Little is known about the habitat and ecology of suburban Barred Owls (Strix varia), a species sometimes considered the nocturnal equivalent of Red-shouldered Hawks (Buteo lineatus). We compared nesting habitat of Barred Owls to that of Red-shouldered Hawks nesting in suburban and urban areas, in and near the city of Cincinnati, Ohio, to determine whether any features distinguished owl nest sites from hawk nest sites. We characterized habitat and land-cover metrics in circular plots of 100 ha and 15 ha, centered on the owl and hawk nests, using ATtiILa software operating within a GIS environment. For the 100-ha plots, the primary cover type in the plots surrounding nests of both species was forest, 41.4 ± 3.4% for Barred Owl plots and 45.9 ± 3.4% for Red-shouldered Hawk plots, followed by low-density residential land: 29.8 ± 4.8% of the Barred Owl plots and 29.3 ± 3.7% of the Red-shouldered Hawk plots. Pasture composed <15% of the plot area for both species and the remainder of the cover types contributed even less. Values of land-cover percentages and metrics did not differ between the species (P > 0.05), for either the large plots or the small (15-ha) plots. Using stepwise binary logit regression analysis, we found that no variables discriminated owl plots from hawk plots. We concluded, based on our methodology, that habitat of suburban Barred Owls differed little from habitat of suburban Red-shouldered Hawks in southwestern Ohio.
Barred Owls (Strix varia) typically have been associated with large tracts of undisturbed mature deciduous or mixed deciduous/conifer forests (Allen 1987, Bosakowski et al. 1987, Bosakowski and Smith 1997, Haney 1997, Mazur and James 2000). Nesting habitats include riparian forests, wooded swamps, and wooded upland areas, often with high canopy closure and located relatively near water (Bosakowski et al. 1987, Laidig and Dobkin 1995, Livezey 2007). Most nesting studies of Barred Owls have been conducted in natural areas such as national and state forests, or national parks. However, Barred Owls also inhabit suburban and urban areas in some regions, although little has been published on the habitat and ecology of these suburban owls. For example, Barred Owls nested in small suburban woodlots in Litchfield, Connecticut (Yannielli 1991), and in urban and suburban Charlotte, North Carolina, in small wooded areas among high-density housing and in larger woodlots near residential and industrial development (Harrold 2003, Mason 2004, Bierregaard et al. 2008).
Red-shouldered Hawks, described as the diurnal equivalent of Barred Owls (Temple and Temple 1976), also nest in undisturbed mature deciduous forests, typically in riparian or upland areas, and often near water (Titus and Mosher 1981, Bednarz and Dinsmore 1982, Armstrong and Euler 1983, Dykstra et al. 2000, 2008). They are similar in size to Barred Owls and both species consume a similar variety of small mammals, amphibians, reptiles, and invertebrates (Livezey 2007, Dykstra et al. 2008, Strobel and Boal 2010). Both species have been considered indicator species for the management of old forests (USDA 1985, 1986, Mazur and James 2000, Hess and King 2002, Crewe and Badzinski 2006).
Red-shouldered Hawks, particularly the western subspecies (B. lineatus elegans), also inhabit suburban and urban areas (Bloom and McCrary 1996, Rottenborn 2000). The eastern subspecies, B. lineatus lineatus, nests in suburban areas in a variety of regions, including the Cincinnati area of southwestern Ohio (Dykstra et al. 2000, 2001a, 2003) and Charlotte, North Carolina (noted anecdotally by Harrold 2003 and Mason 2004). In southwestern Ohio, Red-shouldered Hawks built nests a mean of 75 m from human residences (Dykstra et al. 2000), and their 90-ha breeding-season home ranges contained an average of 169 residences each (Dykstra et al. 2001b).
