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Habitat Preferences of Nesting Southeastern American Kestrels in Florida: The Importance of Ground Cover
Karl E. Miller, Ryan Butryn, Erin Leone, and Jason A. Martin

Southeastern Naturalist, Volume 18, Issue 2 (2019): 192–201

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Southeastern Naturalist K.E. Miller, R. Butryn, E. Leone, and J.A. Martin 2019 Vol. 18, No. 2 192 2019 SOUTHEASTERN NATURALIST 18(2):192–201 Habitat Preferences of Nesting Southeastern American Kestrels in Florida: The Importance of Ground Cover Karl E. Miller1,*, Ryan Butryn1,2, Erin Leone1, and Jason A. Martin1,3 Abstract - Habitat associations for Falco sparverius (American Kestrel) have been quantified for agricultural landscapes dominated by pastures and fields, but little is known about the species’ habitat requirements in natural plant communities such as forests, savannas, and grasslands. Prescriptions for habitat management for the threatened F. s. paulus (Southeastern American Kestrel) in sandhills remain unclear. We assessed how habitat features affected occupancy rates and nest success of Southeastern American Kestrels on 4 conservation lands in peninsular Florida. We assessed habitat relationships at 3 spatial scales (patch, territory, landscape) around 58 nest boxes. We identified a reduced habitat-patch model with 1 variable (percent grass cover) as the best fit for predicting Southeastern American Kestrel occupancy, but none of the habitat models predicted nest success better than the null model. Occupied patches averaged more grass cover (52%), and unoccupied patches averaged relatively little grass cover (32%). Habitat characteristics within nest box territories occupied by Southeastern American Kestrels (i.e., open tree canopy with few woody shrubs and a graminoid-dominated low groundcover) were consistent with ecological reference conditions for sandhills and habitat conditions recommended for other fire-dependent bird species of conservation interest. The loss of suitable foraging habitat (e.g., open ground cover) has received little attention in regional or continental efforts to arrest population declines of the American Kestrel. Additional effort toward maintaining suitable groundcover in native pyrogenic plant communities for Southeastern American Kestrels appears to be warranted. Introduction Ongoing population declines of Falco sparverius L. (American Kestrel) have been documented throughout eastern North America (Farmer and Smith 2009, Sauer et al. 2012, Smallwood et al. 2009a) and especially in the southeastern US (Hoffman and Collopy 1988). American Kestrel habitat associations have been quantified in agricultural landscapes dominated by pastures, farms, and fields (e.g., Rohrbaugh and Yahner 1997, Smallwood and Collopy 2009, Smallwood and Wargo 1997, Toland and Elder 1987), but little is known about American Kestrel habitat requirements in natural plant communities such as forests, savannas, and grasslands. Falco sparverius paulus Howe and King (Southeastern American Kestrel), a non-migratory subspecies, was once widely distributed throughout 7 southeastern states but today is patchily distributed in Florida and the coastal plain of 1Fish and Wildlife Research Institute, Florida Fish and Wildlife Conservation Commission, 1105 Southwest Williston Road, Gainesville, FL 32601. 2Current address: 579 South Willard Street, Burlington, VT 05401. 3Current address: Kleenco Environmental, 8239 North State Road 9, Alexandria, IN 46001. *Corresponding author - Manuscript Editor: Frank Moore Southeastern Naturalist 193 K.E. Miller, R. Butryn, E. Leone, and J.A. Martin 2019 Vol. 18, No. 2 neighboring states (FWC 2003, Schneider et al. 2010, Smallwood and Bird 2002). Range contractions and population declines of the subspecies have been attributed to habitat changes associated with fire suppression and conversion of Pinus palustris Mill. (Longleaf Pine) sandhills to pine plantations (Gault et al. 2004, Hoffman and Collopy 1988). Roadside surveys in Florida during the 1980s recorded more Southeastern American Kestrels in landscapes that included remnant patches of sandhills than in landscapes without sandhills (Bohall-Wood and Collopy 1986). Breeding populations still occur in an old-growth Longleaf Pine forest in the Florida panhandle (Blanc and Walters 2008, Gault et al. 2004) and less commonly in second-growth pine forest in peninsular Florida (FWC 2003). However, no quantitative information is available on the habitat requirements of Southeastern American Kestrels in sandhills or other native plant communities. Prescriptions for habitat management will remain unclear until habitat relationships are assessed within plant communities and across landscapes. We compared nest box use of Southeastern American