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Effects of Brush Management on the Reproductive Ecology of Endangered Black-capped Vireos
Daniel G. Kovar, David A. Cimprich, and Jinelle H. Sperry

Southeastern Naturalist, Volume 17, Issue 2 (2018): 270–285

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Southeastern Naturalist D.G. Kovar, D.A. Cimprich, and J.H. Sperry 2018 Vol. 17, No. 2 270 2018 SOUTHEASTERN NATURALIST 17(2):270–285 Effects of Brush Management on the Reproductive Ecology of Endangered Black-capped Vireos Daniel G. Kovar1,*, David A. Cimprich2, and Jinelle H. Sperry1,3 Abstract - Terrestrial habitats are frequently managed to improve perceived economic or aesthetic value of the land and to improve habitat quality for wildlife species. In central Texas, removal of native Juniperus asheii (Ashe Juniper) is a common landscape-management practice due to the species’ propensity for invasion of rangeland, vigorous growth leading to dominance of habitats, and reputation for high water-use. Ashe Junipers provide habitat for wildlife species of economic (e.g., Odocoileus virginianus [White-tailed deer], Meleagris gallopavo [Wild Turkey]) and conservation concern (e.g., the endangered Setophaga chrysoparia [Golden-cheeked Warbler]); however, the relationship between Ashe Junipers and the federally endangered Vireo atricapilla (Black-capped Vireo) is less clear. Blackcapped Vireos breed in early successional shrublands where Ashe Junipers are often able to invade, grow quickly, and shade out the deciduous shrubby vegetation preferred by vireos. Although Ashe Juniper removal in Black-capped Vireo habitat is common practice, relatively little is known about the impacts of brush management on Black-capped Vireo use and reproductive success. Here we present results of a study on the effects of an Ashe Juniper removal treatment, in which juniper trees were removed but surrounding deciduous vegetation was largely left intact, on Black-capped Vireo habitat use and reproductive success. Comparing before and after Ashe Juniper removal, we found that the number of Blackcapped Vireos settling in manipulated habitats remained similar, and we saw no significant changes in the size of the average territory or reproductive success. We conclude that, when the amount of damage to the surrounding deciduous vegetation is limited, selective Ashe Juniper removal is unlikely to negatively affect Black-capped Vireos. Introduction Brush management, or the control and removal of woody plants, is one method frequently employed by land managers to reverse the encroachment of trees and shrubs into grasslands, shrublands, and savannahs (Archer et al. 2012). However, because the methods and goals of brush management, as well as the habitats where it is applied, vary so greatly, it is difficult to make broad statements about the effects of brush management on avian populations. In studying bird responses to forest management and harvest, Crawford et al. (1981), concluded that silvicultural practices cannot be categorically described as beneficial or detrimental to birds. Similarly, brush management can impact bird communities, but the effects depend on the type of management activity carried out, and are almost always species 1Engineer Research and Development Center, US Army Corps of Engineers, Champaign, IL 61826. 2Environmental Division, Fort Hood Directorate of Public Works, 4612 Engineer Drive, Room 76, Fort Hood, TX 76544. 3Department of Natural Resources and Environmental Sciences, University of Illinois at Urbana-Champaign, Urbana, IL 61801. *Corresponding author - Manuscript Editor: Frank Moore Southeastern Naturalist 271 D.G. Kovar, D.A. Cimprich, and J.H. Sperry 2018 Vol. 17, No. 2 specific, with some species benefiting and others apparently harmed by woody-plant removal (Castrale 1982, Easton and Martin 1998, Hanowski et al. 1999, Rodewald and Smith 1998, Schulz et al. 1992). However, few studies have examined the effects of brush clearing on disturbance-dependent shrubland habitat. Brush removal can have widely varying effects on species of interest; thus, it is critical to conduct quantitative studies of how woody-plant removal will affect species of conservation concern that use disturbance-dependent shrublands. In central Texas, Juniperus ashei J. Buchholz (Ashe Juniper, hereafter Juniper) is one of the most common targets of brush management (Lyons et al. 2009). This native evergreen tree species provides habitat for a variety of species, including culturally and economically important species such as Odocoileus virginianus (Zimmerman) (White-tailed Deer) and Meleagris gallopavo L. (Wild Turkey), as well as species of conservation concern such as the