Based on published reports of land-cover types inhabited by Barred Owls and Red-shouldered Hawks in remote natural areas and our observations of both species in suburban regions in and near the city of Cincinnati, Ohio (C. Dykstra, M. Simon, and J. Hays unpubl. data), we believed the two species might use similar suburban habitats. However, suburban habitat of the Barred Owl has been little studied. Knowledge of the ecology and habits of these common species will give us insight into some aspects of community dynamics of raptors, which have been studied in various species in natural settings (e.g., Hakkarainen and Korpimäki 1996, Sergio et al. 2003a), but not in urban/suburban regions, to our knowledge. Thus, we studied Barred Owls nesting in suburban and urban settings in and near the city of Cincinnati, Ohio, in southwestern Ohio, and compared land-cover types and environmental features surrounding owl nests to those surrounding Red-shouldered Hawk nests in the same area. Our goal was to evaluate whether any features distinguished owl nest sites from hawk nest sites.
Southwestern Ohio is a hilly, unglaciated area in the Interior Plateau ecoregion (Omernik 1987). The hills are dissected by many small streams located in ravines and by two large tributaries of the Ohio River, the Great Miami and the Little Miami rivers. Native forests are dominated by second-growth oak-hickory (Quercus spp., Carya spp.) and American beech-sugar maple (Fagus grandifolia, Acer saccharum) associations, with lowland riparian forests characterized by American sycamores (Platanus occidentalis) and beech.
Our study area centered on the city of Cincinnati, Ohio, and included Hamilton County, Clermont County, and southwestern Warren County, Ohio, and one owl nest in northern Campbell County, Kentucky. Many of the nests studied were located in the wide band of suburban development surrounding the city of Cincinnati. Suburban areas varied from densely populated (residential lots approximately 20 × 35 m) to sparsely-populated (>2.5-ha residential lots, as well as undeveloped private land); all of these suburban areas were classified as “low-density residential” (see below). Human population density in the study area was approximately 480 people/km2, based on population in the six suburban townships containing the greatest numbers of nests (Ohio Department of Development 2008, U.S. Census Bureau 2008). Most residences and other buildings were surrounded by lawns and other nonnative vegetation, but residences tended to be located on level ground, with steep slopes and riparian areas left in native vegetation. Areas of public land within the study area contained no residences, but were heavily used for sports and other recreation.
Nest Location Techniques
We defined “nests” as locations with a known nest and “nest sites” as areas where preflighted owl young (“branchers”) were present but no nest was identified. We assumed that preflighted owl young were still located relatively close to their natal nest. For our analyses, we included only nests and nest sites where pairs attempted breeding. We considered that a breeding attempt occurred if there was evidence that eggs had been laid (i.e., observations of an incubating adult, presence of small down feathers on the edges of nest or cavity, broken eggshells below nest, nestlings in nest, or preflighted owl young; Steenhof and Newton 2007).
Barred Owl nests
We found nests and nest sites between 17 April and 1 June, 2006–09, using primarily two techniques. We received reports of nests or sightings from members of a local raptor-rehabilitation association, RAPTOR, Inc. (“RAPTOR reports”). We solicited information on nest locations and owl sightings from birders and others by word-of-mouth and by announcements at meetings of local ornithological societies, nature centers, county parks, and on a local birding list-serve (“public reports”). We also found three Barred Owl pairs using stick nests that had been previously occupied by Red-shouldered Hawks. We visited all reported locations to find owl nests and confirm breeding attempts. Locations of nests were confirmed by our observation of an incubating owl, an owl flying from the nest cavity, nestlings in the nest, or broken eggs on the ground below a nest cavity.
Red-shouldered Hawk nests
Nests included in this study were previously known to us from an ongoing long-term study (Dykstra et al. 2009). We originally found the nests in that study primarily between February and early April, using several techniques (Dykstra et al. 2000). As for Barred Owls, we received reports of nests or sightings from members of a local raptor-rehabilitation association (RAPTOR reports), and we solicited and received information on nest locations and hawk sightings from birders and other members of the general public (public reports). Other nests were found as a result of our own searches and observations (Dykstra et al. 2000). We visited all known nest areas at least twice (and sometimes three or more times) between mid-February and mid-April and viewed nests from the ground using 8× or 10× binoculars or a 20–60× spotting scope.