Kestrels in sandhills at different spatial scales with a structured hypothesis-testing approach. Assessment of habitat relationships across multiple spatial scales can improve understanding of limiting factors (Mayor et al. 2009). Our objective was to determine what is most important in determining occupancy of nest boxes and nest success by assessing vegetation structure within the immediate habitat patch, the amount of suitable habitat within the nesting territory, and the pattern and configuration of habitat patches in the landscape around the territory. Our ultimate objective was to provide quantitative habitat management guidelines to land managers. Study Area We assessed habitat relationships for Southeastern American Kestrels at 58 nest boxes on 4 public conservation lands in north-central Florida: Ichetucknee Springs State Park (Suwannee and Columbia counties), Camp Blanding Training Center and Wildlife Management Area (Clay County), Gold Head Branch State Park (Clay County), and Ordway–Swisher Biological Station (Putnam County). We selected these public lands because they were dominated by sandhills and had long-standing programs (established in 2002 or earlier) of providing nest boxes for Southeastern American Kestrels. Nest boxes were installed in sandhills primarily by FWC and the University of Florida during the 1990s. Longleaf Pine sandhill is a pyrogenic, xeric, upland plant community composed of widely spaced pines that form an open canopy, a sparse midstory layer of xerophytic Quercus spp. (oaks), a poorly developed shrub layer that includes oaks and Serenoa repens (W. Bartram) Small (Saw Palmetto), and herbaceous ground cover of grasses and forbs (Myers 1990, Platt et al. 1988, Provencher et al. 2002). Once found throughout the southeastern US, Longleaf Pine sandhills have declined primarily due to fire suppression and land-use conversion (Ware et al. 1993). The vegetative structure of this subclimax community is naturally maintained by lowintensity fires at frequent intervals, described variously as 1–3 y (FNAI 2010) and 2–5 y (Provencher et al. 2002). Prescribed burning occurred at our study sites at Southeastern Naturalist K.E. Miller, R. Butryn, E. Leone, and J.A. Martin 2019 Vol. 18, No. 2 194 a similar frequency, although intervals varied locally, and fire histories were not available for all sites. The Longleaf Pine forests were uneven-aged, with a significant component of trees older than 70 y and scattered relict trees aged 125–250 y. Other plant communities in the landscape included mesic pine flatwoods, wetlands, and hammocks. Methods We typically visited nest boxes once every 7–10 d during March–June 2009 and inspected nest boxes with either an aluminum extension ladder or a video camera mounted on a telescoping fiberglass pole. We made more-frequent visits close to the anticipated hatching date to ensure accurate estimation of hatching and fledging dates, with incubation and nestling periods of 30 d and 28 d, respectively, used as guidelines (Smallwood and Bird 2002). For this analysis, we defined occupancy as evidence of at least 1 egg having been laid and success as evidence of at least 1 nestling having fledged. We considered nestlings known to be alive ≤7 d prior to fledging to have fledged (Smallwood and Collopy 2009, Steenhof and Newton 2007). We used the Breeding Biology Research and Monitoring Database framework to sample vegetation within a ~1-ha habitat patch around each nest box (T.E. Martin, Montana Wildlife Cooperative Research Unit, University of Montana, Missoula, MT, unpubl. data). We established 4 vegetation sampling plots at each nest box. We centered 1 plot on the nest box, while remaining plots were located 30 m from the central plot at 120° intervals. We recorded percent cover of bare ground, grass, and shrubs; maximum height of ground cover; and maximum height of horizontal visual obscurity (robel pole) in a 5 m × 5 m area centered on each plot. We measured several aspects of forest structure, including counting snags, pines, and hardwoods in 2 size classes (8–15 cm diameter at breast height [dbh] and >15 cm dbh), and determining median height of pines and hardwoods in a 0.04-ha circle (11.3-m radius) centered on each plot. At each plot, we also estimated percent canopy closure with a concave densiometer and basal area of pine and hardwood trees using a prism. We averaged these vegetation variables across plots associated with each nest box to determine composite values for that habitat patch. We used the Florida Cooperative Land Cover Map version 2.3 (FWC and FNAI, Tallahassee, FL) and ArcGIS Desktop (version 10.3, ESRI, Redlands, CA) to quantify habitat variables surrounding nest boxes. We used FragStats version 4 (McGarigal et al. 2012) to measure landscape attributes at 2 scales (500-m buffer [78.5 ha] and 1-km buffer [314 ha]) around nest boxes. The smaller scale encompassed the entirety of a typical American Kestrel territory, while the larger scale effectively encompassed a landscape several times larger than a typical territory (Bird and Palmer 1988; K.E. Miller, pers. observ.; Smallwood and Bird 2002). We measured the total area, number of patches, and mean patch size of breeding habitat (FNAI classification categories: 1240 Sandhill, 1231 Upland Pine). Southeastern American Kestrels sometimes avoid nest boxes near dense woodlands (K.E. Miller, pers. observ.; Wilmers 1983), so we also measured the distance to Southeastern Naturalist 195 K.E. Miller, R. Butryn, E. Leone, and J.A. Martin 2019 Vol. 18, No. 2 unsuitable woodland habitat (1110 Upland Hardwood Forest, 1111 Dry Upland Hardwood Forest, 1112 Mixed Hardwoods, 1120 Mesic Hammock, 1122 Prairie Mesic Hammock, 1123 Live Oak, 1124 Pine-Mesic Oak, 1140 Slope Forest, 1150 Xeric Hammock, 1220 Upland Mixed Woodland, 1311 Mesic Flatwoods, 1400 Mixed Hardwood–Coniferous, 1410 Successional Hardwood Forest, 183231 Hardwood Plantations). In addition, we calculated Edge Density (ArcGIS Desktop, version 10.3), which is a measure of habitat interspersion. We constructed all models in SAS 9.2 (SAS Institute Inc., Cary, NC). We used PROC CORR to identify highly correlated (>.0.7) habitat-patch variables and then reduced the number of variables in the habitat-patch structure model from 15 to 9 by removing some of the correlated variables. When winnowing down a list of highly correlated variables, we retained those parameters that habitat managers could most easily interpret. We then explored combinations of the 9 habitat-patch structure variables using PROC LOGISTIC, with the SCORE option, to identify the most influential variables; the top-performing model from this exercise had 1 variable (percent grass cover). For each response (occupancy and success), we tested 7 competing models against a null model: habitat patch structure, reduced habitatpatch structure (percent grass cover), habitat amount (500-m scale), habitat amount (1-km scale), habitat distance, habitat pattern (500-m scale), and habitat pattern (1-km scale). See Table 1 for the variables included in each model. We used PROC GLIMMIX to predict occupancy and nest success of Southeastern American Table 1. Variables included in models used to describe Southeastern American Kestrel occupancy and nest success. Models Habitat Reduced Habitat Habitat Habitat Habitat patch habitat amount pattern Habitat amount pattern Variables structure patch 500 m 500 km distance 1 km 1 km Pine basal area X Pines 8–15 cm X Pines > 15 cm X Oak basal area X Oaks 8–15 cm X Oaks >15 cm X Percent grass X X Percent shrub X Maximum height cover X Habitat area 500 m X Habitat area 1 km X Woodland distance X Mean patch 500 m X Mean patch 1 km X Total patches 500 m X Total patches 1 km X Edge density 500 m X Edge density 1 km X Southeastern Naturalist K.E. Miller, R. Butryn, E. Leone, and J.A. Martin 2019 Vol. 18, No. 2 196 Kestrels, assuming a binary distribution. We included site as a random effect to account for potential correlation between nest boxes within sites, but this term was estimated at zero and subsequently dropped from the models. We did not model nest success with exposure-based models, given our small sample sizes and given that studies of nest box use by Southeastern American Kestrels typically treat nest success as a binary variable. We calculated Akaike information criterion (AICc) weights (Burnham and Anderson 2002) to determine the best fit model for each response variable. Results Eighteen (31%) of the 58 monitored nest boxes were occupied by nesting Southeastern American Kestrels. Clutch size was 4.4 ± 0.8 (mean ± SD). Eleven (65%) of 17 nesting attempts were successful, yielding 2.2 ± 1.8 (mean ± SD) fledglings per nest and 3.4 ± 1.0 fledglings per successful nest. We were unable to determine the fate of 1 nest. We performed logistic regression analysis on data from the locations of 51 nest-boxes. Logistical constraints precluded collection of habitat-patch data at 5 nest boxes in Camp Blanding and 2 nest boxes at Ordway–Swi sher. We identified the reduced habitat-patch model (percent grass cover) as the best fit for predicting occupancy by Southeastern American Kestrels (Table 2), but none of Table 3. Model comparisons, logistic regression of habitat and nest success of Southeastern American Kestrels, northcentral Florida. Competing models -2LL k AIC Delta Weight Null 22.07 1 24.34 0.00 0.29 Habitat amount 500 m 19.82 2 24.68 0.34 0.25 Habitat amount 1 km 19.85 2 24.71 0.37 0.24 Reduced habitat-patch structure 21.60 2 26.46 2.12 0.10 Distance 21.92 2 26.78 2.44 0.09 Habitat pattern 1 km 18.96 4 30.30 5.96 0.01 Habitat pattern 500 m 20.30 4 