endangered Setophaga chrysoparia (P.L. Sclater & Salvin) (Golden-cheeked Warbler; Bryant 1991, Kroll 1980, Rollins and Armstrong 1997). However, in the absence of natural disturbance, Juniper has invaded grasslands and early successional areas (Barnes et al. 2008, Fuhlendorf et al. 1996, Jessup et al. 2003, Smeins et al. 1997). Although some of these areas likely represent recolonization of areas cleared by harvest (Diamond and True 2008, Diamond et al. 1995), these invasions are of economic concern to ranchers because Juniper woodlands contain sparse broadleaf understory, produce almost no grass or herbaceous vegetation (Bryant 1991), and Juniper foliage is not particularly palatable or nutritious to livestock (Huston et al. 1994). Additionally, landowners in general have a strong preference for less Juniper cover (Olenick et al. 2005), and the most commonly cited concerns are forage production and water conservation (Kreuter et al. 2005). Although Juniper removal could have deleterious impacts on species that require mature Junipers or closed-canopy woodlands, it is possible that selective Juniper removal, which leaves the surrounding deciduous vegetation largely intact, could improve habitat quality for early-successional species. The endangered Vireo atricapilla Woodhouse (Black-capped Vireo, hereafter Vireo) is 1 potential example. The Vireo, which has declined in large part because of habitat loss, prefers to nest in early-successional to mid-successional shrublands with notable spatial heterogeneity, including a mix of deciduous shrubs interspersed with areas of open ground or bare rock (Grzybowski 1995). One key feature of this type of habitat is the openings between vegetation components that allow light to reach low branches and promote high foliage-density within the zone (0.4 m to 1.25 m above the ground) in which Vireos most commonly place their nests (Grzybowski 1986). Certain aspects of Juniper removal are likely to be compatible with Vireos for 2 main reasons. First, Juniper does not appear to be a necessary component of high-quality Vireo habitat (Grzybowski et al. 1994). Where Vireo habitat includes both Juniper and broadleaf shrubby vegetation, Vireos prefer to place their nests in broadleaf substrates (Bailey and Thompson 2007). Vireos can, in some cases, successfully breed in Juniper woodlands (Pope et al. 2013), but this habitat appears to be marginal, and far fewer male Vireos establish territories in mature Juniper stands Southeastern Naturalist D.G. Kovar, D.A. Cimprich, and J.H. Sperry 2018 Vol. 17, No. 2 272 than in adjacent shrublands (Kovar et al. 2018). Second, maturing Juniper can drive the transition from open, early-successional habitat to closed-canopy woodland. Although disturbance-dependent shrublands are transient ecosystems in general, Juniper invasion might increase the speed at which shrubland habitat patches become unsuitable for Vireos. As Junipers become dominant in a shrubland, their closed canopy causes the disappearance of the “apron” of leafy vegetation that typically conceals Vireo nests (Barlow 1977). However, before concluding that Juniper removal is neutral or beneficial for Black-capped Vireos, it is critical to ensure that Vireos still recognize the modified areas as habitat, and that fitness does not suffer as a result of selecting that habitat. Brush management has the potential to negatively affect Black-capped Vireos in at least 3 ways. First, snakes (especially Pantherophis spp. [rat snakes]) are the primary predators of Vireo nests (Stake and Cimprich 2003), and snakes often prefer habitats with high heterogeneity (“edge”), which they use to thermoregulate (Blouin-Demers and Weatherhead 2001). Removing vegetation could create more edge habitat and bring more snakes into contact with breeding Vireos (Klug et al. 2010, Sperry and Weatherhead 2009). Second, any loss of low shrubby vegetation has the potential to reduce the number of potential Vireo nest sites, and birds may respond to this loss by reducing their use of treatment sites (Chalfoun and Martin 2007). Finally, Black-capped Vireos are foliage gleaners and mainly eat insects and spiders during the breeding season (Grzybowski 1995). Therefore, the loss of broadleaf (through incidental shrub cover removal) and Juniper foliage could reduce food availability for Vireos by removing potential feeding substrates as well as the food base for arthropod prey (Burke and Nol 1998, Marshall and Cooper 2004). To address these concerns, we examined how a brush-management process intended to facilitate training activities on a military installation, which primarily involved selective removal of Junipers, affected habitat use and reproductive success of