Cavity-nesting Bird Use of Snags in
Eastside Pine Forests of Northeastern
California1
William F. Laudenslayer, Jr.2
Abstract
Relationships between snags (standing dead trees) and cavity-nesting birds were examined in
the breeding seasons of 1989, 1990, 1991 on the Modoc and Lassen National Forests and
Lassen Volcanic National Park, California. Transects, that differed by snag density, were
randomly placed in eastside pine habitat patches dominated by either ponderosa (Pinus
ponderosa) or Jeffrey pine (P. jeffreyi). Snags with active nests had greater diameters and
were taller than random alternative snags; both differences were significant (P > 0.05). Snags
with historical nest cavities generally were of larger diameter than snags without historical
nest cavities. Despite the heavier nesting use of larger snags, many large snags, with similar
visual deterioration characteristics, showed no indication of historical nesting use.
Introduction
Snags (standing dead trees) are an important component of forests and play a
crucial role in the continuation of soil fertility as well as perpetuation of species that
depend on snags for parts of their life histories (e.g., snag associated insects,
substrates for vertebrate nesting, and roosting cavities) (Bull and others 1997,
Machmer and Steeger 1995, Parks and others 1997). Some bird species, because of
their need for cavities in which to nest (i.e., natural cavities, existing excavated
cavities, or suitable conditions for the excavation of new cavities), find suitable
nesting substrate in snags or live trees with patches of decay or cavities resulting
from limb breakage. Some 85 species of North American birds, not to mention
numerous other vertebrates and invertebrates, construct nests in snags, or nest in
natural cavities, or previously excavated holes in snags (Scott and others 1977).
Availability of nest sites may limit the numbers of cavity-nesting species.
Snag size, especially diameter, and to a lesser extent height, are also thought to
be important characteristics related to bird usage. Generally, larger diameter snags
are used in preference to smaller diameter snags (Bull 1975, Cunningham and others
1980, Mannan and others 1980, McClelland and Frissell 1975, Milne and Hejl 1989,
Raphael and White 1984, Scott 1978, Scott and Oldemeyer 1983). To the contrary,
Hay and Guntert (1983) concluded that smaller snags are preferred by some species
1
An abbreviated version of this paper was presented at the Symposium on the Ecology and Management
of Dead Wood in Western Forests, November 2-4, 1999, Reno, Nevada.
2
Research Wildlife Ecologist, Pacific Southwest Research Station, USDA Forest Service, 2081 E.
Sierra Ave., Fresno, CA 93710 (e-mail: blaudenslayer@fs.fed.us)
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Nesting Bird Use of Snags in Eastside Pine Forests—Laudenslayer
for purposes other than nesting. Many authors consider the presence of snags to be
essential for the continued presence of cavity-nesting birds.
The eastside pine forest of California is characterized by relatively open stands
of pine with relatively few snags compared to other western forests. Preliminary
surveys of this forest in 1988 indicated that very large portions of the Modoc and
Lassen National Forests had well under one snag to the acre. The paucity of snags is
related to the lower tree densities inherent in these stands as well as past management
activities. In the late 1980s, the USDA Forest Service became concerned about the
effects of this perceived current and future snag deficit on cavity-nesting birds.
To gain a better understanding of the effects of snag densities on cavity-nesting
birds, a study was designed to examine the relations between snag numbers and
numbers of cavity-nesting birds. The study also collected information on the
characteristics of snags used by these nesting birds. This paper contrasts the
characteristics of snags used by actively nesting birds with the characteristics of
snags available in close proximity but not used; and it will contrast the characteristics
of snags used currently with those used by nesting birds in the past.
