Urban Ecosyst DOI 10.1007/s11252-016-0554-0
Spatial distribution and the value of green spaces for urban red-tailed hawks Joan L. Morrison 1 & Isabel G. W. Gottlieb 1,2 & Kyle E. Pias 1,3
# Springer Science+Business Media New York 2016
Abstract Raptors increasingly live and nest successfully in urban areas. In the urban landscape of Hartford, CT, red-tailed hawks established home ranges in large green spaces such as parks, golf courses, and cemeteries but also nested successfully in the commercial district of downtown and in densely built urban and suburban neighborhoods. Data collected from 11 radio-tagged breeding adult hawks indicated that year-round home ranges averaged 107.7 ha, much smaller than home ranges reported for hawks inhabiting rural areas. Most hawk home ranges had multiple core areas that were usually associated with favored perches or larger patches of ‘usable’ green space, defined as patches ≥0.25 ha in size, and home range size was positively associated with larger usable green space patches in core areas. Most nests were located in the largest core area and were within a larger patch of green space within the largest core area. Rather than just the amount or size of green space patches, the value of urban green spaces for these hawks likely also varies with the number and proximity of suitable perches such as buildings or tall trees, types and density of prey, and amount of human activity in and adjacent to these spaces. Territoriality and intraspecific competition may also influence home range size and dispersion of red-tailed hawks nesting in Hartford. In this urban area, mortality due to ingestion of rodenticides and collisions with vehicles affected hawk reproductive success. Keywords Red-tailed hawk . Urban raptor . Home range . Green space . Connecticut . territoriality
* Joan L. Morrison
[email protected]
1
Department of Biology, Trinity College, Hartford, CT, USA
2
Present address: Department of Wildlife Ecology and Conservation, University of Florida, Gainesville, Florida, USA
3
Present address: Hono o Na Pali Seabird Mitigation Project, Lihue, Hawaii, USA
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Introduction Urbanization affects wildlife populations primarily through conversion and fragmentation of natural habitats. These conversions have lasting influences on food and other resources and introduce pollution and other factors that can lead to direct mortality (Collins et al. 2000, McKinney 2006). A common outcome of urban development is densely built environments dominated by human construction and infrastructure with few remaining small and isolated green spaces, areas that retain some natural habitat. Because of these changes, urban areas are typically characterized by reduced wildlife species richness and community diversity, high abundances of mostly non-native species, and relatively fewer native species, sometimes termed ‘biotic homogenization’ (Chace and Walsh, 2006, McKinney 2006). Although human development and urbanization have been described as major threats and deterrents to many raptor populations (Bird et al. 1996, Richardson and Miller 1997, Hager 2009), some species inhabit and nest successfully in urban environments, for example, in municipal parks, suburban neighborhoods, and business parks (Dykstra et al. 2001, Hogg and Nilon 2015), where buildings and urban trees may provide suitable nest substrates and perches and remaining green spaces may provide suitable foraging habitat. Peregrine falcon (Falco peregrinus) populations have increased over the past few decades owing in large part to successful nesting in urban areas, particularly on buildings and bridges (Cade et al. 1996, Gahbauer et al. 2015). Red-shouldered hawks (Buteo lineatus, Bloom et al. 1993, Rottenborn 2000) and Mississippi kites (Ictinia mississippiensis, Parker 1996) nest in a variety of habitats in urban landscapes and seem tolerant of high levels of human activity. Cooper’s hawks (Accipiter cooperii) nest in high densities in several urban areas (Rosenfield et al. 1995, Boal and Mannan 1998, Stout et al. 2007). Rapid expansion of an urban population of merlins (Falco columbarius) may be facilitated by high abundance and availability of their house sparrow (Passer domesticus) prey (Sodhi et al. 1992). Of the larger buteos, red-tailed hawks (B. jamaicensis) are regular inhabitants of urban and suburban landscapes (Minor et al. 1993, Stout et al. 1998); even Swainson’s hawks (B. swainsoni) have been reported nesting in cities (James 1992). Raptors vary in their tolerance of urbanization, however (Berry et al. 1998, Schmidt and Bock 2004), and individuals living in urban environments often differ from conspecifics living in rural environments in home range