Vireos. First, we determined how the brush-clearing process, including the removal of almost all Junipers as well as a limited amount of broadleaf growth, affected habitat characteristics. Next, we examined how habitat use, as measured by the number of male Vireos settling at treatment sites and the size of their territories, changed before and after brush management. Finally, we determined whether reproductive success, as measured by the daily survival rates of nests, the rate of territory success, and number of fledglings produced per territory, differed before and after the Juniper-removal process. Based on the life-history traits outlined above, we predicted that the Juniper-removal process would have limited negative effects on these aspects of Vireo breeding ecology. Field-site Description We conducted all field work at Fort Hood Military Reservation, a US Army post in central Texas, which supports a large breeding population of Vireos (Cimprich and Kostecke 2006). The installation contains extensive sections of undeveloped land, some of which undergoes frequent habitat modification. These Southeastern Naturalist 273 D.G. Kovar, D.A. Cimprich, and J.H. Sperry 2018 Vol. 17, No. 2 undeveloped areas are sometimes intentionally modified to improve conditions for training, or to improve habitat quality for wildlife, such as Vireos. Habitat modification can occur as a side effect of certain training activities, such as heavy vehicle traffic suppressing the growth of woody vegetation, or when unintentional fires are set by artillery practice. We collected data at 2 study areas at Fort Hood (abbreviated MM and LF) in 2013 and 2014. These 2 sites were relatively close geographically (~5 km apart), but occupied separate and distinct patches of shrubland habitat. Between the 2013 and 2014 breeding seasons, Fort Hood training-area managers conducted brushmanagement activities at these sites and others in order to alter the vegetation for specific training exercises. In these modified areas, brush was cleared to enhance the movement of tracked vehicles by clearing a single, wide lane (~40 m across). In addition, a network of smaller trails was cleared or expanded, and most Junipers were targeted for removal regardless of whether they were in the path of a trail. Although Juniper was the only species targeted by the brush-clearing process (its evergreen foliage blocks the laser sighting-devices of weapons), it was expected that some level of disturbance would be inevitable as trails were widened or created, and as a side effect of using heavy equipment. Juniper slash was masticated to avoid creating potential habitat for common predators of Vireo nests (e.g., small mammals and rat snakes). Juniper removal was not carried out specifically for Vireos; thus, the exact area where removal would occur was not known ahead of time. We therefore monitored birds in areas that we expected would be treated. During the summer of 2014, following brush clearing, field personnel mapped the extent of the modified areas using GPS receivers. The modified area overlapped with 22.2 and 48.2 ha of Vireo habitat monitored in 2013 at LF and at MM, respectively. We considered this subset of the total area monitored (which only included treated areas) to be the boundaries of our study sites when analyzing data (Fig. 1). Methods Field methods We suspected that habitat modification would likely involve some removal of both Juniper and broadleaf foliage; thus, we counted Junipers and measured an index of broadleaf shrub cover on the 2 study sites. We used a systematic vegetation sampling design, consisting of 15-m–radius vegetation-sampling plots centered on the intersection of UTM gridlines. We used this method because the intersections were random with respect to Juniper abundance and shrub cover while ensuring even sampling across the extent of the study areas. We measured vegetation characteristics at these plots before and after the brush-management treatment was carried out. The 2 study sites occupied different spatial scales; MM contained a larger amount of potential Vireo habitat (Fig. 1), and consequently, we monitored a larger area at that site, even before Juniper removal. We therefore used different grid spacing at the 2 study sites (90-m grid at MM and 75-m grid at LF). We initially sampled every other grid intersection, and then randomly sampled at additional intersections as time allowed, for a total of 24 plots at MM and 30 at LF. Within Southeastern Naturalist D.G. Kovar, D.A. Cimprich, and J.H. Sperry 2018 Vol. 17, No. 2 274 Figure 1. Distribution of Black-capped Vireo territories monitored at 2 sites (LF at left, and MM at right) in Central Texas, before (2013, top) and after (2014, bottom) Ashe Juniper removal treatment was carried out. The dashed line indicates the areas where Junipers were removed. Dark polygons represent monitored territories that were less than 50% within the treatment area and were not included in territory-scale analyses. Background imagery is from 2011 and is presented only for reference. Southeastern Naturalist 275 D.G. Kovar, D.A. Cimprich, and J.H. Sperry 2018 Vol. 17, No. 2 each sampling plot, we counted the number of Juniper stems >2 m in height within a 15-m radius. We operationally defined shrub cover as broadleaf vegetation from 0.5 m to 2.0 m above the ground because most Vireo nests are placed in vegetation within this range. Within each sampling plot, we recorded the presence or absence of shrub cover at 20 points (5 points each, at 3-m intervals, in each cardinal direction from the center). For each plot, we calculated the proportion of these 20 points that had shrub cover within 0.5 m. In order to delineate territories and accurately determine productivity at the territory level, we uniquely marked adults. We captured Vireos using mist nets and recordings of conspecific vocalizations and Megascops asio (L.) (Eastern Screechowl) calls. Once captured, we banded the birds with unique combinations of Darvic or Acetal bands (Avinet, Inc., Portland, ME) and an aluminum USGS band. Approximately 89% of the territorial males we monitored on the treatment sites were banded, which allowed us to accurately determine the number of territories present and their success. We resighted males throughout the breeding season and used GPS receivers to record these locations. We attempted to collect ≥15 locations per male and produced spot maps, which allowed us to determine approximate territory size (International Bird Census Committee 1970). Throughout the breeding season (mid-March through the end of July), we searched for nests using behavioral cues and systematic searches of likely vegetation. We monitored nests every 2–3 d until fledging became imminent (10–11 d), when we checked nests daily. We considered nests failed if they contained only dead nestlings, if evidence of depredation (e.g., destroyed nest, egg shell fragments) was present, or if they were empty before the nestlings could be expected to survive outside the nest. We considered nests successful if we observed any of the following in the vicinity of the nest: young fledglings, adults carrying food, or adults scolding intensely on at least 2 separate days after the presumed fledging. Brood parasitism by Molothrus ater (Boddaert) (Brown-headed Cowbird) was rare on our study sites because of the ongoing Brown-headed Cowbird control program at Fort Hood (typical base-wide average of less than 10% per year; Cimprich and Heimbuch 2013). We considered parasitized nests failed for our survival analyses if all host young died. We considered a territory successful in a year if ≥1 nest belonging to the territorial male was successful. We considered the number of nestlings present the day before fledging as the number of offspring produced by that nest. Checking nests daily prior to fledging allowed us to consistently assess the number of fledglings produced per nest. Several Vireos in our study successfully raised multiple clutches in a season, so we calculated territory productivity by summing the number of fledglings produced in each successful nest in a territory . Data analysis We used ArcGIS software 10.3 (Environmental Systems Research Institute, Redlands, CA) to plot the boundaries of the study areas, to examine the spot-mapping data, and to create minimum convex-polygon hulls around the cluster of locations for each territorial male. We calculated the area of the polygons as a representation of territory size, and determined how much of each territory overlapped with the Southeastern Naturalist D.G. Kovar, D.A. Cimprich, and J.H. Sperry 2018 Vol. 17, No. 2 276 boundaries of the study area (i.e., the area where Juniper removal occurred between the 2013 and 2014 breeding seasons). We conducted all statistical analyses in R 3.1.3 (R Foundation for Statistical Computing, Vienna, Austria) and report estimated mean and 95% confidence intervals. We calculated the estimates of habitat use and reproductive success in the lsmeans package (Lenth 2016) while holding other parameters at their mean values. First, to examine how the brush-clearing process changed vegetation at the study site, we used paired t-tests to determine whether the abundance of Junipers and amount of shrub cover differed between years. Although the Juniper abundance data violated the assumption of normality, the results from a nonparametric Wilcoxon signed-rank test (using the coin