Methods
Study Areas
The 24 study areas are located in eastside pine forests of Modoc, Lassen, and
Shasta Counties, California. These forests are dominated by ponderosa pine (Pinus
ponderosa), Jeffrey pine (P. jeffreyi) in some locations, and white fir (Abies
concolor) with smaller amounts of incense cedar (Calocedrus decurrens), western
juniper (Juniperus occidentalis), California black oak (Quercus kelloggii), and
lodgepole pine (P. contorta). Dominant shrubs include big sagebrush (Artemisia
tridentata), bitterbrush (Purshia tridentata), and mahala mat (Ceanothus prostratus);
with lesser amounts of Nevada manzanita (Arctostaphylos nevadensis), curl-leaf
mountain-mahogany (Cercocarpus ledifolius), and silver sagebrush (Artemisia cana).
Study Design
Snag densities were estimated for approximately 1,000 land management
polygons on the Modoc and Lassen National Forests, and suitable areas within
Lassen Volcanic National Park. Study areas were randomly selected from the pool of
potential study areas in seven snag density classes: 0, >0 to <0.20, >0.20 to 0.40,
>0.40 to <0.61, >0.61 to <0.81, >0.81 to <1.21, and >1.21/hectare. All of the lesser
snag density classes had sufficient numbers of potential study areas from which to
randomly draw the study areas. For the two densest snag density classes, all areas
found with such densities were used in the study. One 100 x 500 m long strip transect
was placed randomly within each of the 24 selected study areas; the bounds of these
areas were used as the basis of all subsequent work.
Active Nests
From 1989 through 1991, each transect was searched for nests, especially of
cavity-nesting birds. Nests were located by following birds back to their nests,
visually examining potential nest holes for indication of recent excavation, and
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Nesting Bird Use of Snags in Eastside Pine Forests—Laudenslayer
watching and listening for nesting activity (e.g., carrying nesting material, food
delivery, hearing the sounds of young birds).
At active nests, information was collected on tree or snag species, diameter at
breast height (DBH), total height, nest height, nest hole diameter, compass direction
the nest hole faces, and whether the nest was constructed in living or dead portions of
the tree. For each active nest tree, the nearest random alternative snag or live tree,
without an active nest, was also identified. Information collected from this snag or
tree was species, DBH, height. If either the nest snag or alternative site was a snag
included in the snag inventory, the snag number was also recorded linking the nest or
alternative site to the snag characteristics data collected under snags.
Historic Nest Holes
All snags with diameters in excess of 15 cm were permanently marked, mapped,
and information including species, diameter, height, percent bark remaining, and
number of nest holes were taken on 23 of the 24 study plots in 1989. On one plot, the
number of small snags (<25 cm in diameter) was so great that information was taken
only on those snags with diameters in excess of 25 cm.
Analysis
Primary and secondary cavity-nesting bird species found nesting in the study
areas were red-breasted sapsucker (Sphyrapicus ruber), Williamson’s sapsucker
(Sphyrapicus thyroideus), hairy woodpecker (Picoides villosus), white-headed
woodpecker (Picoides albolarvatus), black-backed woodpecker (Picoides arcticus),
northern flicker (Colaptes auratus), pileated woodpecker (Dryocopus pileatus), tree
swallow (Tachycineta bicolor), mountain chickadee (Poecile gamblei), red-breasted
nuthatch (Sitta canadensis), white-breasted nuthatch (Sitta carolinensis), pygmy
nuthatch (Sitta pygmaea), brown creeper (Certhia americana), house wren
(Troglodytes aedon), and mountain bluebird (Sialia currucoides).
Comparisons of individual species were based on the four with the greatest
number of nests found: hairy woodpecker, mountain chickadee, red-breasted
nuthatch, and pygmy nuthatch. Characteristics of snags with active nests were
compared to alternative nest snags using t-tests and box plots. Information on
historical nest holes and nesting snags (of snags >15 inches in DBH) was taken from
data collected in 1990.
Results
Active Nests
Over the 3-year bird study period, 110 active nests of cavity-nesting birds were
located. All but four of these nests were in ponderosa or Jeffrey pines. The majority
of nests were of hairy woodpecker (17), mountain chickadee (12), red-breasted
nuthatch (11), and pygmy nuthatch (16).
