Abstract
Space use information can be integral in the creation of effective conservation and management strategies. However, avian territories (defended areas) are studied far more frequently than home ranges (entire use areas), and few studies have compared the two. This is the case for the cerulean warbler Setophaga cerulea, a declining Neotropical migrant songbird. There is an extensive record of cerulean warbler territory estimates, while the home range has only recently been explored. Studies of these space use areas differ in their sampling and estimation techniques and location. Consequently, comparison of both space use areas is difficult. We used radio telemetry to delineate cerulean warbler diurnal space use areas in southern Indiana. Our primary objective was to describe the relationship between the home range and territory. Kernel density home range estimates of sampled adult male cerulean warblers (n = 14, mean ± SE = 2.33 ± 0.29 ha) were significantly larger (0.54 ± 0.18 ha, P = 0.006) than territory estimates of the same group of individuals (n = 14, mean ± SE = 1.79 ± 0.39 ha; P = 0.006). Minimum convex polygon home range estimates of the same group of birds (n = 14, mean ± SE = 3.45 ± 0.55 ha) were also significantly larger (1.38 ± 0.19 ha, P < 0.001) than territory estimates (n = 14, mean ± SE = 2.07 ± 0.58 ha). Additionally, the territory estimates that we report are considerably larger than other published estimates for this species, which were delineated using spot mapping methods. Cerulean warbler home ranges contain territorial and extraterritorial space, and the latter has not been studied in detail. Area and habitat requirements likely vary throughout this species' range, and regional conservation management might benefit from study in peripheral space use areas.
Introduction
Boundaries of avian space use are often categorized into two major classifications, the home range and territory (Whitaker and Warkentin 2010). The home range is the entire area used by an animal in its regular activities of acquiring food and mates and caring for offspring (Burt 1943), and the territory, a subset of the home range, is “any defended area” (Noble 1939). Breeding territories are the principal location for most home range activities, and they are generally defended from conspecifics through vocalization (Nice 1941). Territory size can vary greatly among individuals of the same species depending on conspecific density, habitat characteristics, resource distribution, and other factors (Wiens et al. 1985; Smith and Shugart 1987; Flockhart et al. 2016). The significance of avian home ranges has not been studied in detail relative to avian territories, which are often assumed to represent a bird's entire space use area (Whitaker and Warkentin 2010). Studies that have examined the relationship between home ranges and territories in birds showed that home ranges tend to be larger and contain different habitat characteristics (Naguib et al. 2001; Anich et al. 2009; Streby et al. 2012; Tomasevic and Marzluff 2018). Extraterritorial movements may provide additional opportunities to forage, mate, and investigate neighboring habitats (Stutchbury 1998; Naguib et al. 2001; Norris and Stutchbury 2001; Celis-Murillo et al. 2017). If this extraterritorial space use increases breeding success or individual fitness, home ranges should also be used to approximate space use requirements for a species.
Territories are commonly delineated using the spot-mapping method. This method typically involves recording the locations of a singing individual, often over multiple outings, and using those locations to generate a map of the territory (Bibby et al. 2000). Although spot mapping has been the standard for mapping breeding territories, it can vastly underestimate territory size (Streby et al. 2012). Radio telemetry, however, is subject to less observer bias and may provide more accurate data that are not influenced by prior assumptions of a species' space or habitat use (Anich et al. 2009; Streby et al. 2012).
A home range contains territorial and extraterritorial space (area outside the defended territory), and it has the potential to be significantly larger than the defended territory. Spot mapping, which generally records locations of territorial behavior (e.g., song), is less useful when demarcating extraterritorial space in the home range. Rather, avian home ranges are measured using other techniques, including radio telemetry, mist netting, song playback, and direct observation (Falls 1981; Ferry et al. 1981; Samuel et al. 1985; Naguib et al. 2001; Anich et al. 2009; Carpenter and Wang 2018; Tomasevic and Marzluff 2018). Relatively few studies have compared avian home ranges and territories, and in those that have, the territory was significantly smaller than the home range (range = 8–70%; Ferry et al. 1981; Naguib et al. 2001; Bas et al. 2005; Anich et al. 2009; Tomasevic and Marzluff 2018).
In Indiana and elsewhere throughout its breeding distribution, researchers study and use silvicultural strategies as methods for managing cerulean warbler Setophaga cerulea habitat (Hamel and Rosenberg 2007; Boves et. al 2013; Islam et al. 2013; Wood et al. 2013; Buehler et al. 2020). Space use information is critical when assessing the effects of forest management on this species and when identifying microhabitat characteristics and how they influence fitness (Boves et al. 2013; Kaminski and Islam 2013; Nemes and Islam 2017; Wessels and Boves 2021). Habitat characteristics for this species are typically determined within a delineated area, which is often the defended territory (Perkins and Wood 2014; Nemes and Islam 2017; Wessels and Boves 2021). The space use area that is delineated (e.g., territory or home range) and the methods used to delineate and estimate the area (e.g., spot mapping or radio telemetry and minimum convex polygon [MCP] or kernel density estimate [KDE]) may influence the determined habitat. Cerulean warbler breeding territory size estimations are known in many locations throughout its range. The average area of territories derived from MCPs (∼0.2–1.0 ha) (Kaminski and Islam 2013; Buehler et al. 2020) and KDEs (∼0.3–1.1 ha; Barg et al. 2005; Perkins and Wood 2014; Wessels and Boves 2021) varies regionally (Nemes and Islam 2017; Buehler et al. 2020). To our knowledge, no one has published information on cerulean warbler territory estimates based off radio telemetry locations; however, advancements in technology allow researchers to fasten radio transmitters to small birds, including the cerulean warbler (Carpenter and Wang 2018; Delancey and Islam 2019). Carpenter and Wang (2018) used this technology to generate the only published estimates of cerulean warbler home ranges (mean of 6.7 ± 0.7 ha).
