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2010 SOUTHEASTERN NATURALIST 9(4):813–820
Flying Squirrel Removal Does Not Reduce Their Use of
Simulated Red-cockaded Woodpecker Nest Clusters
Jennifer S. Borgo1 2,*, Michael R. Conover1, and L. Michael Conner3
Abstract - Reproductive success of the endangered Picoides borealis (Red-cockaded
Woodpecker) is thought to be reduced by the presence of Glaucomys volans (Southern
Flying Squirrels); hence, these squirrels are often removed when found inside
woodpecker cavities. For this management practice to benefit Red-cockaded Woodpeckers,
however, squirrel removal must both reduce the future probability of a flying
squirrel re-occupying cavities and increase reproductive success for Red-cockaded
Woodpeckers. In this study, using simulated Red-cockaded Woodpecker clusters
(pseudo-clusters), we tested the first assumption regarding squirrels reoccupying nest
cavities. We found no differences between removal and control pseudo-clusters in
the amount of time that flying squirrels were present in pseudo-clusters, the proportion
of nest boxes occupied by flying squirrels, or the mean number of total squirrels
and individual squirrels (counting each squirrel only once in the analysis) present in
the pseudo-clusters. Thus, removing flying squirrels from nest clusters did not reduce
the future probability of a flying squirrel occupying either a cavity or a cluster.
These results indicate a need to re-evaluate flying squirrel removal as a management
technique to enhance Red-cockaded Woodpecker reproduction.
Picoides borealis Vieillot (Red-cockaded Woodpecker) is a cooperatively
breeding bird that occurs in old-growth pine stands of the southeastern United
States. It has been listed as an endangered species since 1973 (USFWS 2003).
Red-cockaded Woodpeckers excavate cavities in living Pinus spp. (pines)
that they use for nesting and roosting (Lennartz et al. 1983, Steirly 1957).
Cavities in pine forests are scarce and Glaucomys volans L. (Southern Flying
Squirrels) often exclude Red-cockaded Woodpeckers from their own
cavities (Conner et al. 1996, Harlow and Lennartz 1983, Kappes and Harris
1995, Laves and Loeb 1999, Loeb 1993). Cavity usurpation by flying squirrels
prevents Red-cockaded Woodpeckers from nesting and successfully raising
offspring. In fact, Red-cockaded Woodpeckers are less likely to nest or successfully
fledge even if only one cavity in a cluster (i.e., aggregation of cavity
trees) is occupied by a squirrel (Loeb and Hooper 1997). Hence, much effort
has gone into trying to keep flying squirrels from occupying Red-cockaded
Woodpecker cavities. Usually these efforts have relied upon the removal of
flying squirrels at the nest-cluster needing protection (Franzreb 1997, Laves
and Loeb 1999, Mitchell et al. 1999, Richardson and Stockie 1995).
1Jack H. Berryman Institute, Utah State University, 5230 Old Main Hill, Logan, UT
84322. 2Current address - Coker College, 300 East College Avenue, Hartsville, SC
29550. 3Joseph W. Jones Ecological Research Center, Route 2, Box 2324, Newton,
GA 39870. *Corresponding author - firstname.lastname@example.org.
814 Southeastern Naturalist Vol. 9, No. 4
It is unclear, however, whether removing flying squirrels results in higher
reproductive success of Red-cockaded Woodpeckers. There has only been
two studies on the topic, and their results were contrary (Laves and Loeb
1999, Mitchell et al. 1999). In Georgia, Mitchell et al. (1999) compared the
reproductive success of Red-cockaded Woodpeckers in clusters where flying
squirrels had been removed to that of untreated clusters. They found no difference
in the number of woodpecker eggs, nestlings, or fledglings produced
in the first or second nest attempts in either year. Squirrel removal also did
not affect the percentage of successful clusters (i.e., those fledging ≥1 chick;
Mitchell et al. 1999). Laves and Loeb (1999) evaluated the effects of Southern
Flying Squirrel removal on a population of Red-cockaded Woodpeckers
in the Carolina Sandhills, SC. They found that removal had no effect on the
likelihood of nesting by Red-cockaded Woodpeckers. Reproductive success,
however, based on both hatching and fledging rates, was higher in removal
clusters than control sites in their study.
