
sites are reported in Table 2. All woodpeckers exclusively selected snags for nest excavation,
although live trees were available, and average burn severity at nests was approximately 92%
while average burn severity at available sites was 69%. Decay class for selected snags ranged
from 1.49 for Black-backed Woodpecker, 1.5 for Hairy Woodpeckers, 2.8 for White-headed
Woodpeckers, and 0.7 for available sites. Mean decay class was lower in available sites due to
the random selection of live trees, which generally have a decay class value of zero.
12
Table 2. Mean and standard deviation (SD) values for habitat parameters at nests for each
species of woodpecker and available sites.
Nest tree
Black-backed
Woodpecker
(n=39)
Hairy
Woodpecker
(n=78)
White-headed
Woodpecker
(n=41)
Available sites
Mean
Mean
Mean
Mean
91 ± 16
93± 17
91 ± 15
69 ± 37
DBH (cm)
34.7 ± 9.3
39.0 ± 8.6
47.2 ± 44.9
43.6 ± 24.5
Height (m)
17.4 ± 7.8
16.8 ±. 48
8.7 ± 6.9
17.5 ± 8.0
Decay Class
1.49 ±0.97
1.50 ±0.86
2.80 ±1.35
0.70 ± 0.79
Scorch (%)
91 ± 18
91 ± 16
97 ± 9
60 ± 41
Black-backed
Woodpecker
(n=39)
Hairy
Woodpecker
(n=78)
White-headed
Woodpecker
(n=41)
Available sites
Mean
Mean
Mean
Mean
Herb (%)
13 ± 12
13 ± 16
15 ± 19
12 ± 19
Shrub (%)
23 ± 17
27 ± 25
24 ± 22
17 ± 22
2±3
3±3
2±2
2±2
Small tree density
(stems/ha)
5.63 ± 18.10
3.44 ± 13.46
3.57 ± 12.86
42.63 ± 60.99
Medium tree density
(stems/ha)
3.13 ± 11.44
2.19 ± 8.04
4.76 ± 12.46
10.41 ± 19.55
Large tree density
(stems/ha)
0.00 ± 0.00
0.00 ± 0.00
1.78 ± 6.43
3.36 ± 10.26
Small snag density
(stems/ha)
180.11 ± 97.98
138.52 ± 90.45
90.42 ± 79.83
87.61 ± 104.20
Medium snag density
(stems/ha)
13.76 ± 17.51
18.45 ± 28.40
10.11 ± 21.11
11.41 ± 21.64
Large snag density
(stems/ha)
4.38 ± 9.48
3.44 ± 10.19
4.76 ± 9.79
3.69 ± 9.73
Burn severity (%)
91 ± 15
93 ± 17
91 ± 15
70 ± 36
Variable
Burn severity (%)
Nest site
Variable
Coarse woody debris
(%)
(n=62)
(n=73)
The best model predicting presence of Black-backed Woodpecker nests at the nest tree
spatial scale included DBH, height, decay, and scorch (wi=0.25, Table 3), with a positive
relationship with density of decay, scorch, and height and a negative relationship with DBH. The
only other model with substantial support (∆AICc <2) included these same parameters plus year,
which had a positive influence on nest presence. The most influential parameters at the nest tree
scale included DBH, decay, and height. In order to obtain a 0.75 probability of nest presence for
Black-backed Woodpeckers, the minimum DBH was approximately 10cm and the minimum
decay class was approximately 1.8 (Fig. 7). Height of nest tree did not show a relationship with
13
the probability of nest presence, therefore a threshold could not be determined. At the nest site
scale, the best model included the positive influence of increasing density of small snags and the
negative influence of increasing density of small trees (Table 3). Density of small snags was the
most important variable for Black-backed Woodpeckers, with summed model weight of 0.89. In
order to obtain 0.75 probability of presence of Black-backed Woodpecker nests, the minimum
density of small snags was 260 stems/ha (Fig.7).
Table 3. Summary of results of logistic regression with AICc model selection for models of
Black-backed Woodpecker nests. Model-averaged coefficients with summed weights greater
than 0.80 are included.