Our present knowledge of cerulean warbler habitat and space use is based off studies that vary considerably in methodology, and none compare the territory and home range. Other studies highlighted the differences between avian home ranges and territories (Anich et al. 2009; Streby et al. 2012; Tomasevic and Marzluff 2018). A similar understanding of cerulean warbler spatial use can benefit this species' management by providing context for known cerulean warbler habitat requirements and highlight areas that may need further study. Here, we sought to determine estimations of, and the spatial relationship between, cerulean warbler home range and territory in southern Indiana. We used radio telemetry to create KDEs of cerulean warbler territories and home ranges to test the hypothesis that cerulean warblers use a home range larger than the defended territory. We discuss the intensity of use within these space use areas and the potential influence of sampling and estimation methods.
Study Site
We conducted our research in study units of the Hardwood Ecosystem Experiment (hereafter HEE; Figure 1) during the 2018 and 2019 breeding seasons (May to mid-July). The HEE is a 100-y study that is taking place in Yellowwood and Morgan-Monroe state forests in southern Indiana, which have dissected uplands with steep slopes, ephemeral streams, and bottomland drainages (Homoya et al. 1985). Initiated in 2006, the HEE is a collaborative, multidisciplinary project examining impacts of several forest management practices on a variety of floral and faunal taxonomic groups and communities. There are nine core study units (range = 78–110 ha) that are surrounded by buffer areas that separate core study units from other state forest management activities. Buffers include the management units and range in size from 303 to 483 ha (Kalb and Mycroft 2013). Three units are undergoing uneven-aged management, three units are undergoing even-aged management, and the remaining three units are not managed (control; Swihart et al. 2013). We conducted our research on cerulean warblers in nine 225-ha study units that overlay the core research units (Islam et al. 2013).
We conducted research in nine 225-ha cerulean warbler Setophaga cerulea research units (blue squares) in Yellowwood (light green) and Morgan-Monroe (dark green) state forests in Brown, Morgan, and Monroe counties, Indiana, from May to July 2018 and 2019. The research units are part of the Hardwood Ecosystem Experiment (HEE; red star), a long-term study on the impacts of forest management on plant and animal communities of a central hardwood forest.
We conducted research in nine 225-ha cerulean warbler Setophaga cerulea research units (blue squares) in Yellowwood (light green) and Morgan-Monroe (dark green) state forests in Brown, Morgan, and Monroe counties, Indiana, from May to July 2018 and 2019. The research units are part of the Hardwood Ecosystem Experiment (HEE; red star), a long-term study on the impacts of forest management on plant and animal communities of a central hardwood forest.
Methods
Study species
The cerulean warbler breeds throughout the eastern and central United States and southern Ontario, Canada. During the breeding season, cerulean warblers are most strongly associated with mature deciduous forests that occur in wet bottomlands or upper mesic slopes and ridgetops (Buehler et al. 2020). The cerulean warbler is one of the fastest declining Nearctic-Neotropical migrant songbirds, despite being once common throughout its breeding distribution (Buehler et al. 2008). Breeding Bird Survey data depict an approximately 3% per year rate of cerulean warbler population decline in Indiana from 1966 to 2019 (Sauer et al. 2020). Habitat loss at the wintering and breeding grounds may be the primary cause for this decline (Robbins et al. 1992; Buehler et al. 2020). Cerulean warblers are a state endangered species in Indiana (Indiana Department of Natural Resources 2020) and a national species of Conservation Concern by the U.S. Fish and Wildlife Service (USFWS 2021), and it is considered near threatened throughout its range (BirdLife International 2020).
Selection of study subjects
We conducted point counts during mid- to late May of the 2018 and 2019 breeding seasons to locate cerulean warblers as part of a long-term study of this species in the HEE (Islam et al. 2013). Survey points were 200 m apart along a 7 × 7 grid (49 points total), covering the entirety of each cerulean warbler research unit. During and following point count surveys, we identified adult male cerulean warblers that appeared to be paired and on established territories. We chose paired adult males with established territories because they are more likely to maintain a territory. Adult male cerulean warblers are distinguishable by their cerulean blue plumage, which is not shared with female or juvenile birds (Buehler et al. 2020). We used behavioral cues (e.g., singing, interactions with a female, and observed nesting behaviors) that we observed over multiple visits, if necessary, to determine if a male was paired and on a territory.
Attachment of radio transmitters
We targeted all adult males in the study units by luring birds toward strategically placed mist nets using playbacks of conspecific calls, songs, or recordings of songbirds mobbing an Eastern screech owl Megascops asio. We banded each bird with a U.S. Geological Survey aluminum band and a year-specific color band (green in 2018 and yellow in 2019) on the left leg and a unique combination of two color bands to identify the individual on the right leg. We affixed a 0.33-g (<4% average body mass, ∼9.0 g) radio transmitter (Blackburn Transmitters, Nacogdoches, TX) to paired males chosen for the study using the attachment method described by Rappole and Tipton (1991) and modified by Streby et al. (2015).
Radio telemetry tracking regime
We tracked male cerulean warblers fitted with a radio transmitter and recorded locations and data over 6 d of the lifespan of the radio transmitter (<21 d). We used a three-element folding Yagi antenna connected to a TRX-1000 telemetry receiver (Wildlife Materials, Inc.) to track our subjects using the homing method (White and Garrott 1990). Cerulean warblers spend much of their time high in the canopy (Buehler et al. 2020), and if we were unable to obtain visual confirmation, we circled the area and obtained readings until we were confident of the bird's location. We tracked individuals following a burst sampling design, which other researchers used to study cerulean warbler space use (Barg et al. 2005; Wood and Perkins 2012; Carpenter and Wang 2018; Wessels and Boves 2021). Burst sampling occurred for 60 min, and each burst was separated by an interval of ≥1 d. We began each session when a bird was located, and we recorded locations every 3 min (or when we located the bird after 3 min). We opted for 3-min intervals rather than 1-min intervals that others used to study this species (Barg et al. 2005; Wood and Perkins 2012; Perkins and Wood 2014; Wessels and Boves 2021). We hypothesized that home ranges and the use of radio transmitters would result in a larger space use area, and we wanted to provide additional time for the birds to move about the home range. These locations conformed to the principle of biological independence, where sampling intervals need only be long enough for an animal to travel from any point in their home range to any other point within their home range (Lair 1987; Barg et al. 2005). Biological independence differs in concept from the traditional statistical independence, which is determined empirically (Swihart and Slade 1985; Swihart and Slade 1997). We assert that biological independence is appropriate for the study of cerulean warblers, which are highly mobile animals (Buehler et al. 2020). Our sampling regime provided sufficient time for an individual to travel across its entire home range or territory between each successive sampling point. In addition, other authors determined that biological independence is more relevant than statistical independence, especially when creating utilization distributions that determine relative space use (de Solla et al. 1999).