size, survival, reproductive success, habitat use, and responses to human activity. Individuals of some species experience similar or higher reproductive success, nest at higher densities, and have smaller home ranges in urban environments (Parker 1996, Coleman et al. 2002, Dykstra et al. 2000). These findings suggest that urban landscapes can provide high quality habitat perhaps due to a variety of potential nesting substrates, high density of prey items, and the absence of persecution by humans (Sodhi et al. 1992, Bloom et al. 1993, Boal and Mannan 1998). On the other hand, urban landscapes can contain many potential hazards, thus, despite higher nesting densities in urban environments some species suffer reduced reproductive success owing to factors such as starvation, predation by domestic pets on young, intraspecific competition, toxins, or disease (Botelho and Arrowood 1996, Tella et al. 1996, Hindmarch and Elliott, 2015, Miller et al. 2015). While urban environments may provide sufficient resources for some raptors, differences in species’ responses to urbanization indicate that the life history and ecology of raptors living in urban environments are not well understood. The red-tailed hawk is one of North America’s most common large raptors and occurs in a wide variety of habitats characterized primarily by a mixture of open grasslands and woodland
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and including urban areas (Preston and Beane 2009). Red-tailed hawks have a generalist diet, preying on small- to medium-sized mammals, birds, and reptiles (Preston and Beane 2009), and breeding pairs maintain exclusive territories across their range (Newton 1979). In some regions of North America, red-tailed hawks migrate in response to weather and food supply, while in other regions some hawks move only locally in winter and others stay in a welldefined home range throughout the year (Preston and Beane 2009). More than some species, red-tailed hawks are generally tolerant of urbanization in the landscape (Berry et al. 1998, Schmidt and Bock 2004). Urban hawks nest successfully on a variety of human-made and natural substrates in both highly urbanized and suburban neighborhoods (Runyan 1987, Minor et al. 1993, Stout et al. 1998) and employ novel foraging techniques that facilitate their utilization of locally abundant prey (Tumlinson 2012). While the breeding biology of red-tailed hawks has been described for hawks nesting in both rural and urban environments, few studies have focused on characterizing hawk home ranges or habitat use within urban areas. Several studies have identified the importance of perches to hawks nesting in open landscapes (Fitch et al. 1946, Bednarz and Dinsmore 1982, Bosakowski et al. 1996). Janes (1984) showed that reproductive success of nesting pairs was correlated not with home range size but with the dispersion and density of perches that hawks used while foraging. Stout et al. (2006a) reported that natural cover comprised about 16 % of the area around nest sites of red-tailed hawks nesting in an urban landscape. Forty percent of this area was wooded and the remaining 60 % consisted of herbaceous cover. From limited telemetry data, Petersen (1979) found that hawk home ranges containing large areas of unbroken woodlots were larger than home ranges containing small woodlots or scattered mature trees and that hawk movements were generally confined to small woodlots and upland pasture and grassland habitats. We studied the spatial distribution and home ranges of red-tailed hawks nesting in the urban landscape of Hartford, CT, USA. Our objectives were to investigate the distribution of red-tailed hawk nests and home ranges throughout this landscape, to estimate core area and overall home range sizes, to identify variables characterizing hawk home ranges, and to monitor reproductive success of these urban raptors. We were particularly interested in the hawks’ association with urban green spaces, which include non-built, vegetated areas such as local gardens, formal urban parks, golf courses, cemeteries, small woodlots, overgrown vacant lots and areas of contiguous yard surrounding houses in neighborhoods (Blair 1996, Gill et al. 2009, Lerman and Warren 2011). Many wildlife species in urban environments can benefit from green spaces as these areas can provide nesting and denning sites, food resources, migratory stopovers, refuges, and connections to other areas (Mörtberg and Wallentinus 2000, Pennington et al. 2008, Veríssimo and Bernard, 2015). For urban red-tailed hawks, open grassy areas with scattered trees such as city parks and cemeteries may be important since they are similar to this hawk’s preferred habitat in more rural areas (Bednarz and Dinsmore 1982).