On occasion, live trees were used for nesting and most of the cavity-nesting bird
species took advantage of such sites. Eight nests (7 percent of total) were found in
live trees; these sites include the dead tops of large diameter pine trees, a crack in the
bole of a large tree, other damaged areas where the bark was lost, and a dead limb in
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a western juniper. Half of the live tree nests were on transects with more than 0.4
snags greater than 38 cm in diameter during the bird study time period (see Landram
and others 2002 for a discussion of snag demography on these study areas).
Mean DBH of snags used for nesting were in excess of 70 cm for the four birds
of interest (fig. 1). The smallest diameter used by all four species was approximately
40 cm. Although the smallest diameters used by each species approximated 40 cm,
the largest diameters were quite variable ranging from <100 (mountain chickadee) to
approximately 180 cm (red-breasted nuthatch).
Figure 1—Mean diameters (cm) of nesting snags used by hairy woodpecker (HaWo n = 17), mountain chickadee (MoCh - n =10), red-breasted nuthatch (RbNu - n = 11),
and pygmy nuthatch (PyNu - n = 16) + 1 standard error and values for the largest and
smallest nest tree diameter.
Heights of snags used for nesting ranged from less than 5 m (mountain
chickadee) to more than 45 m (red-breasted nuthatch) (fig. 2). The shortest trees used
by each species was between 7 and 9 m in height except for mountain chickadee—
one nest at 3 m and two at essentially 0 m (both in downed logs). Tallest trees used
approached the maximum available. Despite this variation in the ranges in nest tree
height by species, mean heights of nest trees selected by each species were not very
different with the exception of hairy woodpecker, which generally used shorter trees
than the other three species.
Nest heights were quite variable: hairy woodpecker nests generally were higher
than the other three species, and mountain chickadee nests generally were lower than
the other three species (fig. 3). Range of nest heights was from about 2 m for all
except mountain chickadee to 20 m and above. Several mountain chickadee nests
were in logs nearly at ground level.
Trees selected for nesting differed significantly in diameter (fig. 4) (t-value =
6.774; df = 106; P-value <0.0001) and height (fig. 5) (t-value = 2.916; df = 106; Pvalue = 0.0043) from the nearest randomly selected trees.
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Nesting Bird Use of Snags in Eastside Pine Forests—Laudenslayer
Figure 2—Mean heights (m) of nesting snags used by hairy woodpecker (HaWo - n
= 17), mountain chickadee (MoCh - n =10), red-breasted nuthatch (RbNu - n = 11),
and pygmy nuthatch (PyNu - n = 16) + 1 standard error and values for the tallest and
shortest nest trees.
Figure 3—Mean nest heights (m) of hairy woodpecker (HaWo - n = 17), mountain
chickadee (MoCh - n =10), red-breasted nuthatch (RbNu - n = 11), and pygmy
nuthatch (PyNu - n = 16) + 1 standard error and values for the highest and lowest
nest locations.
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Nesting Bird Use of Snags in Eastside Pine Forests—Laudenslayer
Figure 4—Comparison of nest snag diameters (cm) to diameters of alternative snags
(n = 107). The box plots display the 10th, 25th, 50th, 75th and 90th percentiles of
each variable.
Figure 5—Comparison of nest snag heights (m) to heights of alternative snags (n =
107). The box plots display the 10th, 25th, 50th, 75th and 90th percentiles of each
variable.
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Nesting Bird Use of Snags in Eastside Pine Forests—Laudenslayer
Historic Nest Holes
In 1989, a total of 1,459 snags of 9 snag species were found on the 24-5 ha study
plots (table 1). Nest holes were not found in any of the California black oak, incense
cedar, lodgepole pine, red fir, or western juniper snags on any study plot. Of the tree
species with nest holes, Jeffrey pine, ponderosa pine, and white fir, only about 15
percent of the snags of each species had at least one nest hole (fig. 6). For the fourth
category, unknown pine (Jeffrey or ponderosa pine but too deteriorated to determine
which species), some 35 percent of the snags had at least one nest hole. Snags with
more than eight nest holes were very rare and found only in Jeffrey and ponderosa
pines.