Because these two studies produced contrary results, few conclusions can
be drawn about the utility of flying squirrel removal as a management technique.
One limitation on further testing is that Red-cockaded Woodpeckers
are endangered and wildlife managers do not want to risk losing their existing
populations by stopping squirrel removal from active nest clusters; yet, further
research on this topic is clearly warranted. Therefore, we tried an alternative
approach to determine whether flying squirrel removal is a useful management
practice. For flying squirrel removal to increase the reproductive success of
Red-cockaded Woodpeckers, two assumptions must be true. First, the removal
effort must reduce the likelihood that a squirrel will occupy a cluster in the future.
Second, lower levels of flying squirrel encroachment must lead to higher
reproductive success in Red-cockaded Woodpeckers. In this study, we tested
the first assumption.
This study occurred on the 11,736 ha Joseph W. Jones Ecological Research
Center (a.k.a. Ichauway) located in Baker County, about 61 km
southwest of Albany, GA. Currently a privately funded research institution,
the Jones Center originated in the 1920s as a quail plantation. Ichauway is
dominated by Pinus palustris Mill (Longleaf Pine) savannah. Within this
matrix, patches of P. taeda L. (Loblolly Pine) forests, P. elliotti Engelm
(Slash Pine) flatwoods, and mixed hardwoods (predominantly Quercus spp.
[oaks]) are found. While Aristida beyrichiana Trin. & Rupr. (Beyrich Threeawn
or Wiregrass) and old-field grasses (e.g., Andropogon spp.) dominate,
over 1000 species of vascular plants are present in the understory (Drew et
al. 1998, Goebel et al. 1997). Ichauway is managed to maintain a Longleaf
Pine and Wiregrass community through prescribed fire (generally a two-year
rotation) and extensive hardwood removal.
2010 J.S. Borgo, M.R. Conover, and L.M. Conner 815
Actual Red-cockaded Woodpecker nest-clusters could not be used for
this study because management policy at Ichauway requires removal of
all flying squirrels present in active Red-cockaded Woodpecker clusters
(i.e., there could be no control sites in active Red-cockaded Woodpecker
clusters). Thus, we created 20 “pseudo-clusters” using nest boxes instead
of tree cavities. Each pseudo-cluster had the same habitat qualifications as
Red-cockaded Woodpecker cluster sites (open midstory, <500 m apart, and
≥4 potential cavity trees; USFWS 2003). All pseudo-clusters were >4 km
from the nearest active Red-cockaded Woodpecker cluster. Trees utilized for
pseudo-clusters had to be live and ≥38 cm diameter at breast height. Four
nest boxes were attached to trees between 4.7 and 6.8 m above the ground
within the site to create a pseudo-cluster. The nest box design was modified
from Sonenshine et al. (1973). Compared to Red-cockaded Woodpecker
cavities, they are somewhat larger (3630 versus 921 cm³) but have a similar
entrance diameter (4.4 versus 4.5 cm). While providing less protection from
the elements than internal cavities, flying squirrels still readily use them
(Gilmore and Gates 1985). Because flying squirrels selected both nest boxes
and cavities present at Ichauway in the proportion that they were available
1 = 0.03, P > 0.75; Borgo 2004), we considered nest boxes adequate to
simulate Red-cockaded Woodpecker nest cavities. Eighty nest boxes were
placed in the 20 pseudo-clusters. We installed nest boxes from 20–25 February
2003 and left them up until 3 July 2003, concurrent with the time period
that flying squirrels are removed from Red-cockaded Woodpecker clusters
at Ichauway. While other studies on the effect of flying squirrel removal
on Red-cockaded Woodpeckers have done a more intensive removal effort
(Laves and Loeb 1999, Mitchell et al. 1999), our effort was better matched
to a typical management situation.