Methods We studied red-tailed hawks in Hartford County, CT (41.73° N, 72.65° W) from 2007 to 2010. The county covers ~1945 km2 and human population density is ~470 per km2 (U.S. Census Bureau 2013). Within the county, the landscape consists of densely built commercial districts, urban residential neighborhoods characterized by a high density of multi-family homes and paved roads and buildings, and medium-density suburban neighborhoods characterized by
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single family homes on larger lots. While large mature trees are often present along city streets, grounds are typically sparsely vegetated or are characterized by managed lawns and impervious surfaces (i.e., pavement). Green spaces of varying types and sizes occur throughout this urban landscape. The study area includes 8 city parks (19.4–95.9 ha in size), which contain few to many usually sparsely spaced mature trees, shrubby woodlots, managed lawns and ball fields, and areas of pavement (i.e., parking lots and pathways). We located hawk nests by searching throughout the county. During winter months, we drove systematically through neighborhoods so we could view suitable nest substrates while foliage was absent. When access was permitted, trees and woodlots that were not entirely visible from the road were surveyed on foot as were city parks, golf courses, and cemeteries. Observations of nest building activities and reports from members of the public also helped us locate nests. We identified an occupied nest as one where a pair of adult red-tailed hawks was observed multiple times either nest building, perching, incubating (identified by bird posture), or feeding young (Postapulsky 1974). We captured adult hawks at occupied nest sites during the breeding season using a bal. chatri trap with live bait (Bub 1991). Hawks were banded with a standard aluminum U.S. Fish and Wildlife Service band and a green aluminum color band with unique alphanumeric characters (ACRAFT Sign and Nameplate Co., Edmonton, Alberta, Canada). Each captured hawk was fitted with a backpack style VHF radio-transmitter (Advanced Telemetry Systems, Isanti, Minnesota, USA) before being released at the capture site. Hawks were sexed using DNA analysis (Avian Biotech, Tampa, Florida, USA). We located each radio-tagged hawk at least weekly, year round, and ~95 % of locations were verified by a visual observation of the target hawk. Hawk locations were recorded on an aerial photo and later transcribed into ArcGIS ver. 9.3.1 (Environmental Systems Research Institute, Redlands, CA). We monitored nests approximately bi-weekly from February through July, 2007–2010. Most nests were inaccessible so we could not determine clutch size, thus, we used direct observations of adults at nests and behavioral cues (nestling vocalizations, adults feeding nestlings) to determine nesting stage (Martin and Guepel 1993), and we recorded the number of fledglings, when possible. After fledging, young hawks remain near the nest site for several weeks, and their begging calls can be easily heard, thus fledglings can be counted. We constructed home ranges for radio-tagged hawks using the fixed kernel density estimator and the least squares cross-validation smoothing method (Kernohan et al. 2001), in ABODE (Laver 2005) in ArcGIS. We calculated year-round home ranges and separate home ranges that represented the breeding season (March–August) and the non-breeding season (September–February), for all radio-tagged hawks. The seasons were identified based on the life history chronology for red-tailed hawks in the northeastern US (Zeranski and Baptist 1990; Preston and Beane 2009) and our observations. All home range polygons were drawn at the 99 % kernel. Particularly in urban environments, the landscape is spatially heterogeneous in terms of food availability, nest and roost sites, presence of competitors, and other factors. Thus, individuals tend to have one or more core areas of intensive use in their home ranges (Hodder et al. 1998). Rather than using a fixed percentage to describe core areas within hawk home ranges (e.g., Kernohan et al. 2001, Roth et al. 2004), we determined biologically meaningful core areas by examining each probability distribution, also estimated in ABODE. We were particularly interested in the hawks’ association with urban green spaces. While many of the red-tailed hawk’s known prey items, for example Eastern gray squirrels (Sciurus carolinensis), other rodents, and songbirds occur throughout the city, we assumed prey