For the four taxa of snags with nest holes, as snag diameter increased, generally
a higher proportion of snags had nest holes (fig. 7). Almost none of the snags in the
0-30 cm diameter class had nest holes regardless of species but between 10 (white
fir) and 100 percent (unknown pine) of the >120 cm diameter class had nest holes.
Numbers of nest holes relative to the numbers of snags increase as diameter
class increases (fig. 8). Except for white fir, snags in larger diameter classes generally
had greater numbers of nest holes than did snags in smaller diameter classes.
Table 1—Numbers of snags present by snag species across all 24-5 ha study plots in 1989.
Snag species
No. of snags
California black oak
4
Incense cedar
27
Jeffrey pine
843
Lodgepole pine
30
Unknown pine
35
Ponderosa pine
269
Red fir
1
Western juniper
66
White fir
184
Total
1,459
Figure 6—Percentages of snags with historic nest holes, for the snag species that
possessed more than one nest hole, ranging from 0 nest holes to more than 8.
Jeffrey pine = JePi (n = 843), ponderosa pine = PoPi (n = 269), unknown pine = Unk
Pi (n = 35), white fir = WhFi (n = 183). Both JePi and PoPi have snags with >8 nest
holes per snag, but the percentages are too low (0.1 and 0.7 percent respectively) to
be illustrated in the figure.
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Figure 7—Percentages of snags by diameter class with historic nest holes, for the
snag species with more than one nest hole, that had nest holes compared to those
without any nest holes. Jeffrey pine = JePi (n = 843), ponderosa pine = PoPi (n =
269), unknown pine = Unk Pi (n = 35), white fir = WhFi (n = 183).
Figure 8—Numbers of snags by diameter classes with nest holes, for the snag
species with more than one nest hole, compared to the number of nest holes in each
diameter class. Jeffrey pine = JePi (n = 843), ponderosa pine = PoPi (n = 269),
unknown pine = Unk Pi (n = 35), white fir = WhFi (n = 183).
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Discussion
Active Nests
All four bird species nested in trees, generally dead or in the dead sections of
live trees, that were greater than 40 cm DBH and with heights of nest trees that
generally exceeded 10 m. The smallest diameter trees used approximate the lower
limits of snag diameters found in many snag guidelines (e.g., Studinski and Ross
1986). The majority of the nests were in trees substantially larger in diameter than 40
cm DBH. Hairy woodpeckers often used shorter trees than the other species, but the
majority of nests of all species were in trees in excess of 20 m in height. The heights
of the nests, somewhat dependent on the height of the trees, was perhaps more
variable than either the diameters or heights of trees selected for nesting. Hairy
woodpeckers, despite selecting shorter trees on the average, tended to place their
nests higher in the trees than the other species.
The assemblage of cavity-nesting birds generally placed their nests in trees
larger in diameter and taller than randomly selected alternative trees found in the
study areas. The difference in diameters was greater than the difference in heights,
suggesting that tree diameter may be the more important variable driving nest tree
selection. Another explanation is that tree diameters are more stable than tree heights,
as tree height often becomes shorter as trees decay, whereas once the bark is lost tree
diameters do not change measurably.
The selection of larger trees for nesting by cavity-nesting birds in California
eastside pine forests reflects what is known from other forests and is well
documented in the literature (e.g., Bull 1978, Cunningham and others 1980,
Gutzwiller and Anderson 1986, Gutzwiller and Anderson 1987, Mannan and others
1980, Mannan and Meslow 1984, McClelland and Frissell 1975, Raphael and White
1984, Rosenberg and others 1988, Swallow and others 1988). Larger trees are
generally preferred over smaller trees for most cavity-nesting species.