To record individual locations, we used a handheld global positioning system (Garmin GPSmap 62s and Garmin GPSmap 64s) with the appropriate universal transverse mercator projection (UTM 16N NAD83). We recorded locations using the waypoint averaging function of the Garmin unit to minimize positional error. Speaking into a digital voice recorder, we recorded if the bird sang, the type of vocalization (quiet song, short song, or chip), whether we confirmed the observation visually or audibly, the species of tree where we observed the bird, a visually estimated height of the tree, a visually estimated height of the bird in the tree (if possible), and any additional notes, which we later transcribed into an Excel spreadsheet. We used all location points to determine the individual's home range and the subset of points where the male sang to determine the individual's territory.
For each cerulean warbler, we conducted six tracking sessions over the lifespan of the radio transmitter (<21 d). We conducted the sessions at different times to avoid any biases associated with time of day (e.g., singing rate changes throughout the day). We recorded two tracking sessions during each of the following time periods: 0600–0900, 0900–1200, and 1200–1500 hours. We refer to the time periods as “1,” “2,” and “3,” respectively. We randomized the order of each radio telemetry session using a random number generator, but we sampled each unique time period before repeating a time period (i.e., we sampled time periods 1, 2, and 3 once before repeating any period). If we knew the nesting status of the tracked cerulean warbler, we attempted to track the individual three times during the incubation stage and three times during the nesting stage. If we did not know the nesting stage, then the individual was still tracked, and we focused our nest searching efforts to find the nest of that pair. We did not track any birds in multiple years.
Space use estimation
We combined locational data from the global positioning system and observational data from Excel in ArcGIS Pro (ESRI) and constructed 95% isopleth fixed KDEs of home ranges and territories using the package adehabitatHR (Calenge 2006) in R statistical software version 4.1.3 (R Core Team 2022). Kernel density estimators are a probability density function and are the preferred means of estimating space use of birds (Figure 2; Marzluff et al. 2004; Barg et al. 2005; Anich et al. 2009; Whitaker and Warkentin 2010; Flockhart et al. 2016; Tomasevic and Marzluff 2018). We calculated the kernel smoothing parameters using least squares cross-validation, which tests multiple smoothing parameters against the data and selects the one with the lowest estimated error for the KDE (Worton 1989; Seaman and Powell 1996). We generated estimates for all birds that we sampled completely (six tracking sessions) and that had >15 territory locations. This sample size is sufficient to create reasonable KDEs (Said et al. 2005; Börger et al. 2006; Anich et al. 2009; Tomasevic and Marzluff 2018); however, others suggested a minimum of 30 locations (Seaman et al. 1999). We also constructed MCPs of the territories and home ranges for easier comparison to published cerulean warbler MCP space use estimates. MCPs are simple in concept, but they tend to include areas of unused space and are greatly influenced by sample size and peripheral points (Barg et al. 2005; Anich et al. 2009). For each individual, we determined the areas of the space use estimates and calculated the two-dimensional overlap of the territory and home range by dividing the territory area by the home range area. We were also interested in examining the intensity of use within the territory versus extraterritorial space. To accomplish this, we generated core area estimates (50% isopleth fixed KDE; Wood and Perkins 2012) using each bird's home range locations (Figure 2). Next, we used the Intersect tool in ArcGIS Pro to isolate home range locations and core area that were within each bird's territory and calculated the percentage of home range points and core area within each territory.
Utilization distribution for an adult male cerulean warbler Setophaga cerulea that we tracked from 10 to 16 June 2019 in Yellowwood State Forest, Brown County, Indiana. The outer border represents the 95% isopleth of a fixed kernel density estimate that we generated from home range locations (“o” symbol). Inner isopleths range from 10 to 90%, increasing incrementally by 10%. Core areas are within the 50% isopleth (red) of the home range estimate.
Utilization distribution for an adult male cerulean warbler Setophaga cerulea that we tracked from 10 to 16 June 2019 in Yellowwood State Forest, Brown County, Indiana. The outer border represents the 95% isopleth of a fixed kernel density estimate that we generated from home range locations (“o” symbol). Inner isopleths range from 10 to 90%, increasing incrementally by 10%. Core areas are within the 50% isopleth (red) of the home range estimate.
Statistical analysis
We used the Shapiro–Wilk test to assess whether the data violated the assumption of normality (α = 0.05). Differences between territory and home range estimates were normally distributed; consequently, we determined if the mean areas of the territories and home ranges were significantly different using a one-tailed paired t-test (P < 0.05), which can be used for within-subjects study designs. We used a one-tailed paired t-test because we expect that home ranges are larger than territories. We reported all space use estimates as mean ± standard error (SE).
Results
Radio telemetry tracking
We obtained location points from 20 marked male cerulean warblers (n = 4 in 2018 and n = 16 in 2019) in HEE study units over the course of the study. We confirmed warbler locations at 51% of the recorded points (35% aurally and 16% visually), and we recorded the remaining points when we were confident of the bird's location based on the radio transmitter signal. Of the 20 marked birds, 14 (n = 2 in 2018 and n = 12 in 2019) had six completed tracking sessions and met the designated minimum requirement of 15 singing points (mean = 41 and range = 17–66; Table 1; Data S1, Supplemental Material). We did not completely track six birds because their territory dissolved following the fledging of young or a nest failure. The mean number of total home range locations per bird was 77 (range = 60–97; Table 1; Data S1, Supplemental Material). We tracked the 14 birds over an average 11-d period (range = 6–20 d).
Year tracked, number of points, 95% fixed kernel density territory and home range estimates (KDEs), overlap of territory and home range KDEs, minimum convex polygon (MCP) territory and home range estimates, and overlap of territory and home range MCPs of 14 cerulean warblers Setophaga cerulea that we tracked in 2018 and 2019 in the Hardwood Ecosystem Experiment in Morgan, Monroe, and Brown counties, Indiana.