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abundance and availability to be greater in urban green spaces than in densely built neighborhoods. Using 2008 orthophotos of greater Hartford County, in ArcGIS, we digitized all green spaces within the county, areas where the ground was covered by vegetation not by buildings or pavement, and created a ‘green space’ layer. Trees along city streets were not counted as green space because most were single isolated trees and the ground beneath them was most often not vegetated. Also, we were primarily interested in hawk use of green space throughout the city at a scale larger than just individual trees. We calculated various patch and class metrics about green spaces within each hawk home range and core area using the Vector-based Landscape Analysis Tools Extension (available at https://sites.google.com/site/largvlate/gistools/v-late) in ArcGIS. These metrics characterize the configuration and distribution of green space patches from a variety of perspectives such as patch number, density, size, perimeter, and perimeter/area ratio and provide a means to quantify the extent and fragmentation of green space within hawk home ranges. When characterizing size of green space patches, we used area-weighted means because metrics that simply describe patch means consider each patch equally, regardless of patch size and do not address the fact that use by an animal of a particular patch is dependent on patch size (McGarigal and Marks 1995). We overlaid each hawk’s locations on the green space layer and used a G-test for goodness-of-fit to investigate whether hawks used different sized patches of green space as expected by the availability of different patch sizes within their respective home range. We attempted to identify ‘usable’ green space for these urban hawks assuming that a ‘usable’ green space patch was one large enough to accommodate a hawk’s landing and takeoff even if the hawk is carrying a large prey item such as a squirrel or rat. We also quantified road density (km/ha) and building density (# buildings/ha) within each hawk’s home range and core area using ArcGIS. As one common prey item of red-tailed hawks, Eastern gray squirrels often occur at high densities in urban areas (McCleery et al. 2007, Parker and Nilon 2008). Having often observed hawks preying upon squirrels in our study area, we hypothesized that squirrel abundance in green spaces may be an important factor influencing home range size and dispersion of these urban hawks. Using line transect sampling methodology (Hein 1997), during July 2007 through July 2008 we estimated squirrel densities approximately twice monthly, on good weather days, within 8 separate 100 m X 100 m strip transects in hawk home ranges. We recorded all squirrels seen either on the ground or up to 10 m in trees along each transect; Table 1 Red-tailed hawks in Hartford County, CT nested in a variety of natural and human-constructed sites located in city parks, cemeteries, and in densely built urban and suburban neighborhoods Nest structure
Species
Tree
White pine (Pinus strobus) Spruce (Abies sp.)
5 3
Cottonwood (Populus sp.)
1
Oak (Quercus sp.)
1
Maple (Acer sp.) Ledge or top of building
1 2
Back of billboard
1
Substrate height
21.6 m ± 1.8
Distance from nest to nearest road Distance to nearest human structure
93.2 m ± 31.4 (range 2.6–274 m) 170.6 m ± 39.1 (range 9.2–314 m)
M
F
1543
1504
Mean (SE)
M M
1013 1473
F
1094
F
M
684
M
M
1172
784
F
1583
1393
F
1862
74 (4.2)
63
56
105 75
65
64
74
76
92
73
69
107.7 (15.2)
84.7
25.3
84.6 161.4
73.2
120.2
46.1
178.3
158.7
150.2
101.7
94.0 (17.2)
41.0
34.5
75.8 177.1
81.8
164.5
44.7
143.4
160.5
83.5
27.6
115.9 (21.9)
90.7
21.1
92.5 109.4
60.3
198.5
41.0
243.2
105.8
216.6
95.4
0.50 (0.06)
0.61
0.45
0.50 0.23
0.89
0.41
0.68
0.75
0.25
0.40
0.30
0.80 (0.06)
0.67
0.93
0.86 0.53
0.98
0.91
0.93
0.96
0.67
0.92
0.48
0.47 (0.07)
0.59
0.36
0.47 0.20
0.89
0.39
0.64
0.75
0.22
0.38
0.26
0.51 (0.07)
0.53
0.16
0.39 0.51
0.62
0.62
0.26
1.00
0.58
0.55
0.40
Proportion of hawk Proportion of usable Proportion of usable Hawk ID Sex Number of Overall Home Breeding season Non-breeding Proportion of green locations Range home range season home range space in overall home locations in green space green space in overall green space patches in overall home range home range range