For this investigation, we selected all nests in 2007 that were originally found as a result of reports from the raptor-rehabilitation association (RAPTOR reports), or from birders and other members of the general public (public reports). All Red-shouldered Hawk nests found by other methods were excluded from our analyses, in order to minimize bias resulting from differing nest-location techniques. Although it is preferable statistically to find nests in an unbiased way by walking transects through a specific region of habitat, that time-consuming technique probably would not be efficacious in fragmented suburban areas. We believe that our methodology ensured that any potential bias should be roughly equivalent for both species.
Coordinates of nests and nest sites were recorded using a handheld GPS unit and these coordinates were entered into a geographic information system (GIS; ArcView 3.3 with Spatial Analyst 2.0; ESRI, Redlands, California, U.S.A.). We selected one plot size of 100 ha (radius of 564 m) based on the home-range size of Red-shouldered Hawks in this study area (90 ha in the breeding season; Dykstra et al. 2001b) and the home-range size of suburban Barred Owls in Charlotte, North Carolina (113 ha for males in the breeding season; Harrold 2003). We also created smaller plots of 15 ha (radius = 219 m) around the nests, based on the core areas used by Red-shouldered Hawks (13 ha for seven breeding-season males; Dykstra et al. 2001b) and suburban Barred Owls (18 ha for five breeding-season males; Harrold 2003). Both sets of plot circles were overlaid on selected geographic coverages: the National Land Cover Dataset (NLCD) 2001 (for 30-m resolution land-cover metrics; Homer et al. 2007); the National Hydrography Dataset (NHD) 2009 (for the 1∶24 000 scale stream length and densities; Simley and Carswell 2009); and official state roadway coverages from the Ohio and Kentucky Departments of Transportation (M. Thompson, D. Zourarakis pers. comm.) for the road length and densities. Desired metrics were computed using ATtiILa software (USEPA 2004) operating within the GIS environment.
Based on published reports of habitat selection in our study species, we selected land-cover metrics that represented habitat we believed important for nesting and foraging (forests, streams, fields) and those that represented features that might be deterrents (such as residential areas and roads; Appendix).
Data were not normally distributed. Thus, we first used Mann-Whitney U-tests for the univariate comparison of habitat variables between species. We then examined the Pearson correlation matrix. For all pairs of variables with r > 0.70, we selected one variable of the pair, choosing those that were most basic (least derivative). For multivariate analysis, all variables in the reduced dataset were used in stepwise binary logit regression analysis (SYSTAT 11.0), with forward and backward steps, to select the combination of variables that best discriminated nest sites/areas of the two species.
Nests and Nest Sites
We found 21 nests and nest sites of Barred Owls in the study area (14 nests and 7 nest sites; Fig. 1). Nine of these were reported to us by the public, nine by the raptor-rehabilitation group, and three were open stick nests found by us because they were formerly occupied by Red-shouldered Hawks. One of the nests was in a nest box erected 8 m from a cavity nest that had been used by the owls in the previous season, but then usurped by raccoons (Procyon lotor); for all analyses, we included only the nest in the nest box. The remaining 13 nests were in trees. Of 10 nest trees for which species was recorded, five (50%) were American sycamores, and one each were American beech, northern red oak (Quercus rubra), eastern cottonwood (Populus deltoides), ash sp. (Fraxinus americana or F. pennsylvanica), and willow sp. (Salix sp.; 10% each). Of 11 nest trees for which tree condition was recorded, six (55%) were alive (four sycamores [36%], one eastern cottonwood [9%], and one willow sp. [9%]) and five (45%) dead (one each of beech, northern red oak, ash sp., sycamore, and unknown sp. [9% each]).