31.63 7.29 0.00 Habitat patch structure 12.59 10 69.25 44.91 0.00 Table 2. Model comparisons, logistic regression of habitat and occupancy of nest boxes by Southeastern American Kestrels, northcentral Florida. Competing models -2LL k AIC Delta Weight Percent grass cover 54.73 2 58.98 0.00 0.95 Null 64.92 1 67.01 8.03 0.02 Habitat amount 1 km 63.26 2 67.51 8.53 0.01 Habitat amount 500 m 63.52 2 67.77 8.79 0.01 Distance 64.71 2 68.96 9.98 0.01 Habitat pattern 500 m 61.39 4 70.26 11.28 0.00 Habitat pattern 1 km 63.76 4 72.63 13.65 0.00 Habitat patch structure 47.43 10 72.93 13.95 0.00 Habitat patch structure 12.59 10 69.25 44.91 0.00 Southeastern Naturalist 197 K.E. Miller, R. Butryn, E. Leone, and J.A. Martin 2019 Vol. 18, No. 2 the habitat models predicted nest success for Southeastern American Kestrels better than the null model (Table 3). For both occupancy and success, it also appeared that habitat-amount models outperformed habitat-pattern models. Regardless, none of the territory or landscape-scale models had much predictive value. There was a significant effect of grass cover on occupancy by Southeastern American Kestrels (F1,49 = 8.04, P < 0.007; Table 2). Nest boxes located in habitats with a ground cover dominated by grasses (versus shrubs or leaf litter) were more likely to be occupied (Fig. 1). The canopy closure and bare ground variables were highly correlated with grass cover (and thus not included in the habitat-patch structure model; see Methods). Nest boxes occupied by Southeastern American Kestrels were located in patches that had more grass cover (52 ± 22% [mean ± SD]), relatively little bare ground (31 ± 15%), and a more open tree canopy (21 ± 18%) than did unoccupied nest sites, which had less grass (32 ± 18%), more bare ground (42 ± 20 %), and a denser tree canopy (35 ± 12%). Although pine basal area, an important metric widely used by managers in forest assessment, was typically lower around occupied nest boxes (8.9 ± 9.2 m²/ha [38.8 ± 40.3 ft²/acre]) than around nest boxes that were unoccupied (13.4 ± 13.5 m²/ha [58.3 ± 58.7 ft²/acre]), it was not significant in the habitat-patch model for occupancy of nest boxes (F1,41 = 1.82, P = 0.185). Figure 1. Influence of grass cover (%) on occupancy of nest boxes by Southeastern American Kestrels in north-central Florida. Dashed lines indicate 95% confidence intervals around predicted values. Southeastern Naturalist K.E. Miller, R. Butryn, E. Leone, and J.A. Martin 2019 Vol. 18, No. 2 198 Discussion Previous work on habitat relationships for the Southeastern American Kestrel has been limited to anthropogenic landscapes (e.g., pastures and other agricultural habitats; Smallwood and Collopy 2009). Our study is the first quantitative assessment of habitat relationships for this imperiled subspecies using nest boxes in native plant communities. Breeding Southeastern American Kestrels selected nest boxes in habitat patches with an average of 52% grass and rarely used areas with less than 25% grass (Fig. 1). Similar degrees of grass cover were documented in studies of foraging habitat used by wintering American Kestrels in pastures, citrus groves, and scrub in southern Florida (Smallwood 1987) and of breeding habitat used by resident Southeastern American Kestrels in pastures in northern Florida (Smallwood and Collopy 2009). American Kestrels in Florida appear to choose areas with similar habitat structure, regardless of habitat type, location, season, or their migratory status. Habitat characteristics within nest box territories occupied by Southeastern American Kestrels (i.e., open tree canopy with few woody shrubs and a graminoiddominated, low ground cover) were consistent with ecological reference conditions for sandhills (FNAI 2010). These habitat structural features can be created and maintained through hardwood-removal programs designed to benefit Picoides borealis Vieillot (Red-cockaded Woodpecker) and other birds associated with pine ecosystems with a grass–forb herbaceous layer (Conner et al. 2002, Provencher et al. 2002). Similarly, populations of fire-dependent bird species such as Peucaea aestivalis (Lichtenstein) (Bachman’s Sparrow) and Colinus virginianus (L.) (Northern Bobwhite) increase after restoration of fire to southern pin e ecosystems, likely because of increases in grasses in the herbaceous layer and arthropods associated with those grasses (Wilson et al. 1995). We believe managers tracking the impact of habitat restoration on avian communities in Longleaf Pine sandhills can regard the presence of Southeastern American Kestrels as a positive indicator of success. Our findings are consistent with previous evidence that American Kestrels respond to structural features of open habitat that facilitate hunting on the ground from perches (Smallwood and Bird 2002). Occurrence