Table 2 Home ranges (ha) of breeding adult red-tailed hawks (n = 11) living in the greater Hartford, CT area. Breeding season = March – August, Non-breeding season = September through February. ‘Usable’ green space patches are those ≥0.25 ha in size
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squirrels were more difficult to see within branches and higher tree canopies. Squirrel densities were estimated using program DISTANCE ver. 5.0 (Thomas et al. 2010). Because many of the green space metrics were correlated, we used univariate general linear models (GLM) to examine associations between squirrel densities, green space metrics, and the size of hawk home ranges and core areas. We predicted that smaller hawk home ranges would be associated with higher squirrel densities, that higher squirrel densities would be associated with less fragmented (smaller patch perimeter to area ratio) green space, and that hawk home ranges with a greater proportion of and less fragmented green space would be smaller than home ranges with more fragmented (smaller, isolated patches) and smaller overall proportions of green space. High densities of roads and buildings were predicted to be associated with larger home ranges because these features fragment green spaces that are assumed to be useful as foraging areas for hawks. We transformed all proportional data with the arcsine square root transformation to meet assumptions of normality (Zar 1999). Statistics were performed using SYSTAT ver. 8.0 (SPSS 1996). Means ± SE are reported, unless otherwise noted. Biological significance was evaluated at α = 0.10 because of small sample sizes.
Results We found hawks nesting throughout Hartford County, not only in city parks, golf courses, and cemeteries but also in the commercial district of downtown Hartford and in densely built urban and suburban neighborhoods, which had a wide range of amounts and sizes of green space patches, road densities, and building densities. Average distance between occupied hawk nests (n = 13 adjacent pairings) was 1.38 ± 0.07 km (range 0.95 to 1.86 km). Eleven of 14 nests that 0.30 Green space patches in home ranges Green space patches used by hawks
G = 178.5, P < 0.01
0.25
Frequency
0.20
0.15
0.10
0.05
0.00 0.05
0.1
0.25
0.5
1
5
10
25
50
100
> 100
Green space patch size (ha)
Fig. 1 Red-tailed Hawks in Hartford, CT, USA used the larger green space patches in their home ranges disproportionate to the availability of different patch sizes
F F
M
M
M
M
F
784
1013
1473
1543
1504
Mean (SE)
M
3
4
2
4
2
5 3
2
26.37 (5.24)
11.42
6.65
33.17
18.67
16.28
9.81 46.27
59.11
0.24 (0.02)
0.13
0.26
0.21
0.22
0.22
0.21 0.38
0.33
0.20
10.18 (2.53)
2.86
1.66
16.58
4.67
8.14
1.96 15.42
29.55
6.29
0.62 (0.07)
0.58
0.50
0.52
0.68
0.99
0.92 0.60
0.86
0.37
0.92 (0.02)
0.83
0.85
0.93
1.00
1.00
0.95 0.94
0.99
0.85
0.49 (0.07)
0.32
0.18
0.45
1.00
0.67
0.56 0.43
0.33
0.26
0.77
4.34 (2.16)
0.92 (0.36)
0.94 (0.49)
2.38 (1.10)
2.53 (1.33)
8.00 (3.82)
1.71 (0.44) 2.16 (0.60)
25.07 (20.18)
0.82 (0.34)
2.63 (1.12)
0.57 (0)
1094 1393
31.45
0.98
0.38
684
5
10.79
0.81
M
0.29
0.56
1172
43.15
14.07
0.30
4
0.14
F
1583
14.07
F
1862
1
Proportion green Proportion usable Proportion of usable green Mean usable green space space in core green space in core space patches in core patch size in core (SE)
Hawk ID Sex Number of Total core area Core area as % of Mean core core areas overall home range area size
Table 3 Core areas (ha) within year round home ranges of breeding adult red-tailed hawks (n = 11) living in the greater Hartford, CT area. ‘Usable’ green space patches are those ≥0.25 ha in size
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Urban Ecosyst Table 4 Variables characterizing green space within year round home ranges and core areas of breeding adult red-tailed hawks (n = 11) living in the greater Hartford, CT area. ‘Usable’ green space patches are those ≥0.25 ha in size Area
Variable
Year Round Home Range Size Proportion of usable green space patches in home range Density of usable green space patches in core area Mean perimeter/area ratio for all green space patches in home range
Coefficient df F-ratio P 175.39
9
12.79
< 0.01
−1.78
9
4.03
0.08
−0.09
9
31.50
0.00
9
2.82
0.10
9
16.59
< 0.01
9
5.33
0.05
Area weighted mean patch size of 2.12 usable green space patches in core area Overall Core Area Size
Mean perimeter/area ratio for all green space patches in home range
−0.03