Cerulean warbler space use
We generated fixed KDEs and MCPs of the home ranges (KDE = 2.33 ± 0.29 ha, range = 1.02–4.85 ha; MCP = 3.45 ± 0.55 ha, range = 1.23–8.92 ha) and territories (KDE = 1.79 ± 0.39 ha, range = 0.22–5.09 ha; MCP = 2.07 ± 0.58 ha, range = 0.46–8.74 ha) of 14 tracked cerulean warblers (Table 1; Data S2, Supplemental Material). One-tailed, paired t-tests showed that home ranges were significantly larger than their respective territories (KDE = 0.54 ± 0.18 ha, t = 2.95, df = 13, P = 0.006; MCP = 1.38 ± 0.19 ha, t = 7.28, df = 13, P < 0.001; Figure 3). On average, territory KDEs constituted 70% (range = 15–132%) of the individual's home range, and territory MCPs constituted 52% (range = 19–98%) of the home range (Figures 4; Table 1; Figures S1–S12, Supplemental Material). Two birds had territory KDEs that were greater than their respective home range KDEs (Figures S4 and S9, Supplemental Material). We generated territory kernel estimates from fewer locations than home range estimates, and kernel estimates tend to increase with a smaller sample size. Consequently, we investigated whether the smaller sample size used for territory estimates influenced our determined territory–home range relationship. Simple linear regression did not indicate that the territory–home range relationship was predicted by the number of singing locations (slope = 0.352 ± 0.507, adjusted R2 = 0.04, P = 0.500). Core areas of the home range (0.54 ± 0.08 ha, range = 0.25–1.27 ha) were largely contained within the territory (92 ± 4%, range = 46–100%), including six birds' core areas that were entirely within their territories. Most cerulean warbler home range locations (82 ± 4%, range = 45–96%) were also contained within the territory.
Box plot of 95% fixed kernel density estimates (KDEs) and minimum convex polygons (MCPs) of cerulean warbler Setophaga cerulea space use areas (n = 14) that were generated from locations recorded from May to July 2018 and 2019 in Yellowwood State Forest, Brown County, Indiana. KDE home ranges (2.33 ± 0.29 ha) were significantly larger than KDEs of the same individuals' territories (1.79 ± 0.39 ha; t = 2.95, df = 13, P = 0.006, one-tailed paired t-test). MCP home ranges (3.45 ± 0.55 ha) were also significantly larger than MCPs of the same individuals' territories (2.07 ± 0.58 ha; t = 7.28, df = 13, P < 0.001, one-tailed paired t-test).
Box plot of 95% fixed kernel density estimates (KDEs) and minimum convex polygons (MCPs) of cerulean warbler Setophaga cerulea space use areas (n = 14) that were generated from locations recorded from May to July 2018 and 2019 in Yellowwood State Forest, Brown County, Indiana. KDE home ranges (2.33 ± 0.29 ha) were significantly larger than KDEs of the same individuals' territories (1.79 ± 0.39 ha; t = 2.95, df = 13, P = 0.006, one-tailed paired t-test). MCP home ranges (3.45 ± 0.55 ha) were also significantly larger than MCPs of the same individuals' territories (2.07 ± 0.58 ha; t = 7.28, df = 13, P < 0.001, one-tailed paired t-test).
Territory and home range estimates of adult male cerulean warblers Setophaga cerulea tracked in a research unit of the Hardwood Ecosystem Experiment in Yellowwood State Forest, Brown County, Indiana. (A) A 95% fixed kernel density estimate of the territory (hatched) was 1.16 ha, 55% of the corresponding home range estimate (2.10 ha; light blue). A minimum convex polygon of the territory (dashed line boundary) was 1.28 ha, 56% of the corresponding home range polygon (2.28 ha; solid line boundary). We generated the home range using all location points (74; “o” symbol), and we generated the territory using location points where we observed the bird singing (40; “+” symbol). We tracked this individual (bird ID: Ope) using radio telemetry from 10 to 16 June 2019 and observed that this individual had a nest (yellow star). (B) A 95% fixed kernel density estimate of the territory (light blue) was 2.14 ha, 58% of the corresponding home range estimate (3.71 ha; light blue). A minimum convex polygon of the territory (dashed line boundary) was 3.58 ha, 65% of the corresponding home range polygon (5.50 ha; solid line boundary). We generated the home range using all location points (77; “o” symbol), and we generated the territory using location points where we observed the bird singing (38; “+” symbol). This bird appeared to have two simultaneous mates and exhibited polyterritorial behavior. We tracked this individual (bird ID: Emu) using radio telemetry from 8 to 18 June 2019.
Territory and home range estimates of adult male cerulean warblers Setophaga cerulea tracked in a research unit of the Hardwood Ecosystem Experiment in Yellowwood State Forest, Brown County, Indiana. (A) A 95% fixed kernel density estimate of the territory (hatched) was 1.16 ha, 55% of the corresponding home range estimate (2.10 ha; light blue). A minimum convex polygon of the territory (dashed line boundary) was 1.28 ha, 56% of the corresponding home range polygon (2.28 ha; solid line boundary). We generated the home range using all location points (74; “o” symbol), and we generated the territory using location points where we observed the bird singing (40; “+” symbol). We tracked this individual (bird ID: Ope) using radio telemetry from 10 to 16 June 2019 and observed that this individual had a nest (yellow star). (B) A 95% fixed kernel density estimate of the territory (light blue) was 2.14 ha, 58% of the corresponding home range estimate (3.71 ha; light blue). A minimum convex polygon of the territory (dashed line boundary) was 3.58 ha, 65% of the corresponding home range polygon (5.50 ha; solid line boundary). We generated the home range using all location points (77; “o” symbol), and we generated the territory using location points where we observed the bird singing (38; “+” symbol). This bird appeared to have two simultaneous mates and exhibited polyterritorial behavior. We tracked this individual (bird ID: Emu) using radio telemetry from 8 to 18 June 2019.
Male cerulean warblers typically defended a single territory, but several individuals appeared to defend multiple, disjointed territories (Figure 4B; Figures S3 and S4, Supplemental Material). One of these males (Figure 4B) maintained territories around two concurrent nests that were 275 m apart. This individual was observed mate-feeding female cerulean warblers at both nests and was the only male at either location.