Black-backed Woodpecker models for nest trees
Model
DBH Height Decay Scorch
Year DBH Height Decay Scorch
∆AICc
0
0.76
Model-averaged coefficients
wi
Variable
0.25 Intercept
0.17 DBH
Decay
Height
Black-backed Woodpecker model for nest sites
Model
Small tree density Small snag
density
∆AICc
0
∑ wi
β
1 -803.63
0.95
-0.05
0.93
0.84
0.87
0.07
± SE
474.43
0.02
0.36
0.04
Model-averaged coefficients
wi
Variable
0.54 Intercept
Small snag
density
14
∑ wi
β
1 -347.93
0.89
0.01
± SE
148.93
0.00
Figure 7. Probability of presence of Black-backed Woodpecker nests. The shaded region indicates the 95% confidence interval for the
predicted probability line. The red arrows indicate the75% probability of presence of Black-backed Woodpecker nests.
15
Several models were substantially supported (∆AICc <2) to predict nest presence of Hairy
Woodpeckers at the nest tree scale (Table 4). The best model included decay class and percent
scorch of nest trees. Other supported models included DBH, burn, year and height. The most
influential variables included decay class and percent scorch, with summed model weights of
0.98 and 0.96, respectively. Thresholds at the nest tree scale were indicated by minimum decay
class of 1.3 and minimum scorch of approximately 90% (Fig. 8). The best model predicting
presence of Hairy Woodpecker nests at the site scale included small tree density and large tree
density (wi=0.17, Table 4). Nest presence was negatively influenced by density of both small and
large trees. An alternative, strongly supported model (∆AICc =1.97) included density of small
trees and burn severity. Burn severity was a positive influence on nest presence. The most
influential variable on presence of Hairy Woodpecker nests was small tree density, with summed
model weight of 0.99. In order to obtain 0.75 probability of presence of Hairy Woodpecker
nests, there would be no small diameter trees in proximity of the nest (Fig. 8).
Table 4. Summary of results of logistic regression with AICc model selection for models of
Hairy Woodpecker nests. Model-averaged coefficients with summed weights greater than 0.80
are included.
Hairy Woodpecker models for nest trees
Model
∆AICc
Decay Scorch
0
DBH Decay Scorch
0.84
Burn Decay Scorch
1.22
Year Decay Scorch
1.37
DBH Height Decay Scorch
1.51
Model-averaged coefficients
wi Variable
∑ wi
β
± SE
0.17 Intercept
1 -461.49
397.59
0.11 Decay
0.98
1.05
0.34
0.09 Scorch
0.96
2.71
0.86
0.09
0.08
Hairy Woodpecker models for nest sites
Model
Small tree density Large tree density
Small tree density Burn
Model-averaged coefficients
∆AICc wi
Variable
0 0.53 Intercept
1.97 0.20 Small tree
density
16
∑ wi
1
0.99
β
-205.68
-0.03
± SE
122.13
0.01
Figure 8. Probability of presence of Hairy Woodpecker nests. The shaded region indicates the 95% confidence interval for the
predicted probability line. The red arrow indicates the75% probability of presence of Hairy Woodpecker nests.
17
Models and model-average coefficients at the nest tree scale for White-headed
Woodpeckers are given in Table 5. The best model predicting presence of White-headed
Woodpecker nests included DBH, height, decay, and scorch of nest trees. The most influential
variables predicting nest presence included decay class, height, and percent scorch. Whiteheaded Woodpecker nest presence was positively associated with decay and percent scorch of
nest tree and negatively associated with nest tree height. Predicted probability of presence of
White-headed Woodpeckers is 0.75 with minimum decay class of 2, minimum percent scorch of
100%, and maximum tree height of 8 meters (Fig. 9). The best model predicting presence of
White-headed Woodpecker nests at the nest site scale included year and density of small trees
(Table 5), however several other models were strongly supported by the data. The most
important variable predicting nest presence was density of small trees, a negative influence
resulting in a threshold of no small diameter trees (Fig. 9) to obtain 0.75 predicted probability of
presence.
Table 5. Summary of results of logistic regression with AICc model selection for models of
White-headed Woodpecker nests. Model-averaged coefficients with summed weights greater
than 0.80 are included.