Area weighted mean patch size of 1.48 usable green space patches in core area
we monitored closely were located in trees, and three were located on manmade structures. Nests were often within 100 m of a road or human structure (Table 1). Over the 4 year period, we radio-tagged 11 adult hawks (6 M and 5F) from 11 different nest sites; all hawks were year-round residents that occupied distinct non-overlapping home ranges (Table 2). We estimated each hawk’s home range with more than 50 locations (Table 2), which increases the reliability of estimates (Kernohan et al. 2001) but combined data for all years because of the small sample size of hawks tagged each year. Overall home range size as defined by the 99 % kernel averaged 107.68 ± 15.17 ha and did not differ between male (113.58 ± 25.02 ha, n = 6) and female hawks (100.60 ± 17.42 ha, n = 5, t9 = 0.43, P = 0.68). Although non-breeding season home ranges were overall larger than breeding season home ranges, these did not differ (t9 = −0.78, P = 0.44). In addition, neither breeding season (t9 = −0.60, P = 0.56) nor non-breeding season (t9 = 0.31, P = 0.76) home ranges differed from year-round hawk home ranges (Table 2). Hawk home ranges contained from 23 % to 89 % green space, and patches of green space within home ranges ranged from 0.01 to 115.2 ha in size (Table 2). Eighty percent of all hawk locations were located within green space patches. Hawks did not use different sized green space patches in proportion to their availability within home ranges (G = 178.5, P < 0.01, Fig. 1). While 84 % of all green space patches identified in hawk home ranges were ≤1 ha, for example small yards associated with multiple-family homes, only 18 % of hawk locations were in those patches. Green space patches 0.25 ha and larger contained 95 % of all hawk locations (Fig. 1). Based on the distribution of hawk locations in green space patches, we considered ‘usable’ patches to be those ≥0.25 ha in size. On average, ~51 % of green space patches in hawk home ranges were considered ‘usable’ (Table 2). Each hawk’s overall home range was one continuous area, but most home ranges had multiple core areas (range 1 to 5 cores) that represented an average of 24 % of overall home range areas (Table 3) and usually incorporated the largest green spaces in that hawk’s home range. The number of cores in a home range was not correlated with home range size but was negatively correlated with mean core area size (r = −0.62, P = 0.04). The total amount of core area within a home range did not differ between male (27.55 ± 7.50 ha, n = 6) and female hawks (24.94 ± 8.11 ha, n = 5, t9 = 0.24, P = 0.81). All nests were located in a core area. Most
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nests (73 %) were located in the largest of multiple core areas and were within the largest or second largest patch of green space within this core area. On average, core areas contained 62 % green space of which 92 % was considered ‘usable.’ An average of 49 % of green space patches in core areas was considered ‘usable’ (Table 3). Associations among various green space metrics and hawk home ranges and core areas were not clear-cut. The GLMs identified the amount and density of usable green space patches in core areas as having an influence on both overall home range size and the overall amount of core area within the home range (Table 4). Hawks with a larger proportion of usable green space in the home range and larger green space patches in the core areas had larger home ranges. Smaller home ranges were associated with greater fragmentation of green spaces (larger mean perimeter/area ratio for all green space patches) and with higher density of usable green space patches in the core areas (Table 4). We found no influence of either road density or building density on home range or core area size. We also did not find an association between squirrel density and any of the green space metrics or home range size. Red-tailed hawks in greater Hartford incubated eggs in March and April, and all fledglings were out of the nest by the end of July. Some nests fledged 3 young each year, and the mean number of fledglings produced across all 4 years was 2.08 ± 0.12. We found no associations between the mean number of young fledged for all nests during our study period and overall home range size, core area size, squirrel density in green spaces associated with respective hawk home ranges, or any of the green space metrics.