package; Hothorn et al 2008) offered a qualitatively similar conclusion, and so we present only the t-test results for ease of interpretation and consistency with our shrub-cover analysis. Second, we conducted several analyses at the territory scale, including a measure of habitat use and 2 measures of reproductive success. Edge effects can negatively influence songbird reproductive success (Gates and Gysel 1978, Paton 1994, Wilcove 1985); thus, we did not assume that territories had to completely overlap with the treatment area in order to be impacted by the Juniper removal. Therefore, we conducted all territory-scale analyses on a subset of territories which included only those that overlapped the area of Juniper removal by ≥50% (Fig. 1). We used a general linear model with a normal distribution, which included the effects of year and study area to examine whether territory size differed before and after Juniper-removal treatment and between study sites. This model also included an effect of the number of locations collected per territory to account for the fact that observed-territory size can be related to the number of points (Odum and Kuenzler 1955). To examine the 2 measures of reproductive success (territorial success rate and territorial productivity), we used generalized linear models. We used logistic regression (with a binomial distribution and a logit link) to determine whether the probability of a territory success (fledging ≥1 young) differed before and after Juniper clearing, and included the effect of study site as a covariate to account for possible patch-level effects. To examine territorial productivity, we fit the number of fledglings produced per territory (i.e., the total number of nestlings present the day before fledging in each successful nest) to a negative binomial model (using the MASS package; Venables and Ripley 2002), again including the effects of year and study site as covariates. We used a negative-binomial model as an alternative to Poisson regression to account for overdispersion in the productivity-count data. In all cases, we also tested for the presence of a significant interaction effect between the year and study-site variables. Finally, we used Shaffer’s (2004) method of logistic exposure to estimate daily survival rate (DSR) of nests as a measure of reproductive success at the scale of individual nests. For this analysis, we used all monitored nests that were ≤25 m from the treatment-area boundary. We constructed a candidate model set consisting of combinations of variables we selected a priori which could affect nest survival on our study sites. This list consisted of the effects of year (i.e., before or after Southeastern Naturalist 277 D.G. Kovar, D.A. Cimprich, and J.H. Sperry 2018 Vol. 17, No. 2 Juniper removal treatment), study area (MM or LF), ordinal date, and nest height. We compared logistic-exposure models using AICc (Akaike’s information criterion, corrected for small sample size) and calculated model-averaged parameter values and DSR based on the relative support (Akaike weight, wi) for models in the candidate set (Arnold 2010). Results Both Juniper abundance and broadleaf-shrub cover differed significantly before and after brush clearing. The change in the number of Juniper stems at the sampling locations was large and negative (Fig. 2). The average point had 17.7 (CI = 11.1–24.2) fewer Juniper stems after the treatment was carried out (t = 5.41, df = 53, P < 0.0001). The change in shrub cover between years (Fig. 2) was comparatively small (mean reduction of 8.1%, CI = 3.6%–12.5%) although statistically significant (t = 3.60, df = 53, P = 0.0006). The number of Vireos that settled at the treatment study-sites was similar between years (Fig. 1). In 2013, 19 male Vireos established territories that overlapped Figure 2. Change in Juniper stem abundance (top) and percent broadleaf-shrub cover (bottom) before and after brush management treatment. Dots represent the change in vegetation characteristics at individual sampling plots from 2013 to 2014 on treatment sites in central Texas. Plots created using the beeswarm package (Eklund 2015). Southeastern Naturalist D.G. Kovar, D.A. Cimprich, and J.H. Sperry 2018 Vol. 17, No. 2 278 ≥50% with the treatment areas, compared to 18 in 2014 (~5% fewer), yielding a total of 37 territories. For one of these territories, we were only able to obtain 6 locations, and so we excluded this territory from our analysis of territory size. We recorded ≥14 locations for all the remaining territories. For the other 2 analyses at the territory scale, we used the full set of n = 37 territories. The linear regression model describing territory size fit the data well (multiple