White-headed Woodpecker models for nest trees
Model
DBH Height Decay Scorch
Height Decay Scorch
Burn DBH Height Decay Scorch
Burn Height Decay Scorch
∆AICc
0
0.10
1.55
1.56
wi
0.22
0.21
0.10
0.10
White-headed Woodpecker models for nest sites
Model
Year Small tree density
Small tree density Burn
Small tree density
Model-averaged coefficients
Variable
Intercept
Decay
Height
Scorch
β
-416.49
1.42
-0.20
5.18
± SE
586.09
0.36
0.06
2.32
Model-averaged coefficients
∆AICc wi
Variable
0 0.28 Intercept
1.28 0.15 Small tree
density
1.69 0.12
18
∑ wi
1
1.00
0.99
0.86
∑ wi
1
0.96
β
-381.51
-0.03
± SE
145.70
0.01
Figure 9. Probability of presence of White-headed Woodpecker nests. The shaded region indicates the 95% confidence interval for the
predicted probability line. The red arrows indicate the75% probability of presence of White-headed Woodpecker nests.
19
Secondary Cavity Use
Because very few detections were observed in the winter, non-breeding season during
both years of this study, analysis was limited to detections that occurred between March and
August, the breeding season. Nests discovered in 2009 were monitored in the breeding season of
2010 and nests discovered in 2010 were monitored in the breeding season of 2011. Because
many of the nest trees fell during winter, our sample size was reduced to 8 Black-backed
Woodpecker nests, 13 Hairy Woodpecker nests, and 11 White-headed Woodpecker nests
monitored in 2010 and 10 Black-backed Woodpecker nests, 14 Hairy Woodpecker nests and 21
White-headed Woodpecker nests monitored in 2011 (Table 6).
Table 6. Nests monitored for secondary cavity use and percentage of nests with detections of
secondary cavity users.
2010
Excavator
Black-backed Woodpecker
Hairy Woodpecker
White-headed Woodpecker
Nests
monitored
Nests
with
detection
8
12
11
88%
75%
82%
2011
Nests
Nests with
monitored
detection
10
14
21
90%
71%
100%
Grand Total
Both years combined
Total nests
Total
monitored nests with
detection
18
26
32
76
89%
73%
94%
86%
Nine species of secondary cavity users were detected by camera or Treetop Peeper (Table
7). Eighty-one percent of nests monitored in 2010 were used at least once by secondary cavity
users. A total of 34 detections were observed at these nests, of which 65% were birds and 35%
were small mammals. For nests monitored in 2011, 89% of cavities were used at least once, with
a total of 77 detections. Fifty-three percent of detections were birds and 36% were small
mammals and 10% were not identified. In both years, Western Bluebirds (Sialia mexicana) and
chipmunks (Tamias species) were most commonly detected. These results showed that newly
excavated cavities had nearly 100% occupancy, indicating that they were a limited resource.
The mean species richness (and standard deviation) of secondary cavity users detected at
cavities excavated by Black-backed Woodpeckers was 0.94±0.54 (n=28). Cavities excavated by
Hairy Woodpeckers had mean species richness of 0.88±0.71 (n=26).) Mean species richness of
secondary cavity users for cavities excavated by White-headed Woodpeckers was 1.41±0.98
(n=32).There was a significant difference in mean rank of species richness (Kruskal-Wallis,
H2=7.10, p=0.03).
All species of woodpeckers created cavities that were used by both birds and small
mammals (Table 7, Figure 10). Cavities created by White-headed and Black-backed
Woodpeckers were utilized by seven species each. Hairy Woodpecker nests had the lowest
secondary cavity user diversity, with only five species utilizing these cavities. White-headed
Woodpecker cavities supported the greatest diversity on secondary cavity users (57.7% of the 10
species) based on the proportion of use across all secondary cavity species. Black-backed
Woodpecker supported the second greatest diversity of secondary cavity species (25.6%)
compared to Hairy Woodpecker (16.8%). Given that Black-backed and Hairy woodpeckers
excavate cavities in similar habitats, the Black-backed Woodpecker is likely supporting a larger
number of species in this habitat type.
20
Table 7. Proportional use of cavities by secondary cavity users, calculated as number detected
per excavator divided by the total number of detections.