Discussion
Previous studies estimated cerulean warbler home range size (Carpenter and Wang 2018) and territory size (Barg et al. 2005; Wood and Perkins 2012; Kaminski and Islam 2013; Perkins and Wood 2014; Wagner et al. 2015; Buehler et al. 2020; Wessels and Boves 2021), but the relationship between the two has not been examined. Our findings of a home range that extends beyond a defended territory substantiates similar conclusions for other species (Ferry et al. 1981; Naguib et al. 2001; Bas et al. 2005; Anich et al. 2009; Tomasevic and Marzluff 2018). Cerulean warblers are highly mobile, and their extraterritorial movements were likely motivated by a variety of purposes. We frequently observed tracked birds foraging throughout their home range, but during some of these extraterritorial movements, the tracked individual bypassed preferred foraging trees and appeared to have other motivations. For instance, we caught and banded two of the males near neighboring pairs' nests after investigating the sound of a “new neighbor” (our cerulean warbler playback recordings). Cerulean warblers likely use extraterritorial movements to acquire information on neighboring conspecifics and habitat, which Betts et al. (2008) suggested for other species. Similarly, opportunities to engage in extrapair copulations are another motivation for extraterritorial movements (Stutchbury 1998; Whitaker and Warkentin 2010), and cerulean warblers engage in polygyny (Boves and Buehler 2012; this study). Despite opportunities that extraterritorial movements may provide, the majority of cerulean warbler activity was within the defended territory. There are potential costs to extraterritorial movement, such as the intrusion of neighbors and cuckoldry (Whitaker and Warkentin 2010). Extrapair mating may be somewhat common, as it is in some congeneric species (e.g., American redstart Setophaga ruticilla; Kappes et al. 2009), but it has not been studied in detail in cerulean warblers.
On average, territories were 70% (KDE) of the size of the total home range. Interestingly, Anich et al. (2009) and Tomasevic and Marzluff (2018) found an identical relationship between the territories and home ranges of their focal species while using similar methodologies (radio telemetry and kernel density estimators). The same relationship within three species (cerulean warbler, Swainson's warbler Limnothlypis swainsonii, and pileated woodpecker Dryocopus pileatus) with varied life histories suggests that this may also be a common relationship across other birds. Tomasevic and Marzluff (2018) suggested that the similar ratio across taxa might represent comparable costs and benefits of using extraterritorial space. However, this study (Table 1) and Anich et al. (2009) reported great variation in territory and home range overlap among individuals, which may be influenced by many factors. Cerulean warblers often exhibit clustered territoriality (Roth and Islam 2007), and high species density may compress individual territories but have less of an influence on home ranges, resulting in less territory–home range overlap (Anich et al. 2009). There can also be considerable variation in the distance and duration of extraterritorial movements among and within species (Whitaker and Warkentin 2010). Some variation may have arisen from analytical methods; two cerulean warbler territory KDEs were greater than their corresponding home ranges (Figures S4 and S9, Supplemental Material), and Anich et al. (2009) reported similar instances of larger Swainson's warbler territory kernel estimates. Further comparisons with other species may help determine whether the similarity among these findings is more than coincidental.
Ford (1996) documented polyterritoriality in many passerines, including several species closely related to the cerulean warbler. To our knowledge, we report the first instance of a polyterritorial cerulean warbler. We observed an apparent polygynous male that fed two different females at concurrent nests that were 275 m apart. Around each of these nests, the male held a fairly distinct territory (Figure 4B). Furthermore, this male was the only male that we observed around either nest. KDEs of two other males (Figures S3 and S4, Supplemental Material) appeared similarly polyterritorial, but we did not observe those birds engaging in polygynous behavior. The latter two individuals were in study units with lower cerulean warbler density (Islam et al. 2013), which may have permitted them access to more distant patches of preferred habitat.
It is important to note that these estimates represent space use during a relatively short period of time and for a particular demographic (paired male birds). The average home range KDE was 35% of the size of the only previously published estimate from a single study on cerulean warblers in Alabama (6.7 ± 0.7 ha, range = 3.8–10.4 ha; Carpenter and Wang 2018). This difference may highlight changes in space use over a period of time. We tracked cerulean warblers over an average of 11 d (range = 6–20 d), substantially shorter than Carpenter and Wang (2018), who tracked birds over an average of 28 d (range = 18–59 d). Space use areas can shift throughout the breeding season as a result of changes in space use determinants (e.g., location of nest or resources), resulting in a larger estimate (Whitaker and Warkentin 2010). Furthermore, following nesting, cerulean warblers use different areas and habitat types while caring for fledglings (Delancey and Islam 2019; Raybuck et al. 2020).
Territory estimates can be influenced by methodology, such as choice of estimator (Barg et al. 2005; Anich et al. 2009) and sampling technique (Streby et al. 2012), or by biological factors, including resource availability (Wagner et al. 2015), population density (Brown and Orians 1970), and microhabitat (Anich et al. 2012). Territory KDEs in this study were considerably larger than estimates reported by others who used spot mapping and burst sampling regimes to create fixed KDEs of cerulean warbler territories (0.70 ± 0.16 ha, Barg et al. 2005; 0.34 ± 0.06 ha, Perkins and Wood 2014; 1.79 ± 0.39 ha, Wessels and Boves 2021). Several other researchers generated MCPs of cerulean warbler territories using spot-mapping methods (1.04 ± 0.1 ha, Oliarnyk and Robertson 1996; 0.96 ± 0.18 ha, Barg et al. 2005; 0.9 ± 0.1 ha, Robbins et al. 2009; <0.34 ± 0.32 ha, Kaminski and Islam 2013; 0.21 ± 0.03 ha, Wagner et al. 2015; 0.28 ha, Nemes and Islam 2017; 0.45 ± 0.05 ha, Buehler et al. 2020), all of which were also less than territory MCP estimates that we observed here (2.07 ± 0.58 ha). Regional differences, such as varying population densities or environmental characteristics, may be responsible for some of the disparities between our estimates and other published territory estimates. However, other researchers demonstrated that spot mapping can underestimate space use areas. Streby et al. (2012) found that golden-winged warbler Vermivora chrysoptera territories, which were delineated using radio telemetry, were three times larger than the same territories that were concurrently delineated using spot-mapping methods. Similarly, Anich et al. (2009) reported higher territory estimates for Swainson's warblers than had previously been described using spot mapping. Several researchers used spot-mapped territories to study cerulean warbler biology within southern Indiana (Jones and Islam 2006; Roth and Islam 2007; Kaminski and Islam 2013; Wagner et al. 2015; Nemes and Islam 2017). Kaminski and Islam (2013) examined the effect of timber management on territory size and characteristics and found that territory sizes were unaffected. However, territory sizes were considerably smaller than those that we observed here and were possibly underestimated. Studies where the size of the territory is an important variable would benefit from radio telemetry, which provides more exact determinations of their study subject's space use.
