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Flying Squirrel Removal Does Not Reduce Their Use of Simulated Red-cockaded Woodpecker Nest Clusters
Jennifer S. Borgo, Michael R. Conover, and L. Michael Conner

Southeastern Naturalist, Volume 9, Issue 4 (2010): 813–820

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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. Introduction 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 - jborgo@coker.edu. 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. Methods Field-site description 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 Pseudo-nest clusters 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 (χ2 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. Statistical analyses 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 without removal. Results 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. Discussion 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. Acknowledgments 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. Literature Cited Borgo, J.S. 2004. Reducing Southern Flying Squirrel use of Red-cockaded Woodpecker cavities. M.Sc. Thesis. 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