Nest boxes were checked biweekly from 11 March to 3 July 2003 using a
Treetop Peeper® (Sandpiper Technologies, Inc., Manteca, CA), for a total of
nine visits of each cluster. We gained access to nest boxes using a Swedish
sectional ladder. The entrance hole was then blocked with a cloth so squirrels
could not escape through it. We subsequently removed squirrels through a
wire screen in the box. All captured squirrels were ear-tagged and weighed.
All squirrels found in control pseudo-clusters were released on the tree
from which they were captured. In contrast, squirrels captured in treatment
pseudo-clusters were removed and euthanized (Scientific Collecting Permit,
Georgia Department of Natural Resources, 19-WNB-02-86). Whether a
pseudo-cluster was treatment (removal; n = 10) or control (no removal; n =
10) was determined randomly.
We considered the pseudo-cluster the sampling unit (n = 20). The variables
analyzed were the proportion of visits in which no squirrels were
present in any nest box within each pseudo-cluster (success), the proportion
of nest boxes occupied by flying squirrels in each pseudo-cluster per visit,
816 Southeastern Naturalist Vol. 9, No. 4
and the number of squirrels found within each pseudo-cluster per visit. The
latter two variables, though related, are not the same because sometimes ≥1
squirrel used a nest box. We used a Wilcoxon rank sums test (SAS 2003)
to compare the proportion of visits no squirrels occupied a pseudo-cluster
between treatment and control pseudo-clusters due to violation of normality
assumptions. We used the same statistic to compare success between
removal pseudo-clusters and active Red-cockaded Woodpecker clusters on
Ichauway to evaluate if there was similar use by flying squirrels in both
areas. If this was so, this indicates that our use of pseudo-clusters instead of
active clusters was justified. We used a repeated-measures analysis of variance
(Zar 1999) to compare both the proportion of nest boxes occupied in a
pseudo-cluster (transformed using arcsine of the square root) and the number
of squirrels found at pseudo-clusters (square root transformation) with and
Nest-box occupants included flying squirrels (n = 46), birds (n = 34),
and frogs (n = 2; Hyla sp.). Myiarchus crinitus L. (Great Crested Flycatchers),
Parus bicolor L. (Eastern Tufted Titmice), Sitta carolinensis Latham
(White-breasted Nuthatches), and Melanerpes carolinensis (Red-bellied
Woodpeckers) used the boxes for nesting attempts. Given that the pseudoclusters
were outside Red-cockaded Woodpecker cluster areas at Ichauway,
we did not expect to find Red-cockaded Woodpeckers utilizing our boxes.
We removed a total of 58 flying squirrels from the 10 treatment clusters.
The total number of flying squirrels captured and released in the 10 control
clusters was 54, which included 37 individuals. Flying squirrels were present
in control pseudo-clusters 16 separate times. They were present in removal
pseudo-clusters 18 separate times. The number of total squirrels (all squirrels
found each visit) using nest boxes per pseudo-cluster per visit in the
control pseudo-clusters (mean ± SE = 0.60 ± 0.22) was similar (F1,18 = 0.06,
P = 0.81) to the number using the removal pseudo-clusters (0.64 ± 0.16).
Regarding the number of individual squirrels, any previously marked squirrel
was not included a second time in the analysis. The number of individual
squirrels using nest boxes per pseudo-cluster per visit was also not signifi-
cantly different (F1,18 = 0.78, P = 0.39) between the control and treatment
pseudo-clusters (0.41 ± 0.13 for the control pseudo-clusters, 0.64 ± 0.16 for
the removal pseudo-clusters).
Flying squirrel use of nest boxes increased over time at a similar rate in
both treatment and control pseudo-clusters (Fig. 1). Success (percent of visits
no flying squirrels were present) was similar between control (82.4% ± 0.05)
and treatment (80.2% ± 0.05) pseudo-clusters (S = 109.50, two-sided P =
0.81). Success (% visits no squirrels were present in clusters or pseudo-clusters)
was also similar (S = 104.00, two-sided P = 0.2650) between treatment
pseudo-clusters (80.2% ± 0.05) and active Red-cockaded Woodpecker clusters
(87.2% ± 0.03) at Ichauway (Borgo et al. 2006a). The proportion of nest
2010 J.S. Borgo, M.R. Conover, and L.M. Conner 817
boxes occupied by flying squirrels per pseudo-cluster per visit was not significantly different (F1,18 = 0.07, P = 0.79) between control (0.06 ± 0.02 nest
boxes) and treatment (0.07 ± 0.01 nest boxes) pseudo-clusters.