R-squared of 0.3639). The only significant effect was number of locations collected (P = 0.0027, β = 0.0823, CI = 0.0307–0.1340). With this effect taken into account, the effects of year (representing before and after clearing) and study site (representing potential patch-scale effects) were small and not significant (Table 1). In both our logisticregression model of territory success and our negative-binomial model of territorial productivity, the effects of year and study site were also nonsignificant (Table 1). Concordantly, estimated values of territory size, success rate, and productivity were similar between years, and confidence intervals overlapped broad ly (Table 2). Apparent nest survival rates were similar between years at the treatment sites. At treatment sites, 41.2% (17 of 41) of monitored nests were successful in 2013, compared to 41.7% (20 of 48 nests) in 2014. In our logistic-exposure analyses, we used 89 nests from the treatment sites (n = 1461 exposure days). The effect of year was present in highly ranked models (Table 3), but confidence intervals for the model-averaged parameter estimates of this factor overlapped zero (β = 0.063, CI = -0.633–0.759), indicating a weak relationship to daily nest survival. Table 2. Aspects of reproductive success for Black-capped Vireos at treatment sites in central Texas, before (2013) and after (2014) juniper clearing. Variables other than the year (i.e., treatment) effect were held at their means when calculating estimates. 2013 2014 Estimate 95% CI Estimate 95% CI Territory sizeA 1.56 ha 1.06–20.5 ha 1.26 ha 0.80–1.71 ha Territory success rateB 0.758 0.499–0.908 0.509 0.276–0.737 Territory productivityC 2.7 young 1.7–4.3 young 2.2 young 1.4–3.6 young Daily survival rateD 0.959 0.938–0.972 0.963 0.946–0.974 ATerritory size estimates derived from linear regression modeling. BTerritory success-rate estimates derived from logistic-regression modeling. CTerritory productivity estimates derived from negative binomial regression modeling. DDaily survival rate estimates derived from logistic exposure methods and model averaging. Table 1. The effects of year (before and after Juniper removal treatment) and study site on the size, success rate, and productivity of Black-capped Vireo territories in central Texas, in 2013 and 2014. β = effect size. Year (treatment) effect Study-site effect β 95% CI P β 95% CI P Territory size -0.298 -0.985–0.388 0.38 0.329 -0.428–1.085 0.38 Territory success rate -1.106 -2.729–0.358 0.15 1.176 0.328–2.778 0.13 Territory productivity -0.196 -0.845–0.451 0.55 0.425 -0.287–1.134 0.24 Southeastern Naturalist 279 D.G. Kovar, D.A. Cimprich, and J.H. Sperry 2018 Vol. 17, No. 2 Figure 3. Estimates of daily survival rates (and 95% confidence i n t e r v a l s ) for Black-capped Vireo nests in central Texas, in 2013 and 2014. DSR at sites where brush-management activities were carried out (treatment sites) are statistically indistinguishable before (2013) and after (2014) Juniper removal. For comparison, survival declined marginally from 2013 to 2014 at a nearby reference site where no vegetation modification occurred, although confidence intervals again broadly overlapped. These DSR estimates were model-averaged over the candidate model sets listed in Table 3. The effect of study site was present in several highly ranked models (Table 3). The model-averaged parameter estimate for this factor (β = 0.617, CI = -0.012–1.247) only narrowly encompassed zero, indicating a potential difference in daily survival between the treatment sites. Concordantly, the estimated nest survival rate was marginally higher at MM (43.0%, CI = 30.7–54.8%), compared to LF (20.7%, CI = 8.5–36.8%) when averaged across both years. Estimates of daily survival of nests, although numerically higher in the year after clearing, were generally similar and confidence intervals overlapped broadly (Fig. 3). Table 3. Candidate models of daily survival of Black-capped Vireo nests at sites where brush management was carried out between breeding seasons. Models are ranked by distance to the best model based on AICC. LL = log likelihood of the model, K = number of parameters in the model, AICC = Akaike’s information criterion value of the model, corrected for sample size, ΔAICC = Difference in AICC from the most likely model, and wi = model weight. LL K AICC ΔAICC wi Study area -187.31 2 378.64 0.00 0.42 Study area + Year -187.23 3 380.50 1.86 0.17 Constant survival (null) -189.54 1 381.10 2.46 0.12 Study area * Year -187.04 4 382.14 3.51 0.07 Date -189.21 2 382.44 3.80 0.06 Year -189.44 2 382.89 4.26 0.05 Nest height -189.54 2 383.10 4.46 0.05 Date + Year -189.06 3 384.16 5.52 0.03 Nest substrate type -189.48 3 385.00 6.36 0.02 Nest