We found 103 nests of Red-shouldered Hawks in the study area in 2007. Of these, 29 were originally reported to us by the public, 22 by the raptor-rehabilitation association, and 52 were found by us as a result of our own searches and observations. For all analyses, we included only the 51 nests in the first two categories. Of these, 11 of 51 Red-shouldered Hawks nests were in sycamores (22%), five were in northern red oak (10%), five in ash spp. (10%), three in tuliptree (Liriodendron tulipifera; 6%), three in white oak (Q. alba; 6%), three in pin oak (Q. palustris; 6%), three in unidentified oaks (Quercus spp.), two each (4%) in shingle oak (Q. imbricaria), burr oak (Q. macrocarpa), shagbark hickory (Carya ovata), sugar maple, silver maple (A. saccharinum) and unidentified maples (Acer spp.), and one each in black cherry (Prunus serotina), cottonwood, red maple (A. rubrum), black walnut (Juglans nigra), elm (Ulmus sp.), and hickory (Carya sp.). All these nest trees were alive.
Habitat Around Nests and Nest Sites
Large 100-ha plots
The primary cover type surrounding nests and nest sites of both species was forest, 41% for Barred Owl nests and nest sites, and 46% for Red-shouldered Hawk nests (Table 1), but low-density residential land made up 30% of the Barred Owl plots and 29% of the Red-shouldered Hawk plots. Pasture composed <15% of the plot area for both species and the remainder of the cover types contributed even less (Table 1). Total road length within the plots was 4899 m for owls and 4070 m for hawks, more than three times the total stream length within the plots. Univariate comparisons indicated no differences in cover types and other attributes between Barred Owl plots and Red-shouldered Hawk plots (Table 1; all P > 0.2).
Small 15-ha plots
As in the larger plots, the primary cover type surrounding nests and nest sites was forest, followed by low-density residential land. We found no differences in any cover-type variables between hawk and owl plots (Table 2; all P > 0.2).
For the 100-ha plots, road length and number of forest patches were correlated with other variables and were excluded from the analysis (Table 1); the other 11 variables were used in the logit regression model. However, no variables were retained in the final model, indicating that none of these could be used to differentiate hawk plots from owl plots.
For the 15-ha plots, the low-density residential cover type (%) was correlated with total forest cover (%), so we excluded low-density residential. Similarly, the number of forest patches and mean area of forest patches were excluded (Table 2). The remaining 10 variables were used in the logit regression model, but, as above, no variables were retained in the final model.
Habitat Around Nest Sites
Barred Owls nested in suburban areas of Cincinnati, in locations averaging 41–48% forested and 30–31% residential cover type, depending on the spatial scale considered. In some cases, their nests were within about 20 m of suburban and urban residences (C. Dykstra unpubl. data). The cover types and environmental features surrounding owl nests did not differ from those surrounding Red-shouldered Hawk nests in the same study area (Table 1). Although Barred Owl habitat in suburban regions has been little studied, we suspect that Barred Owls are not uncommon in suitable suburban regions of eastern North America (Yannielli 1991, Harrold 2003, Mason 2004, Bierregaard et al. 2008).
Habitat of suburban Barred Owls differed little from that of suburban Red-shouldered Hawks in southwestern Ohio, based on our methodology. The lack of difference was not surprising, as the Barred Owl is sometimes said to be the nocturnal equivalent of the Red-shouldered Hawk (Temple and Temple 1976). In more than one case, Barred Owls apparently usurped an occupied Red-shouldered Hawk nest in our study area; owls were found in historical Red-shouldered Hawk nests with fresh green vegetation, indicating that hawks had likely worked on the nest within the previous month. In addition, we often heard Barred Owls calling while we were visiting Red-shouldered Hawk territories (C. Dykstra unpubl. data), although we did not search for owl nests in these areas in order to preserve the independence of the two datasets. Additionally, we found Red-shouldered Hawks or their nests at five sites where we had reports of Barred Owls and were searching for owl nests.
Barred Owls in northern Michigan also apparently used the same forests frequented by Red-shouldered Hawks, because seven of 114 owl breeding attempts were in old Red-shouldered Hawk nests (Postupalsky et al. 1997). Barred Owls and Red-shouldered Hawks also coexist in urban/suburban Charlotte, North Carolina (Mason 2004), and in the second-growth Allegeny hardwood forests of the central Appalachian Mountains, where they are the most common nocturnal and diurnal raptors, respectively (Smith 2003).