Although we did not explicitly examine habitat use, study design can influence determinations of habitat characteristics. Radio telemetry forces observers to abandon assumptions about space use and can reveal previously unknown habitat types used by their subject (Streby et al. 2012). We observed cerulean warblers using ∼10-y-old patch cuts, albeit infrequently, and Delancey and Islam (2019) radio tracked adult and fledgling cerulean warblers into HEE patch cuts and clear cuts in previous years. Cerulean warblers seldom forage in the understory and are most strongly associated with mature forest canopies (Wood and Perkins 2012). However, this species' attraction to anthropogenic and natural gaps in forest canopy is well documented (Oliarnyk and Robertson 1996; Boves et al. 2013; Farwell et al. 2019). The radio-tracked birds appeared to use the patch cuts sparingly for additional foraging opportunities and remained near the patch cut's edge; however, the availability of these habitats may increase in importance during the fledgling stage. We frequently tracked birds to locations that were out of audible range from their previous location, and they would have likely been missed had we been spot mapping. Cerulean warblers spend most of their time in the forest canopy and can be difficult to observe. Consequently, at half of the recorded locations, we neither heard nor saw the radio-tracked bird and would not have been able to determine its location without radio telemetry. The advent of small radio transmitters has proven very beneficial for the study of secretive species like the cerulean warbler. Choice of estimation technique, specifically KDE or MCP, can also affect determinations of habitat characteristics. Many of the MCPs that we generated included large areas where we never recorded our subjects, and some of these areas were likely unused because of their habitat characteristics. In the field, we often recorded cerulean warblers around or traversing distinctly different habitats (e.g., patch cuts, clear cuts, and bottomland stand of eastern white pine Pinus strobus) but rarely within those habitats. Due to their inclusion of unused space, we believe that the MCPs that we generated were more likely to include larger areas of these habitats than their corresponding KDEs, which more accurately reflected our field observations. Radio telemetry, in conjunction with KDE, allows researchers to produce detailed patterns of spatial behavior that excludes unused habitat.
Likely owing to its practicality, many have considered the territory rather than home range when examining space use characteristics (Whitaker and Warkentin 2010). Space use information is particularly important when estimating population densities (Newell et al. 2013), examining impacts at the landscape level for management (Kaminski and Islam 2013), and identifying associated microhabitat characteristics (Barg et al. 2006; Anich et al. 2012; Streby et al. 2012; Wood and Perkins 2012; Nemes and Islam 2017) and how those attributes influence fitness (Boves et al. 2013). It can also be useful when developing and interpreting other sampling techniques (Falls 1981). Here and elsewhere, comparisons of home ranges and territories have made it apparent that these two categories are not interchangeable, and using territory estimates alone misrepresents total area and habitat use of birds.
Management recommendations
The cerulean warbler is a well-studied species, and previously described spatial and habitat characteristics are undoubtedly relevant. However, it is likely that cerulean warbler space use is often underestimated due to limitations that arise from sampling and estimation techniques. Limited study of space use has the potential to exclude information on this species' behaviors and habitat selection that may advise cerulean warbler management (Hamel and Rosenberg 2007; Wood et al. 2013). Accurate estimations are also important for species like the cerulean warbler that are presumed to be negatively impacted by activities beyond their space use boundaries, such as forest fragmentation (Buehler et al. 2020). Cerulean warbler space requirements and habitat selection varies across and within regions (Barg et al. 2005; Boves et al. 2013; Carpenter and Wang 2018; Buehler et al. 2020). Consequently, region-specific management strategies would benefit from additional study of this species' space use behaviors and habitat selection, particularly within extraterritorial space.
Many aspects of cerulean warbler biology in southern Indiana are associated with hickories and white oaks, but the occurrence of these tree species may decline (Kalb and Mycroft 2013; Wagner et al. 2015; Barnes et al. 2016). Continued study of the effects of forest management strategies should consider a more comprehensive space use area. Additionally, estimates reported here represent a snapshot of a paired male bird's space use over a 1- to 3-wk time period. Total space use throughout the breeding and postbreeding periods may be substantially larger.
Supplemental Material
Please note: The Journal of Fish and Wildlife Management is not responsible for the content or functionality of any supplemental material. Queries should be directed to the corresponding author for the article.
Data S1. Locations of 14 adult male cerulean warblers Setophaga cerulea that we tracked using radio telemetry during the 2018 and 2019 breeding seasons in research units of the Hardwood Ecosystem Experiment, Yellowwood State Forest, Brown County, Indiana. We used coordinates to construct 95% fixed kernel density estimates and minimum convex polygons of each individual bird's territory and home range.
Available: https://doi.org/10.3996/JFWM-21-100.S1 (10 KB XLSX)
Data S2. Text file containing R version 4.1.3 (2022-03-10) code that we used to generate 95% fixed kernel density estimates and minimum convex polygons of cerulean warbler Setophaga cerulea home ranges and territories. We tracked cerulean warblers using radio telemetry during the 2018 and 2019 breeding seasons in research units of the Hardwood Ecosystem Experiment, Yellowwood State Forest, Brown County, Indiana.