substrate type + Height -189.48 4 387.01 8.37 0.01 Global model -186.56 8 389.32 10.68 0.00 Southeastern Naturalist D.G. Kovar, D.A. Cimprich, and J.H. Sperry 2018 Vol. 17, No. 2 280 Discussion Our results show that the brush-management treatment carried out was effective at reducing the number of Junipers, and also resulted in a detectable, but comparatively minor, decrease in the amount of broadleaf-shrub cover. We did not find evidence that an overall reduction in vegetation impacted breeding vireos. At treatment sites, we were not able to detect any decrease in Vireo habitat use or reproduction, as measured by the number of territorial males present, the size of the territories established, the success rate of and number of fledglings produced per territory, or the DSR of nests between years. Vireo nest success can vary between years (Cimprich and Heimbuch 2013); thus, we were initially concerned that a decrease in DSR due to Juniper clearing could potentially be hidden by higher overall success in 2014. To ensure this was not the case, we compared the DSR values calculated here to those from a separate study area on Fort Hood (~19 km away) where no vegetation modification took place (D. Kovar, pers. observ.). At this reference site, where 177 nests were monitored, daily survival and apparent nest-success rates declined slightly from 2013 to 2014, although the confidence intervals of all estimates overlapped broadly (Fig. 3). This comparison with daily survival of nests at a control site confirms that the pattern observed at the treatment sites is not due to higher overall productivity in the second year obscuring a negative effect of vegetation removal. We predicted that Juniper removal would not negatively impact Vireo use or productivity because Juniper is not considered an important component of Vireo habitat. However, it was surprising that the reduction in broadleaf-shrub cover also had no negative effects on Vireo reproduction because Vireos are so closely associated with shrub habitats (Grzybowski et al. 1994). Previous work on shrub-associated bird species has found a preference for higher shrub densities (Chalfoun and Martin 2007), although strength of response is likely species-specific. In addition, although Vireos tend to avoid nesting in Junipers (Bailey and Thompson 2007), we might still expect that the overall reduction in vegetative density (Juniper and broadleaf shrubs) could impact Vireo density or productivity through several mechanisms. One possible explanation for the lack of response is that the decrease in broadleaf shrub cover we detected was statistically, but not biologically, significant. Black-capped Vireos are known to successfully breed in habitats with a wide variation of shrub cover, from ~30 to ~60% (Bailey and Thompson 2007, Grzybowski et al. 1994). The change we observed here (~8% reduction) could have been insufficient to move the habitat outside of the ideal range for Vireos. Another defining feature of Vireo habitat is the open spaces of grass or bare ground between the clumps of dense vegetation (Bailey and Thompson 2007, Grzybowski et al. 1994). The brush-management process employed here could have enhanced the heterogeneity of the shrubland habitat, which in turn made it more desirable to Vireos. Furthermore, any negative effects of clearing would be most severe in the first season following brush removal, and would likely attenuate over time as the vegetation regrows. Conversely, there could also be positive effects of Juniper removal Southeastern Naturalist 281 D.G. Kovar, D.A. Cimprich, and J.H. Sperry 2018 Vol. 17, No. 2 which could increase over time. Juniper is an effective competitor for light and water resources, and in its absence, the remaining shrubs, forbs, and grasses could respond with more vigorous growth and increased foliage density (Kane et al. 2011, Yager and Smeins 1999). The additional foliage could support larger arthropodprey populations and increase concealment around nest sites. Consequently, we believe that, if anything, this study was more likely to overestimate the potential harm and underestimate potential benefits of selective brush management on breeding Vireos. We tested multiple aspects of reproductive success here because a single measure can be misleading. The lower (although statistically indistinguishable) territory success rate seen at treatment sites in 2014 is an example of how a single aspect can be deceptive. Taken by itself, this result could be cause for concern because the consequence of making a Type II error here would entail underestimating harm to an endangered species. However, when taken into context with our other analyses of reproductive success, we believe