Nest-site selection of suburban Barred Owls has been studied in only a single location. In Charlotte, North Carolina, researchers found no variables to distinguish nest plots from randomly placed plots (Harrold 2003), but found that suburban Barred Owl nests were only 11 m from the nearest human dwelling and 41 m from the nearest road (Harrold 2003). In our study area, we did not quantify nest-site selection of Barred Owls. However, suburban Red-shouldered Hawks in our study area selected nest trees that were larger than the surrounding trees, in areas of large trees, and close to permanent or seasonal water (Dykstra et al. 2000). Their nest sites were no further from roads and human residences (primarily single-family homes) than were randomly placed plots, averaging only 86 m from roads and 75 m from houses (Dykstra et al. 2000), suggesting that Red-shouldered Hawks did not alter their use of habitat in response to the presence of houses and roads. The similarity of cover types and other features of the two species in our current study suggests that Barred Owls also may be indifferent to the presence of suburban houses and roads in this study area, as in suburban Charlotte. The nocturnal habits of Barred Owls may serve to increase their tolerance, as they probably interact less frequently with humans than do diurnal hawks.
Home ranges of Barred Owls in the boreal forest in central Saskatchewan consisted of 86% forest (deciduous, mixed, and coniferous), although the entire study area was only 62% forest (Mazur et al. 1998), and these results were typical of telemetry studies in western North America, where Barred Owls used old forests more than expected (Washington; Hamer et al. 2007) and were associated with areas of high canopy closure and large trees (Washington; Singleton et al. 2010). In contrast, home ranges of suburban Barred Owls in Charlotte, North Carolina, consisted of 81% residential land use; although the authors caution that this land-cover value was based on zoning classifications, they also describe the suburban areas as consisting of residential properties including wooded lawns, small woodlots, and city parks (Harrold 2003). We did not investigate home ranges for Barred Owls in this study, so we were unable to quantify home-range habitats, but we can infer that, within the larger landscapes consisting of 41–48% forested land, Barred Owls were able to find sufficient forest, nest sites, and prey to breed.
Our Barred Owl samples included three nests in urban neighborhoods >75 yr old, and closer to downtown Cincinnati than were any of our Red-shouldered Hawk nests. These neighborhoods tended to be relatively densely populated by humans, and often contained large trees, which can develop cavities of appropriate size for Barred Owls. In addition, Barred Owl nests tended to be in sycamores more frequently than did Red-shouldered Hawk nests (50% of all identified nest trees vs. 22%, respectively, in this study), and sycamores are among the most massive trees in the eastern United States (Sibley 2009). The presence of Barred Owls in the older urban neighborhoods, as well as the greater use of sycamores by Barred Owls, may reflect a possible difference in nest-site characteristics between Barred Owls and Red-shouldered Hawks, one that we did not test in this study: nest-tree diameter. The Barred Owl nest trees we measured in this study (n = 7) averaged 77 cm dbh (diameter at breast height; C. Dykstra unpubl. data), whereas Red-shouldered Hawk nest trees in the same southwestern Ohio study area were slightly smaller, averaging 67 cm in 1997–98 (n = 33; Dykstra et al. 2000). Because Barred Owls generally require large trees that contain cavities of appropriate size, unless nesting in an open stick nest, the presence of such trees in older urban neighborhoods may make those areas—in this case, older urban neighborhoods—sufficiently attractive and suitable for Barred Owls. However, we note that the definition of large trees is relative to the habitat; in boreal forests in Saskatchewan, Barred Owl nest-trees averaged only 47 cm dbh, yet these trees were described as relatively large and the surrounding forests as “mature” (Mazur et al. 1997). In eastern North America, Barred Owl nest trees averaged 61 cm dbh (Maryland; Devereux and Mosher 1984), and in suburban Charlotte, North Carolina, 79 cm dbh (Harrold 2003).