and/or nest success of Southeastern American Kestrels were inversely correlated with the extent of woody vegetation in pasture-dominated landscapes in New Jersey (Smallwood and Wargo 1997) and Pennsylvania (Rohrbaugh and Yahner 1997) and with proximity to closed canopy forest in West Virginia and Pennsylvania (Wilmers 1983) and Missouri (Toland and Elder 1987). In our study, vegetation structure within the habitat patch used by Southeastern American Kestrels was more important than the size or shape of the habitat patch or how the patch fit within the larger landscape, including its proximity to dense forest. Given that the Southeastern American Kestrel is non-migratory and that natal dispersal distances in Florida averaged only 4.9 km (Miller and Smallwood 1997), it is possible that habitat isolation may influence its distribution at a larger landscape scale that we did not measure. However, when measured at such a scale (4.9-kmradius circles), neither habitat extent nor habitat fragmentation influenced occupancy of nest boxes by Southeastern American Kestrels in pasture-dominated landscapes Southeastern Naturalist 199 K.E. Miller, R. Butryn, E. Leone, and J.A. Martin 2019 Vol. 18, No. 2 in Florida (Brown et al. 2014). In contrast, the size of contiguous habitat patches was strongly correlated with occupancy of nest boxes by Southeastern American Kestrels in agricultural landscapes in New Jersey (Smallwood et al. 2009b). The proportion of successful nesting attempts (65%) was typical of the 67% success rate reported by Smallwood and Bird (2002) in their review of kestrels across North America. Our data provide limited support for the influence of habitat on nest success, but given our small sample size and the fact that “habitat amount” nearly overcame the null model (Table 3), further research on the minimum habitat patch size needed for the subspecies seems warranted. Our results can help managers in peninsular Florida in the placement of nest boxes to maximize their potential of being used by Southeastern American Kestrels. Based on our data in sandhills and research on the species elsewhere, we recommend siting nest boxes in locations with open tree canopies (ideally ≤25% closure) and low ground cover (≤25 cm; see Smallwood 1987) dominated by grasses. Predictive habitat modeling for Southeastern American Kestrels using regional land-cover data (e.g., Cox et al. 1994, Endries et al. 2009) can identify potential habitat for Southeastern American Kestrels only at a coarse scale, given that the condition and composition of ground cover within GIS land-cover categories are unknown. The loss of suitable, open-ground foraging habitat has received little attention in regional or continental efforts to arrest population declines of the American Kestrel. Population declines frequently have been attributed to nest-site limitation (i.e., a lack of suitable tree cavities) in the southeastern US (Hoffman and Collopy 1988, Smallwood and Collopy 2009) and, at the continental scale, to factors outside the breeding range (McClure et al. 2017, Smallwood et al. 2009a). However, hypotheses about the breeding range rarely include consideration of the status and quality of ground cover. We recommend that further attention be given to maintaining or reestablishing suitable ground cover in native pyrogenic plant communities for Southeastern American Kestrels. Additional research may be needed to elucidate relationships among fire frequency, fire seasonality, fire intensity, and ground cover suitability for Southeastern American Kestrels in sandhills, scrub, and grasslands in Florida and neighboring states. Acknowledgments We thank the many cooperators and colleagues who helped build, repair, install, or monitor nest boxes, including S. Earl, J. Garrison, A. Hallman, R. Melvin, G. Morgan, and D. Pearson. We are grateful to our colleagues J. Brown, N. Klaus, and J. Smallwood for discussions that helped make this work better. Earlier drafts of the manuscript were improved by the comments of R. Bielefeld, A. Cox, B. Crowder, and 2 anonymous reviewers. Funding for this work was provided by Florida’s Nongame Wildlife Trust Fund, Florida’s State Wildlife Grants program, and in-kind contributions from partners and volunteers. Literature Cited Bird, D.M., and R.S. Palmer. 1988. American Kestrel. Pp. 253–290, In R.S. Palmer (Ed.). Handbook of North American Birds. Volume 5: Diurnal Raptors. Part 2. Yale University Press, New Haven, CT. 448 pp. Southeastern Naturalist K.E. Miller, R. Butryn, E. Leone, and J.A. Martin 2019 Vol. 18, No. 2 200 Blanc, L.A., and J.R. Walters. 2008. Cavity excavation and enlargement as mechanisms for indirect interactions in an avian community. Ecology 89:506–514. Bohall-Wood, P., and M.W. Collopy. 1986. 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