Available: https://doi.org/10.3996/JFWM-21-100.S2 (1 KB TXT)
Figure S1. Territory and home range estimates of an adult male cerulean warbler Setophaga cerulea (bird ID: Abe) that we tracked using radio telemetry between 24 May and 3 June 2019 in a research unit of the Hardwood Ecosystem Experiment in Yellowwood State Forest, Brown County, Indiana. A 95% fixed kernel density estimate of the territory (hatched) was 0.59 ha, 48% of the corresponding home range estimate (1.24 ha; light blue). A minimum convex polygon of the territory (dashed line boundary) was 0.55 ha, 25% of the corresponding home range polygon (2.24 ha; solid line boundary). We generated the home range using all location points (68; “o” symbol), and we generated the territory using location points where we observed the bird singing (33; “+” symbol). We found a nest (yellow star) for this individual.
Available: https://doi.org/10.3996/JFWM-21-100.S3 (2.171 MB TIFF)
Figure S2. Territory and home range estimates of an adult male cerulean warbler Setophaga cerulea (bird ID: Amo) that we tracked using radio telemetry between 31 May and 7 June 2019 in a research unit of the Hardwood Ecosystem Experiment in Yellowwood State Forest, Brown County, Indiana. A 95% fixed kernel density estimate of the territory (hatched) was 1.73 ha, 70% of the corresponding home range estimate (2.48 ha; light blue). A minimum convex polygon of the territory (dashed line boundary) was 0.87 ha, 56% of the corresponding home range polygon (1.56 ha; solid line boundary). We generated the home range using all location points (78; “o” symbol), and we generated the territory using location points where we observed the bird singing (21; “+” symbol).
Available: https://doi.org/10.3996/JFWM-21-100.S4 (2.888 MB TIFF)
Figure S3. Territory and home range estimates of an adult male cerulean warbler Setophaga cerulea (bird ID: Fin) that we tracked using radio telemetry between 5 and 13 June 2019 in a research unit of the Hardwood Ecosystem Experiment in Yellowwood State Forest, Brown County, Indiana. A 95% fixed kernel density estimate of the territory (hatched) was 1.33 ha, 59% of the corresponding home range estimate (2.25 ha; light blue). A minimum convex polygon of the territory (dashed line boundary) was 2.94 ha, 57% of the corresponding home range polygon (5.20 ha; solid line boundary). We generated the home range using all location points (97; “o” symbol), and we generated the territory using location points where we observed the bird singing (47; “+” symbol). We found a nest (yellow star) for this individual.
Available: https://doi.org/10.3996/JFWM-21-100.S5 (1.681 MB TIFF)
Figure S4. Territory and home range estimates of an adult male cerulean warbler Setophaga cerulea (bird ID: Ice) that we tracked using radio telemetry between 24 May and 5 June 2019 in a research unit of the Hardwood Ecosystem Experiment in Yellowwood State Forest, Brown County, Indiana. A 95% fixed kernel density estimate of the territory (hatched) was 5.09 ha, 105% of the corresponding home range estimate (4.85 ha; light blue). A minimum convex polygon of the territory (dashed line boundary) was 8.74 ha, 98% of the corresponding home range polygon (8.92 ha; solid line boundary). We generated the home range using all location points (81; “o” symbol), and we generated the territory using location points where we observed the bird singing (66; “+” symbol). We found a nest (yellow star) for this individual.
Available: https://doi.org/10.3996/JFWM-21-100.S6 (2.699 MB TIFF)
Figure S5. Territory and home range estimates of an adult male cerulean warbler Setophaga cerulea (bird ID: Joe) that we tracked using radio telemetry between 14 and 27 May 2019 in a research unit of the Hardwood Ecosystem Experiment in Yellowwood State Forest, Brown County, Indiana. A 95% fixed kernel density estimate of the territory (hatched) was 1.15 ha, 65% of the corresponding home range estimate (1.78 ha; light blue). A minimum convex polygon of the territory (dashed line boundary) was 1.21 ha, 37% of the corresponding home range polygon (3.29 ha; solid line boundary). We generated the home range using all location points (71; “o” symbol), and we generated the territory using location points where we observed the bird singing (41; “+” symbol). We found a nest (yellow star) for this individual.
Available: https://doi.org/10.3996/JFWM-21-100.S7 (2.114 MB TIFF)
Figure S6. Territory and home range estimates of an adult male cerulean warbler Setophaga cerulea (bird ID: Knope) that we tracked using radio telemetry between 23 May and 7 June 2018 in a research unit of the Hardwood Ecosystem Experiment in Yellowwood State Forest, Brown County, Indiana. A 95% fixed kernel density estimate of the territory (hatched) was 1.63 ha, 75% of the corresponding home range estimate (2.18 ha; light blue). A minimum convex polygon of the territory (dashed line boundary) was 1.90 ha, 60% of the corresponding home range polygon (3.17 ha; solid line boundary). We generated the home range using all location points (78; “o” symbol), and we generated the territory using location points where we observed the bird singing (64; “+” symbol). We found a nest (yellow star) for this individual.
Available: https://doi.org/10.3996/JFWM-21-100.S8 (1.982 MB TIFF)
Figure S7. Territory and home range estimates of an adult male cerulean warbler Setophaga cerulea (bird ID: Max) that we tracked using radio telemetry between 2 and 7 June 2019 in a research unit of the Hardwood Ecosystem Experiment in Yellowwood State Forest, Brown County, Indiana. A 95% fixed kernel density estimate of the territory (hatched) was 0.22 ha, 15% of the corresponding home range estimate (1.47 ha; light blue). A minimum convex polygon of the territory (dashed line boundary) was 0.46 ha, 19% of the corresponding home range polygon (2.46 ha; solid line boundary). We generated the home range using all location points (78; “o” symbol), and we generated the territory using location points where we observed the bird singing (21; “+” symbol). We found a nest (yellow star) for this individual.
Available: https://doi.org/10.3996/JFWM-21-100.S9 (1.654 MB TIFF)
Figure S8. Territory and home range estimates of an adult male cerulean warbler Setophaga cerulea (bird ID: Mud) that we tracked using radio telemetry between 31 May and 5 June 2019 in a research unit of the Hardwood Ecosystem Experiment in Yellowwood State Forest, Brown County, Indiana. A 95% fixed kernel density estimate of the territory (hatched) was 1.61 ha, 64% of the corresponding home range estimate (2.51 ha; light blue). A minimum convex polygon of the territory (dashed line boundary) was 1.04 ha, 44% of the corresponding home range polygon (2.37 ha; solid line boundary). We generated the home range using all location points (79; “o” symbol), and we generated the territory using location points where we observed the bird singing (32; “+” symbol). We found a nest (yellow star) for this individual.