Proportional use by secondary cavity
user for each species of woodpecker
Secondary cavity users
Common name
Scientific name
Total
detections
Blackbacked
Woodpecker
Hairy
Woodpecker
White-headed
Woodpecker
American Kestrel
Falco sparverius
1
0.00
0.00
1.00
House Wren
Troglodytes aedon
2
0.50
0.00
0.50
Mountain Bluebird
Sialia currocoides
4
0.00
0.50
0.50
Mountain Chickadee
Poecile gambeli
8
0.25
0.13
0.63
Northern Flicker
Colaptes auratus
5
0.40
0.00
0.60
White-breasted Nuthatch
Sitta carolinesis
4
0.00
0.00
1.00
Western Bluebird
Sialia mexicana
15
0.08
0.46
0.46
Chipmunk species
Tamias species
11
0.33
0.09
0.58
Douglas Squirrel
Tamiasciurus douglasii
6
0.00
0.50
0.50
Northern Flying Squirrel
Glaucomys sabrinus
2
1.00
0.00
0.00
25.60%
16.80%
57.70%
TOTAL
Figure 10. Nest web for Angora fire. “Other” trees included unknown species and Calocedrus
decurrens.
21
In addition to hosting a high diversity of species, Black-backed Woodpeckers uniquely hosted
Northern Flying Squirrels (n = 2), and co-hosted 50% of the use by House Wrens and Northern
Flickers (another cavity excavator) along with White-headed Woodpecker. In contrast, Hairy
Woodpeckers co-hosted ~50% of the use by the Western Bluebird and Douglas squirrel, which
are complementary species to those hosted by Black-backed Woodpecker. White-headed
Woodpeckers uniquely hosted White-breasted Nuthatches and American Kestrels, primarily
hosted Mountain Chickadees and chipmunks, with overlap in species support with Hairy
Woodpeckers (Western Bluebirds, Mountain Bluebirds, and Douglas Squirrels) and Blackbacked Woodpeckers (House Wrens and Northern Flickers).
Overall sample sizes for most secondary cavity users were small. Although sample sizes
were too small to directly compare differences in species composition, we did compare use of
cavities excavated by particular species of woodpeckers and groups of secondary cavity users
(Table 7, Fig. 10). Species associated with open canopy included House Wrens, Western and
Mountain Bluebirds, White-breasted Nuthatches, and Tamias species. Species associated with
closed canopy included Northern Flying squirrels, Douglas Squirrels, Northern Flickers, and
Mountain Chickadees. There were no significant differences in use of cavities excavated by
particular species for secondary cavity users associated with open canopy (Fisher’s Exact Test,
p=0.32), species associated with closed canopy (Fisher’s Exact Test, p=0.26), birds (Fisher’s
Exact Test, p=0.46), or small mammals (p=0.07). When we grouped all secondary cavity users
together, we found significant differences in cavity selection by excavator (Fisher’s Exact Test,
p=0.04), with more secondary cavity users selecting cavities excavated by White-headed
Woodpeckers, followed by Black-backed Woodpeckers. Cavities excavated by Hairy
Woodpeckers were the least selected. This trend was repeated when Cavity Utilization Indices
were calculated. White-headed Woodpeckers had 19 bird nests, nine detections of other birds,
three small mammal dens, and 14 detections of other small mammals, resulting in a Cavity
Utilization Index of 2.35, with 94% of cavities used. Cavities of Black-backed Woodpeckers had
detections of 11 bird nests, three additional bird, three small mammal dens, and three additional
small mammals; resulting in a Cavity Utilization Index of 1.68, with 89% of cavities used. Hairy
Woodpeckers created cavities that had detections of 15 bird nests, three other birds, one small
mammal den, and four additional small mammals, for Cavity Utilization Index of 1.15, with 73%
of cavities used.
Discussion
Woodpeckers play an important role in post-fire habitats by rapidly colonizing these
areas and creating cavities that are used by many other species for nesting, denning, roosting, and
resting (Aitken and Martin, 2002; Blanc and Walters, 2007). Species of woodpeckers select
habitat based on excavation ability and foraging preferences. Species with weaker excavation
ability, like White-headed Woodpeckers, will rely more heavily on more decayed snags and live
trees for nuts than species with strong excavation ability, like Black-backed Woodpeckers. By
understanding the habitat components that are most important for nest site selection, managers
may conserve habitat that is preferable for a particular species of woodpecker that may in turn,
increase the biodiversity of secondary cavity users. Management that removes most or all small
diameter snags from burned areas will reduce the amount of suitable habitat for all three species
of woodpeckers, resulting in a reduced density and availability of excavated cavities. Because
22
woodpeckers may act as keystone species (Lawton and Jones, 1995; Martin and Eadie, 1999;
Bonar, 2000; Bednarz et al., 2004), loss or degradation of habitat for woodpeckers may influence
the structure and composition of cavity-dependent communities. In areas that have been
disturbed, such as burned forests, the presence and abundance of certain keystone species can
influence the progression of succession by accelerating colonization of some species or altering
species composition. Understanding the relationships between woodpeckers, cavity-dependent
communities, and habitat is crucial for forest management and conservation.