Discussion The spatial distribution of breeding raptors in a landscape and the size and configuration of raptor home ranges are influenced primarily by the abundance and distribution of resources in relation to the raptor’s energetic needs (Newton 1979). In urban environments, high environmental heterogeneity and varying levels of human activity and disturbance complicate our understanding of factors that influence the distribution and success of breeding raptors. Redtailed hawks are a generalist, adaptable species that apparently can nest and thrive in many environments including urban areas. Stout et al. (2006a) reported that red-tailed hawks avoided areas with the heaviest urbanization and suggested this was because of insufficient hunting habitat. In Hartford, while we predictably found hawks nesting in the largest green spaces such as urban parks, other hawks nested successfully throughout the city even in the highly built urban center. In downtown Hartford, a pair that nested on a building frequently foraged in a nearby city park containing mature trees suggesting that hawks can persist even in densely built urban areas as long as suitable foraging habitat and perches are present near a suitable nest substrate. Red-tailed hawk home ranges in our study area were smaller than those reported for hawks nesting in rural areas (Fitch et al. 1946, Janes 1984, Smith et al. 2003) and significantly smaller than home ranges expected based on this species’ body size alone (Newton 1979). Other than Petersen (1979), ours is the only study to estimate red-tailed hawk home ranges using radiotelemetry and the first to do so in an urban environment. Petersen (1979) reported the overall average year round home range for 10 hawks nesting in an agricultural landscape as ~158.64 ha. Stout et al. (2006b) indicated that the 314.2 ha area they used to describe redtailed hawk nesting habitat encompassed most of the typical home range of hawks in their study area but they also acknowledged that these areas overlapped significantly. Two other
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studies of red-tailed hawks nesting in urban/suburban areas did not report home range per se but reported nest density. Nest density (0.08 breeding pairs/km2) reported by Minor et al. (1993) was significantly lower than nest density in our study area estimated from mean home range size (~1 breeding pair/km2). Minor et al. (1993) suggested this apparently low nest density resulted from their inclusion within their study area of large parts of the heavily urbanized areas of the city, which they assumed to lack suitable green space for foraging and nesting. Runyan (1987) reported a nest density of 0.28 pairs/km2 having arbitrarily excluded areas within his total study area that he assumed did not contain suitable nesting habitat. Thorough searching of our study area revealed occupied hawk nests throughout the urban landscape, suggesting that habitat in this highly urbanized area is sufficient to support nesting red-tailed hawks at a higher density compared to rural areas. For raptors, home range placement and size is assumed to be influenced by habitat quality, often defined by the diversity and amounts of certain habitats within the home range (Howell et al. 1978, Smith et al. 2003, Stout et al. 2006b). Additionally, prey abundance and availability are often considered components of habitat quality (Byholm and Kekkonen 2008, Rooney et al. 2015). We initially assumed that urban green spaces would contain abundant squirrels, thus these green spaces, especially larger ones, would constitute an important component of urban hawks’ home ranges. Counter to our prediction, however, higher densities of squirrels were not associated with larger green spaces suggesting that other factors affect squirrel and, likely, other prey populations in urban green spaces, for example, the types, sizes, and density of trees, the availability of natural and human subsidized food resources, or the presence of predators such as dogs and cats in neighborhoods (Parker and Nilon 2008). Our findings do suggest, however, that green space, including what we defined as ‘usable’ green space seems to be an important component of hawk home ranges in Hartford, because we found an association between hawk home range size and the proportion of usable green space in the home range. However, the finding that smaller home ranges were associated with greater fragmentation of green spaces was counter to our predictions. These conflicting results suggest that the value of green space within hawk home ranges is likely influenced by more than just patch size or density. Urban landscapes are typically characterized by highly fragmented and often small green spaces, and habitat fragmentation is well known to affect many avian species, including raptors (Carrete et al. 2009). Yet what constitutes ‘usable’ green space for red-tailed hawks nesting in urban landscapes likely includes other characteristics of these green spaces such as the amount and types of habitats within them, or in the number and types of mature trees that can serve as perches for hawks and as food sources and escape routes for prey such as birds and squirrels. Our results suggest that even some of the smaller green spaces in our study area may be valuable to hawks particularly if they are used by other common prey species such as rodents, house sparrows, and pigeons, all of which are abundant in highly built urban neighborhoods. Some small green spaces in our study area had few trees within them, yet hawks used these small spaces if they were adjacent to tall buildings, trees, or utility poles that could be used as perches. In contrast, some of the larger urban parks in Hartford have been mostly cleared of trees for development of ball fields, so despite their large size, these green spaces may not be as useful for urban hawks perhaps due to fewer prey but probably also due to high levels of human activity. Core areas in raptor home ranges typically include the nest and the most important foraging areas (Kennedy et al. 1994, Hodder et al. 1998). In our study area larger core areas within hawk home ranges were associated with larger patches of usable green space; these may be areas that are important for fledglings to hunt (Kennedy et al. 1994). Some core areas in hawk
Urban Ecosyst
home ranges contained no green space but included tall buildings or rooftops that were favorite perch sites for the vigilant resident hawk. In some cases the core area that included the nest did not necessarily include a foraging area; for example, the nest was on a building or in a single tree on the street of a densely built neighborhood. The absence of association between home range size and many variables that described size and configuration of green spaces suggests that placement and size of hawk home ranges in the urban environment of Hartford is influenced by a much more complex set of factors than those we evaluated. Our results, corroborated by field observations of territorial behaviors such as aerial chases and displays, suggest that territoriality and intraspecific competition may also play an important role in home range size and dispersion of red-tailed hawks nesting in Hartford. These hawks maintain exclusive territories in rural areas, are highly territorial during the breeding season, and territories remain remarkably stable from year to year (Janes 1984, Moorman et al. 1999), a finding corroborated by our telemetry data. Inter-nest distances throughout our study area were much closer than inter-nest distances reported for this species in other studies (compiled in Rothfels and Lein 1983, Stout et al. 2006b), yet hawk nests were spaced fairly regularly, indicating high territoriality. Within a raptor community, members of the same species often competitively exclude one another from their home ranges suggesting that intraspecific competition may drive placement of individual raptor nests and home ranges (Schmutz et al. 1980, Janes 1984, Bosakowski et al. 1996, Sergio and Penteriani 2005). Home ranges of hawks nesting in Hartford showed essentially no overlap and changed little between the breeding season and non-breeding season, suggesting that an important factor driving dispersion of nesting hawks in our study area may be individual pairs’ tolerance of the presence of other nesting hawks. Hawks remained on their home ranges year round perhaps because sufficient food was available but also, perhaps, to defend these areas against takeover by other hawks. Ultimately, in an environment with sufficient food, perches and suitable nest sites that can support a high density of nesting hawks, the presence of other hawks may be a relatively strong influence on the dispersion of hawk nests and on home range size. In urban environments, the landscape diversity provided by the intermixture of green space patches including areas of wooded and grassy habitats, roads, buildings, and other urban structures apparently offers a variety of suitable nesting sites and foraging habitats for redtailed hawks. In urban areas, perch sites abound, and populations of the hawk’s prey species can be high (Parker and Nilon 2008, Mannan and Boal 2000). Common prey species including songbirds, squirrels, and rats often can be found throughout Hartford’s urban neighborhoods as well as in urban parks and other green spaces. As generalists, hawks may take advantage of any abundant or available prey in their home range, and these prey may not necessarily be associated with green spaces. For example, one hawk in our study area that nested in a densely-built urban neighborhood had many small,