For flying squirrel removal to benefit Red-cockaded Woodpeckers, removal
must both reduce the likelihood of squirrels occupying a cavity within
a cluster in the future and increase the reproductive potential of Red-cockaded
Woodpeckers. In this study, we tested whether the removal of flying squirrels
from a cluster reduced the likelihood of their re-occupying cavities in the same
cluster at a later date. We found that it did not. One limitation of our study was
that our pseudo-clusters were not occupied by Red-cockaded Woodpeckers.
Their absence may have made a difference in the rate at which flying squirrels
occupied cavities within a pseudo-cluster. Still, our results provide a hypothesis
regarding why squirrel removal had little impact on nesting success of
Red-cockaded Woodpeckers elsewhere in Georgia (Michell et al. 1999). Our
findings, when combined with those of Mitchell et al. (1999), call into question
the usefulness of this management practice.
The basic problem with Southern Flying Squirrel removal in Red-cockaded
Woodpecker clusters seems to be that the removal efforts are limited to
the immediate area around the cluster. In general, removing animals from a
small area leaves a large adjacent population that may re-invade the targeted
area (Conover 2002, Frey et al. 2003). For many species, rapid colonization
by non-residents into removal areas works against any control effort
Figure 1. Total number of Flying Squirrels found in nest boxes in all pseudo-clusters
(10 control, 10 treatment) during periodic checks from 11 March to 3 July 2003 at
the Joseph W. Jones Ecological Research Center, GA.
818 Southeastern Naturalist Vol. 9, No. 4
(Frey et al. 2003, Knowles 1986, Stenseth et al. 2001, Swihart 1991). Removal
of flying squirrels from clusters might be more successful if done over larger
areas instead of concentrating on the immediate area around the cluster (Conover
2002). This action would lower the potential for immigration, given
immigrants would have more area to travel through before reaching the cluster
(assuming the cluster is at the center of the removal area). Extending removal
areas may not be feasible, however, due to ethical concerns and management
costs. Therefore, managers may want to consider different ways to reduce the
impact of flying squirrels on Red-cockaded Woodpeckers.
One alternative is the supplementation of cluster areas with nest boxes.
Red-cockaded Woodpeckers are not known to use nest boxes; however,
flying squirrels readily do, and their use of nest boxes within Red-cockaded
Woodpecker clusters has been found to increase the reproductive
success of the woodpecker and reduce cavity use by squirrels (Borgo et
al. 2006a, Brady et al. 2000, Loeb and Hooper 1997). Another alternative
may be the application of an odor deterrent to Red-Cockaded Woodpecker
cavities. In a captive study, flying squirrels reduced their use of nest boxes
treated with odor (urine, fur, or musk) from Lynx rufus Schreber (Bobcat),
Procyon lotor L. (Raccoon), Vulpes vulpes L. (Red Fox), Elaphe guttata
L. (Corn Snake), and Lampropeltis getula L. (King Snake) (Borgo et al.
2006b). However, a field trial evaluating the effect of Red Fox urine and
Elaphe obsoleta (Say) (Rat Snake) musk did not find a significant effect
on flying squirrel use of cavities (Stober and Conner 2007). Further field
evaluations are necessary to elucidate factors, such as the type, concentration,
and frequency of scent application, or a combination of nest-box
supplementation and odor deterrents that might make these management
alternatives more successful.
This research was funded by the Jack H. Berryman Institute and the Woodruff
Foundation. We would like to thank staff of the Joseph W. Jones Ecological Research
Center for overall support. The wildlife laboratory and conservation group, especially
Jonathan Stober, at the Jones Center aided in nest-box inspections and training.
Peter Jones and Johnny “Buck” Freeman were invaluable in modifying and building
nest boxes for this study. Helpful criticisms on the manuscript were received from
two anonymous reviewers and the guest editor, Thomas J. Maier.
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