Ponderosa and Jeffrey pines are the tree taxa most used for nesting in this study.
These eastside pine forests are dominated by ponderosa or Jeffrey pine with other
species of trees relatively uncommon. The pines were generally of larger diameter
than the other tree species.
Sapwood decay in ponderosa pine, and perhaps Jeffrey pine also, tends to be
rapid and the decayed sapwood is easily excavated in cavity construction (Parks and
others 1997). Thus, the entire cross-section of potential nest trees may not be suitable
for excavation, and trees larger in diameter than the nominal guidelines suggest may
be necessary for nesting birds. For the larger-bodied species, larger diameter trees
may even be more desirable.
Historic Nest Holes
When the study was initiated, nearly 1,500 snags were available for nesting. The
numbers of snags per plot varied considerably from 0 on several plots to as many as
several hundred in a few cases. The vast majority of snags did not contain any nest
holes, and only a very few snags had more than two-nest holes. Some 10-15 percent
of the ponderosa and Jeffrey pine and white fir snags had nest holes; and 25 percent
of the unknown pine species snags had nest holes. Although many ponderosa and
Jeffrey pine and white fir snags had not decayed to a state that would permit
excavation of nest holes (i.e., obvious separation of the bark from the bole or loss of
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large portions of the bark, absence of needles and branchlets), the majority of
unknown pine species snags had aged to a point where one might expect the birds to
be able to excavate a nesting cavity. These pine snags had decayed to a state where
they could no longer be classified into either ponderosa or Jeffrey pine using our field
methods. However, even in these rather well decayed trees, a large number of snags
did not have any nest holes. This suggests that a rather large number of the snags in
this study were not suitable or capable for excavation of nest holes or that birds select
snags for locating nest holes on criteria not readily apparent from the visual
characteristics of the snags.
Zack and others (2002) reported that the majority of snags at Blacks Mountain
Experimental Forest and Goosenest Adaptive Management Area did not have any
cavities. However, more snags at Blacks Mountain had cavities than at Goosenest.
Welsh and Capen (1992) also noted that relatively large numbers of apparently
suitable nest trees were not used as nesting sites and also found that the number of
excavated cavities per cavity trees were relatively low. Experimental evidence related
to this question is not conclusive. Differences in habitat characteristics may influence
the findings from snag and cavity studies. Waters and others (1990) reported that nest
sites are not limiting to the secondary cavity-nesting bird community in an oak-pine
woodland. However, Brawn and Balda (1988) showed that nest sites can be limiting
for secondary cavity nesters in a ponderosa pine forest.
Birds apparently do not randomly choose snags in which to excavate a nest. The
larger the diameter of the snag, the more probable it is to have had nest holes
excavated in it. This is particularly apparent in ponderosa and Jeffrey pine, and
unknown pine snags and, as indicated earlier, may be related to the portions of the
trees where the decay takes place.
Management Implications
Snags with large diameters and heights (e.g., greater than 25 inches in diameter)
have been identified as very important to many of the cavity-nesting bird species.
Such snags can only be derived from large trees that have been given the time to
grow large. Because of the commercial value of large trees, they appear to be in short
supply across large landscapes in eastside pine forests. To best provide habitat for
these cavity-nesting species, larger parcels of land should be managed to provide
trees of a variety of sizes and permit some of the largest trees to become snags.
Management guidelines for specific snag densities are difficult to propose
because the capability of the land to produce snags, especially large ones, is so
variable, the snag resource is highly dynamic both in space and time, and many snags
do not appear to be used as nesting substrate, suggesting that some snags may be
surplus (as far as bird nesting is concerned). Balda (1975), as well as many others
since, has proposed methods for estimating how many snags are needed by specific
bird species. Application of those methods could form a baseline for specifying snag
densities. However, snag numbers are probably related to the capability of particular
sites to produce and sustain snags through time and related to site-specific variables,
such as the number of large trees existing per acre, the lifespans of the large trees,
and the mean standing duration of large snags. Snag numbers are also related to the
historical consequences and causes of mortality (e.g., fire, insect attack) and their
effects on the residual of large live trees that will eventually become snags. Thus,
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regardless of guidelines proposed, they will need to be tailored to specific landscape
capabilities and conditions at the project scale.