All three species of woodpecker supported the cavity-dependent community in the
burned area, with White-headed and Black-backed Woodpeckers exerting the strongest influence
based on the richness and diversity of secondary cavity users. While cavities of Hairy
Woodpeckers supported fewer species and were used in lower proportion compared to the other
two woodpeckers, they supported unique and complementary species to the Black-backed
Woodpecker in burned habitats. This suggests that while White-headed and Black-backed
Woodpeckers are the most important excavators influencing colonization, the complement of all
three species of woodpeckers appears to have the greatest influence on colonization of secondary
cavity users.
Eighty-six percent of all cavities monitored were occupied at some point in this study,
indicating strong competition among secondary cavity users for cavities within the burned area.
Cavities may be rare immediately after a fire, as existing snags with old cavities are lost (Saab et
al., 2004, 2007; Bagne et al., 2008). Cavities may continue to be limited because burned snags
have lower persistence than unburned snags (Bagne et al., 2008) in the environment, and this
may be exacerbated by woodpecker activity that reduces structural integrity of snags (Farris et
al., 2002, 2004; Jackson and Jackson, 2004). Limited numbers of available cavities may result in
increased competition among cavity users. Anecdotally, two attempts of “cavity usurping” were
observed within the study area at non-focal cavities: once when a pair of Western Bluebirds
harassed a White-headed Woodpecker that was excavating a new cavity and again when a pair of
Tree Swallows (Tachycineta bicolor) attempted to drive a pair of Pygmy Nuthatches (Sitta
pygmaea) out of their nest cavity. Cavities appear to be limited within the burned area and
species are competing to use them. Because population growth of secondary cavity users may
depend upon an adequate number of cavities available (Holt and Martin, 1997), successful
colonization in burned forests will depend on continued presence of woodpeckers to replenish
the supply of cavities.
While nearly all cavities had some secondary cavity use, very little detection occurred in
winter (November through February). Although many secondary cavity users are active residents
in winter, recently burned forests may not provide adequate food to support an overwinter
population. Studies investigating non-breeding use of cavities are rare; however Gentry and
Vierling (2008) did detect use over winter in a burned forest. That study, however, was
conducted between 11-15 years after the fires had burned. Succession and colonization over time
likely resulted in forest conditions capable of supporting over-wintering birds and small
mammals, which may be replicated at Angora in several years.
This study was limited by the number of cavities that were monitored for secondary
cavity use. Many snags fell during the study period, reducing sample sizes. Increasing the
number of nests discovered and monitored for each species of woodpecker will allow us to
determine if individual species of secondary cavity users show a preference for cavities created
by a particular woodpecker, which may support White-headed Woodpeckers as keystone
excavators in this system. This study was also limited by the lack of implementation of PFH. A
23
study examining the effects of PFH on woodpeckers is needed in order to improve our
understanding of how impacts may cascade down onto the community of secondary cavity users
that depend upon them.
Management Implications
Although woodpecker species differed in their influence on recovery of birds and small
mammals, all three species observed in our study played an important role in supporting the
cavity-dependent community through habitat creation for nesting, resting, denning, and roosting.
The Black-backed Woodpecker was a significant contributor to the establishment of bird and
small mammal species and communities in areas with high burn intensities, and it appeared to
have a more narrow range of suitable habitat conditions for nest site selection compared to the
Hairy Woodpecker. Thus, the habitat requirements of the Black-backed Woodpecker serve as a
useful threshold for managing burned sites for wildlife recovery.