As a caveat to the above, we emphasize that our study had some limitations. First, we may have failed to measure an important distinguishing feature of the two species' habitat, such as the age of the forest, because we were unable to differentiate forest age from the data in our database. Second, the plots we used for habitat analysis may not include the complete actual home ranges, which likely were not circles centered on the nest sites. Finally, we were unable to draw any quantitative conclusions about Barred Owl nest-site selection compared to the available cover types within the study area, as our study did not include random plots. These caveats do not invalidate our results, but do highlight areas where additional research can further our understanding of Barred Owl ecology in suburban areas.
Conservation and Management Implications
Barred Owls in southwestern Ohio seemed somewhat flexible in their requirements for habitat, and apparently tolerated humans well. Their ability to accept suburban landscapes and, presumably, human disturbance, and their propensity for range expansion (Livezey 2009a, 2009b) suggests that they are a species that should be less susceptible to extirpation than some others. However, we note that different populations seem to have different tolerances for human disturbance (Bosakowski et al. 1987, Bosakowski and Smith 1997), and the ability of some populations to use suburban regions does not necessarily indicate that all populations will be able to do so.
The successful range expansion of Barred Owls in the western and northwestern portions of North America (Livezey 2009a, 2009b) has brought them into ranges occupied by the California subspecies of the Red-shouldered Hawk, which is generally more suburban in its habits than the eastern subspecies (Bloom and McCrary 1996, Rottenborn 2000). This subspecies of the Red-shouldered Hawk also has recently expanded its breeding range in the west to include Oregon (Marshall et al. 2003), Arizona (Corman 2005) and east of the Sierra mountains in California (P. Bloom pers. comm.). Nonbreeding Red-shouldered Hawks have also been found in Washington (Tweit 2005). Although it would be unwise to extrapolate beyond our study area in southwestern Ohio, the results of our study suggested that areas suitable for Red-shouldered Hawks, including suburban areas, may also be suitable for Barred Owls, and vice versa.
Barred Owls are known to nest in suburban areas of the Pacific Northwest (J. Buchanan pers. comm.), and if further range expansion continues, may soon come into contact with well-established populations of breeding suburban Red-shouldered Hawks. Their natural colonization may provide a rare opportunity for a preinvasion/post-invasion “natural” experiment on the interspecific interactions of these two raptor species in human-dominated landscapes. For raptors, interspecific competition and/or intraguild predation can decrease reproductive success in both competitors (Korpimäki 1987), decrease breeding density (Hakkarainen and Korpimäki 1996), increase egg and nestling mortality (Kostrzewa 1991, Sergio et al. 2003a), delay laying dates (Hakkarainen and Korpimäki 1996), or decrease post-fledging survival (Hakkarainen and Korpimäki 1996). Raptors facing interspecific competition or predation may reduce the effects of these by changing their habitat use (Sergio et al. 2003b), dispersal patterns (Forero et al. 1999), diet (Korpimäki 1987), or other behaviors. The investigation of population responses of Red-shouldered Hawks to the colonizing Barred Owls would give valuable insight into raptor community dynamics, and further illuminate the complex interactions of species coexisting with humans in urban/suburban areas.
Univariate comparison of habitat variables within 100-ha circular plots for suburban Barred Owls and Red-shouldered Hawks in southwestern Ohio.
Univariate comparison of habitat variables within 15-ha circular plots for suburban Barred Owls and Red-shouldered Hawks in southwestern Ohio.
We are grateful to Ann Wegman and Sandra Stone for assistance with fieldwork, and to Kent Livezey, Associate Editor Joe Buchanan, and three anonymous reviewers for useful comments on earlier versions of this manuscript. We thank the many private landowners who reported raptor nests to us and allowed us access to their property, and the volunteers at RAPTOR, Inc., of Cincinnati, for reporting hawk and owl nests and territories to us. The original Red-shouldered Hawk study was supported in part by the appointment of CRD to the Postgraduate Research Participation Program at the National Exposure Research Laboratory administered by the Oak Ridge Institute for Science and Education through an interagency agreement between the U.S. Department of Energy and the U.S. Environmental Protection Agency. Additional funding was provided by the Ohio Biological Survey, RAPTOR, Inc., Marilyn Arn, and Brad and Marcia Lindner.