Snags are dynamic “entities” on dynamic landscapes that emerge and fall at
different rates, depending on a variety of conditions related to cause of mortality and
site characteristics. As such, land managers cannot be expected to understand or
manage the decay trajectories of each snag. Despite the uncertainties involved in snag
management, snag condition, hardness or softness, or simply the age of the snag may
be useful criteria for choosing snags for removal or retention. Generally, it seems that
the longer a snag stands, the more opportunity it has to be used to excavate a cavity
or provide an existing cavity for a secondary cavity nester to use. Old snags are
usually not commercially valuable, and land managers should attempt to retain those
that do not pose a substantial safety or fire risk. Specific snag objectives might then
be based on retention of hard snags.
The removal of hazard trees and reduction of fuels can also reduce snag
numbers substantially, especially in well roaded areas like northeastern California. It
is often necessary to reduce hazards and reduce fuels, but removal of entire snags in
many cases is not justified. Topping trees rather than total removal is an option that
will maintain some, perhaps the majority, of the snag value yet reduce the hazards
inherent in standing snags. Snag topping, as done on the Modoc National Forest
(sawing off the tops; Studinski, pers. comm.) and at Yosemite National Park (pulling
out the tops; Mattos, pers. comm.), may also permit snags to persist for a longer time.
However, it is not yet known if such methods extend snag life, and it is not known
how cavity-nesting species respond to such treated snags.
If there are fewer snags on landscapes of interest than desired, snags can be
created by a variety of methods (see Shea and others 2002 for a discussion on snag
creation methods and results). Depending on the agent used to create snags, such
trees can provide habitat for cavity-nesting birds as well as other organisms. Even
retaining stumps, cut as high as possible, will still retain some of the snag values for
cavity-nesting birds. However, Morrison and others (1983) recommend that high-cut
stumps be employed only when absolutely necessary for safety considerations.
Information about the relationships of cavity-nesting birds with snag
characteristics can be used to craft management approaches to the conservation of the
snag resource and cavity-nesting birds. However, managers must not predicate their
management strategies only on providing nesting substrates. These birds require a
continual sequencing of snags into their habitats for foraging as well as for nesting. In
addition, cavity-nesting birds are not the only organisms that prefer or perhaps
require dying and dead trees for their survival; a myriad of other organisms,
including fungi, invertebrates, and other vertebrates, are also associated with these
trees. Simply managing for the birds’ needs may not fulfill the requirements of other
species that may actually be more essential to the processes in the forest, such as the
recycling of wood.
Acknowledgments
The advice and ideas of James A. Baldwin, Barry R. Noon, David A. Sharpnack,
and Jared Verner were incorporated into the design and methods of the study. Sandra
Arnold, George Banuelos, Craig DeMartini, Erin Deneke, Mary Flores, Sheila Kee,
Bo Larsen, David Lee, Britta Muiznieks, John Sterling, and Ellen VanGelder served
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Nesting Bird Use of Snags in Eastside Pine Forests—Laudenslayer
as field observers for portions of the study. Thoughts of Jon Arnold, Alan Berryman,
Robert Borys, Al Denniston, Gary Eberlein, Kerry Farris, George Ferrell, George
Gittings, R.J. Laacke, Pat Shea, George Steger, George Studinski and Steve Zack all
contributed to this paper. Review comments by Don Behrens, Dean Carrier, Sandy
Hicks, Brad Valentine, and Steve Zack were greatly appreciated and incorporated.
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