All three species of woodpecker nested exclusively in snags, but only presence of nests of
Black-backed Woodpeckers was negatively correlated with nest tree DBH and positively
correlated with density of small snags. Post-fire forest management treatments in this study did
significantly reduce the overall snag density, thereby reducing nesting habitat for woodpeckers
and secondary cavity users. In particular, Black-backed Woodpeckers are likely to be negatively
impacted by post-fire snag removal due to their dependence on a high density of small diameter
snags. Currently, post-fire harvest prescriptions in the Angora fire footprint prescribe the
removal of all small snags and retention of approximately 5-10 large snags per hectare for
wildlife use (Angora Fire Restoration Project, Environmental Assessment, 2010). The removal
of most or all small snags within a burned area is likely to render the site unsuitable for Blackbacked Woodpecker nesting.
Reduction of all small snags may greatly reduce habitat for Black-backed Woodpeckers,
which in turn is likely to impact the recovery of bird and small mammal community recovery in
burned areas. Maintaining an abundance of suitable woodpecker nesting habitat in burned areas
will result in increases in the abundance and diversity of the cavity-dependent community
(Aitken and Martin, 2008). Cavity-dependent communities include seed dispersing birds and
mammals, insectivores, and predators, which play important roles in the overall ecosystem
(Raphael and White, 1978, 1984; Verner and Boss, 1980). This increases the diversity of species
performing a variety of ecosystem services. Diversity has been demonstrated to be an essential
ingredient to ecosystem stability and resilience (Hooper et al., 2005). Further, small mammal
species observed to utilize woodpecker nests also serve as important prey items for mid and
upper-level carnivores in the montane forest animal communities, such as California spotted
owls (Strix occidentalis occidentalis), coyotes (Canus latrans), weasels (Mustela spp.), and
martens (Martes americana). Reductions in the number of important prey items can have
cascading effects on higher trophic levels.
Management plans with multiple objectives of maintaining species diversity while
promoting fire safety may benefit from integrating strategies to maintain a diversity of
woodpecker species in burned forests. White-headed Woodpeckers in burned forests will require
decayed large diameter snags in open areas. Black-backed and Hairy Woodpeckers will require
areas with high densities of small to medium sized snags, especially in highly scorched areas.
This management goal may be achieved by leaving large patches of high density small snags and
harvesting other areas while leaving larger diameter snags.
24
Acknowledgments
We would like to thank our reviewers: Angela M. White and Tray Biasiolli of the Pacific
Southwest Research Station of the US Forest Service, T. Will Richardson of the Tahoe Institute
for Natural Science, and Tricia York of the California Tahoe Conservancy.
Literature Cited
Aitken, K.E.H., Wiebe, K.L., and Martin, K. (2002) Nest-site reuse patterns for a cavity-nesting
bird community in interior British Columbia. The Auk, 119, 391-402.
Aitken, K.E.H and Martin, K. (2007) The importance of excavators in hole-nesting
communities: availability and use of natural tree holes in old mixed forests of western
Canada. Journal of Ornithology 148: S425-S434.
Akaike, H. (1974). "A new look at the statistical model identification". IEEE Transactions on
Automatic Control, 19, 716–723.
Andersen, D.C. and J.A. MacMahon (1985) Plant Succession Following the Mount St. Helens
Volcanic Eruption: Facilitation by a Burrowing Rodent, Thomomys talpoides. The
American Midland Naturalist. 114, 62-69.
Bagne, K.E., Purcell, K.L., and Rotenberry, J.T. (2008). Prescribed fire, snag population
dynamics, and avian nest site selection. Forest Ecology and Management, 255, 99-105.
Bednarz, J.C., Ripper, D., and Radley, P.M. (2004) Emerging concepts and research directions in
the study of cavity-nesting birds: keystone ecological processes. The Condor, 106,1-4.
Blanc, L.A. and Walters, J.R. (2007) Cavity-nesting community webs as predictive tools: where
do we go from here? Journal of Ornithology, 148, S417-423.
Blanc, L.A. and Walters, J.R. (2008) Cavity excavation and enlargement as mechanisms for
indirect interactions in an avian community. Ecology, 89, 506-514.
Blanc, L.A. and Walters, J.R. (2008) Cavity-nest webs in a longleaf pine ecosystem. The
Condor, 110,80-92.
Bonar, R.L. (2000) Availability of pileated woodpecker cavities and use by other species.