this is not especially concerning. The fact that daily and apparent nest survival rates remained almost identical (and territory productivity similar) at treatment sites before and after clearing, while possibly decreasing at the reference site, suggests that we are not concluding that Juniper removal is safer than it truly is. Appropriate consideration must be taken when attempting to apply these results to brush clearing in general. Across their range, Vireos are associated with a wide variety of plant species, some of which appear important to the birds in one region yet are absent in others (Grzybowski et al. 1994). Care should be taken when applying the results of this study to other regions with different vegetation communities. Second, the method of shrub removal employed here, which specifically targeted Junipers, is relatively precise compared to other methods of Juniper clearing. Techniques such as mastication (employing heavy grinding equipment) or chaining (pulling large chains between bulldozers) are often used to clear large patches of Juniper woodland in their entirety. These methods do not discriminate between Juniper and deciduous growth and do not create the kind of heterogeneous habitat that Vireos require. Although these non-selective methods have the potential to create Vireo habitat in 3–5 years if resprouting deciduous vegetation is allowed to regrow into the structure preferred by Vireos (Reemts and Cimprich 2014), non-selective methods will not preserve habitat that is currently in use by Vireos, such as the marginal woodland habitat described by Pope et al. (2013). Additionally, it is important to develop a plan for the disposal of the brush following cutting because the retention of brush piles can provide habitat for nest predators (Sperry and Weatherhead 2010). Finally, on a broader level, it is important to keep in mind that both early- and later-successional habitats, and the needs of the wildlife which depend on them, must be balanced in central Texas and many other regions. Juniper and other targets of brush-control programs are often native species which provide important wildlife habitat and valuable ecosystem services. Through the 20th century, views on woody-plant control became more nuanced, from brush-eradication plans Southeastern Naturalist D.G. Kovar, D.A. Cimprich, and J.H. Sperry 2018 Vol. 17, No. 2 282 with the singular goal of maximizing stocking rates, to management that takes the landscape and multiple uses (e.g., grazing, conservation, hunting) into account (Fuhlendorf et al. 2012, Fulbright 1996, Rollins and Cearley 2004). The philosophy underlying this shift should be considered when deciding on a brushmanagement strategy and choosing the tactics to be employed to achieve the specified goals. This study should not be used to justify or advocate for the indiscriminate removal of Juniper from the landscape. Our study provides evidence that selective methods of Juniper removal are neutral with respect to Vireo habitat quality. Junipers can drive the transition from high-quality shrubland habitat to low-quality woodland habitat; thus, selective Juniper removal could be used as a method to “stop the clock” of ecological succession and increase the amount of time that a patch of habitat can support actively breeding Vireos. Selective Juniper removal has the potential to benefit species with similar habitat requirements. Other shrubland-associated species of concern, such as Icteria virens (L.) (Yellow-breasted Chat), Toxostoma rufum (L.) (Brown Thrasher), Vireo belli Audubon (Bell’s Vireo), and Passerina ciris (L.) (Painted Bunting), are typically lost as the community matures. By removing the woody plant species that increase the turnover of open habitats into closed-canopy woodlands, selective brush-management could also be used to preserve habitat for these species. Acknowledgments We are grateful to J. Balk, M. Chan, P. Cimprich, M. Devlin, A. Gleason, C. Harris, J. Kutylowski, A. Rives, C. Rutt, and M. Singh for their assistance in the field. V. Buxton, B. DeGregorio, E. Dittmar, J. Gleditsch, E. Mulero, A. Purnell, and S. Summers provided additional advice and support. We are grateful for the comments and suggestions from 2 anonymous reviewers, which helped us improve our manuscript. We thank the US Army for funding this project through an agreement between the Natural Resources Management Branch at Fort Hood, US Army Engineer Research Development Center, and the University of Illinois at Urbana-Champaign. None of the funders had any input into the content of the manuscript or required their approval of the manuscript before submission or publication. 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