Journal of Wildlife Management, 64, 52-59.
Brawn, J.D. and Balda, R.P. (1988). Population biology of cavity nesters in Northern Arizona:
do nest sites limit breeding densities? The Condor, 90, 61-71.
25
Bull, E.L., Parks, C.G., Torgersen (1997) Trees and logs important to wildlife in the interior
Columbia River Basin. General Technical Report PNW-GTR-391. Pacific Northwest
Research Station, Forest Service, U.S. Department of Agriculture.
Cline, A.B., Berg, A.B., Wight, H.M. (1980). Snag characteristics and dynamics in Douglas fir
forests, Western Oregon. Journal of Wildlife Management, 44, 773-786.
Connell, J.H. and R.O. Slatyer (1977) Mechanisms of Succession in Natural Communities and
Their Role in Community Stability and Organization. American Naturalist, 111, 11191144.
Covert-Bratland, K., Block, W.M., and Theimer, T.C. (2006). Hairy Woodpecker Winter
Ecology in Ponderosa Pine Forests Representing Different Ages since Wildfire. Journal
of Wildlife Management, 70, 1379-1392.
Czeszczewik, D., Walankiewicz, W., and Stanska, M. (2008) Small mammals in nests of cavitynesting birds: why should ornithologists study rodents? Canadian Journal of Zoology, 86,
286-293.
Dangerfield, J.M, McCarthy, T.S., and Ellery, W.N. (1998) The mound-building termite
Macrotermes michaelseni as an ecosystem engineer. Journal of Tropical Ecology, 14,
507-520.
Farris, K.L., Huss, M.J., and Zack, S. (2004) The role of foraging woodpeckers on the
decomposition of Ponderosa pine snags. The Condor, 106, 50-59.
Gentry, D.J. and Vierling, K.T. (2008) Reuse of woodpecker cavities in the breeding and nonbreeding seasons in old burn habitats in the Black Hills, South Dakota. American
Midland Naturalist, 160, 413-429.
Holt, R.F. and Martin, K. (1997). Landscape modification and patch election: The demography
of two secondary cavity nesters colonizing clearcuts. The Auk, 114, 443-455.
Hooper, D.U., Chapin, F.S., Ewel, J.J., Hector, A., Inchausti, P., Lavorel, S.,
Lawton, J.H., Lodge, D.M., Loreau, M., Naeem, S., Schmid, B., Setälä, H., Symstad,
A.J., Vandermeer, J., Wardle, D.A. (2005). Effects of Biodiversity on Ecosystem
Functioning: A Consensus of Current Knowledge. Ecological Monographs, 75, 3-35.
Hutto, R.L. (1995). Composition of bird communities following stand replacement fires in
northern Rocky Mountain (USA) conifer forests. Conservation Biology, 9, 1041–1058.
Jackson, J.A., Ouellet, H.R. and Jackson, B.J. (2002) Hairy Woodpecker (Picoides villosus), The
Birds of North America Online (A. Poole, Ed.). Ithaca: Cornell Lab of Ornithology;
doi:10.2173/bna.702.
Kotliar, N.B., Hejl, S.J., Hutto, R.L., Saab, V.A., Melcher, C.P., and McFadzen, M.E. (2002).
Effects of fire and post-fire salvage logging on avian communities in conifer dominated
forests of the western United States. Studies in Avian Biology, 25, 49-64.
26
Lawton, JH and Jones, CG. (1995) Linking species and ecosystems: organisms as ecosystem
engineers. Pages 141-150 in Jones CG, Lawton JH, eds. Linking species and ecosystems.
New York: Chapman and Hall
Martin, K. and Eadie, J.M. (1999) Nest webs: A community-wide approach to the management
and conservation of cavity-nesting forest birds. Forest Ecology and Management, 115,
243-257.
Martin, K., Aitken, K.E.H., and Wiebe, K.L. (2004) Nest sites and nest webs for cavity-nesting
communities in interior British Columbia, Canada: nest characteristics and niche
partitioning. The Condor, 106, 5-19.
Martin, T.E. and Geupel, G.R. (1993) Nest-Monitoring Plots: Methods for Locating Nests and
Monitoring Success. Journal of Field Ornithology, 64, 507-519.
Martin, T.E., C. Paine, C. J. Conway, W. M. Hochachka, P. Allen, and W. Jenkins. (1997)
BBIRD field protocol. Biological Resources Division, Montana Cooperative Research
Unit, Missoula, MT.
Paine, R.T. (1969) A Note on Trophic Complexity and Community Stability. The American
Naturalist, 103, 91-93.
Power, M., Tilman, D., Estes, J.A., Menge, B.A., Bond, W.J., Mills, L.S., Daily, G., Castilla,
J.C., Lubchenko, J., and Paine, R.T. (1996) Challenges in the quest for keystones.
Bioscience, 46, 609-620.
Raphael, M.G. andWhite, M. (1978). Snags, wildlife, and forest management in the Sierra
Nevada. Cal-Nevada Wildlife, 23-41.
Raphael, M. G. and M. White (1984) Use of snags by cavity-nesting birds in the Sierra Nevada.
Wildlife Monographs, 86, 1-66.
Rossell, F., Bozser, O. Collen, P., and Parker, H. (2005) Ecological impact of beavers Castor
fiber and Castor Canadensis and their ability to modify ecosystems. Mammal Review, 35,
248-276.
Saab, V.A., Dudley. J,, and Thompson, W.L. (2004) Factors influencing occupancy of nest
cavities in recently burned forests. The Condor ,106, 20-36.
Saab, V. A., Russell, R.E., and Dudley, J.G. (2007). Nest densities of cavity-nesting birds in
relation to postfire salvage logging and time since wildfire. The Condor, 109, 97-108.
Safford, H. D., Schmidt, D.A., and Carlson, C. (2009). Effects of fuel treatments on fire severity
in an area of wildland-urban interface, Angora Fire, Lake Tahoe Basin, California. Forest
Ecology and Management, 258, 773-787.
27
Salvador R, Valeriano J, Pons X , Diaz-Delgado R (2000) A semi-automatic methodology to
detect fire scars in shrubs and evergreen forests with Landsat MSS time
series. International Journal of Remote Sensing, 21, 655–671.
Steele, B.B. (1993). Selection of foraging and nesting sites by Black-throated blue warblers: their
relative influence on habitat choice. The Condor 95, 568-579.
Verner, J. and Boss, A.S. (1980). California Wildlife and their habitats: Western Sierra Nevada.
General Technical Report, PSW-37. Pacific Southwest Research Station, Forest Service,
U.S. Department of Agriculture.
United States Department of Agriculture. (2010). Environmental Assessment, Angora Fire
Restoration Project. Forest Service, Lake Tahoe Basin Management Unit.
Waddell, K.L. (2002). Sampling coarse woody debris for multiple attributes in extensive
resource inventories. Ecological Indicators, 1, 139-153.
28
Appendix A. Bird behavior indicative of nesting. (Martin and Geupel, 1993)
Bird behavior observed
Observer Action
Bird seen with nest material (grass, spider
web, hair, lichens)
Copulation
Female staying in area without actively
foraging
Female foraging quickly, or making rapid
hops, short flights, or rapid wing flicks
Female call notes
Male is quiet
Male carrying food
Female disappears into tree or shrub
Follow bird carefully from a distance while it
is gathering and carrying nest material.
Search immediate area where copulation was
observed. Copulation often occurs in the same
tree above a nest
Watch the female to see if she looks
repeatedly at a certain area. Often females
repeatedly look at their nest when an
observer/predator is detected.
Keep an eye on the female, she may be
returning to her nest soon
Be aware of short “chips”, “chucks” or other
call notes.
May indicate presence of foraging female or
nest nearby
Some males carry food to incubating females
Male or female carrying food
Lightly tap potential nest shrubs with a stick,
listen or watch for female to flush.
Follow bird from a distance to the nest.
Male or female carrying/dropping fecal sacs
Watch for bird to return to the nest site
Bird drops mouthful of food
You are much too close
Bird flushes from tree or shrub upon approach
Search immediate area and adjacent area.
Birds often sneak quietly to adjacent
tree/shrub before making a quick exit.
Nest is very close by. Do not follow bird, it
will try to lead you away from the nest
You are close to the nest. Search immediate
area.
Especially good for detecting activity in
cavities
Bird is feigning injury
Bird is agitated, scolding
Noisy babies
29