Available: https://doi.org/10.3996/JFWM-21-100.S10 (2.824 MB TIFF)
Figure S9. Territory and home range estimates of an adult male cerulean warbler Setophaga cerulea (bird ID: PSL) that we tracked using radio telemetry between 17 and 25 June 2019 in a research unit of the Hardwood Ecosystem Experiment in Yellowwood State Forest, Brown County, Indiana. A 95% fixed kernel density estimate of the territory (hatched) was 4.95 ha, 132% of the corresponding home range estimate (3.74 ha; light blue). A minimum convex polygon of the territory (dashed line boundary) was 2.06 ha, 45% of the corresponding home range polygon (4.58 ha; solid line boundary). We generated the home range using all location points (85; “o” symbol), and we generated the territory using location points where we observed the bird singing (17; “+” symbol). We found a nest (yellow star) for this individual.
Available: https://doi.org/10.3996/JFWM-21-100.S11 (2.737 MB TIFF)
Figure S10. Territory and home range estimates of an adult male cerulean warbler Setophaga cerulea (bird ID: Sansa) that we tracked using radio telemetry between 7 and 26 June 2018 in a research unit of the Hardwood Ecosystem Experiment in Yellowwood State Forest, Brown County, Indiana. A 95% fixed kernel density estimate of the territory (hatched) was 1.83 ha, 92% of the corresponding home range estimate (2.00 ha; light blue). A minimum convex polygon of the territory (dashed line boundary) was 2.87 ha, 81% of the corresponding home range polygon (3.53 ha; solid line boundary). We generated the home range using all location points (60; “o” symbol), and we generated the territory using location points where we observed the bird singing (49; “+” symbol). We found a nest (yellow star) for this individual.
Available: https://doi.org/10.3996/JFWM-21-100.S12 (2.538 MB TIFF)
Figure S11. Territory and home range estimates of an adult male cerulean warbler Setophaga cerulea (bird ID: Syd) that we tracked using radio telemetry between 5 and 12 June 2019 in a research unit of the Hardwood Ecosystem Experiment in Yellowwood State Forest, Brown County, Indiana. A 95% fixed kernel density estimate of the territory (hatched) was 0.74 ha, 73% of the corresponding home range estimate (1.02 ha; light blue). A minimum convex polygon of the territory (dashed line boundary) was 0.55 ha, 45% of the corresponding home range polygon (1.23 ha; solid line boundary). We generated the home range using all location points (88; “o” symbol), and we generated the territory using location points where we observed the bird singing (43; “+” symbol). We found a nest (yellow star) for this individual.
Available: https://doi.org/10.3996/JFWM-21-100.S13 (2.518 MB TIFF)
Figure S12. Territory and home range estimates of an adult male cerulean warbler Setophaga cerulea (bird ID: Ted) tracked using radio telemetry between 14 and 25 May 2019 in a research unit of the Hardwood Ecosystem Experiment in Yellowwood State Forest, Brown County, Indiana. A 95% fixed kernel density estimate of the territory (hatched) was 0.94 ha, 74% of the corresponding home range estimate (1.27 ha; light blue). A minimum convex polygon of the territory (dashed line boundary) was 0.98 ha, 49% of the corresponding home range polygon (2.02 ha; solid line boundary). We generated the home range using all location points (66; “o” symbol), and we generated the territory using location points where we observed the bird singing (56; “+” symbol). We found a nest (yellow star) for this individual.
Available: https://doi.org/10.3996/JFWM-21-100.S14 (1.807 MB TIFF)
Reference S1. Hamel PB, Rosenberg KV. 2007. Developing management guidelines for cerulean warbler breeding habitat. Asheville, North Carolina: U.S. Department of Agriculture, Forest Service, Southern Research Station. e-Gen. Tech. Rep. SRS–101, pages 364–374.
Available: https://doi.org/10.3996/JFWM-21-100.S15 (249 KB PDF) and https://www.srs.fs.usda.gov/pubs/gtr/gtr_srs101/gtr_srs101-63.pdf
Reference S2. Islam K, Kaminski KJ, MacNeil MM, Young LP. 2013. The cerulean warbler in Morgan-Monroe and Yellow State Forests, Indiana: pre-treatment data on abundance and spatial characteristics of territories. Pages 61–68 in Swihart RK, Saunders MR, Kalb RA, Haulton GS, Michler CH, editors. The Hardwood Ecosystem Experiment: a framework for studying responses to forest management. Newtown Square, Pennsylvania: U.S. Department of Agriculture, Forest Service, Northern Research Station. Gen. Tech. Rep. NRS-P-108.
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Reference S4. Sauer JR, Link WA, Hines JE. 2020. The North American Breeding Bird Survey, analysis results 1966–2019. Laurel, Maryland: U.S. Geological Survey Patuxent Wildlife Research Center. U.S. Geological Survey data release.
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Reference S5. Swihart, RK, Saunders MR, Kalb RA, Haulton SG, Michler CH, editors. 2013. The Hardwood Ecosystem Experiment: a framework for studying responses to forest management. Newtown Square, Pennsylvania: U.S. Department of Agriculture, Forest Service, Northern Research Station. Gen. Tech. Rep. NRS-P-108.
Available: https://doi.org/10.3996/JFWM-21-100.S16 (22.279 MB PDF) and https://www.fs.usda.gov/nrs/pubs/gtr/gtr-nrs-p-108.pdf
Reference S6.[USFWS] U.S. Fish and Wildlife Service. 2021. Birds of conservation concern 2021. Falls Church, Virginia: U.S. Department of the Interior, U.S. Fish and Wildlife Service, Migratory Birds.
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Acknowledgments
We thank the Associate Editor and two anonymous reviewers for their many valuable comments on previous versions of this manuscript. This research was made possible by funding from the Indiana Department of Natural Resources through Purdue University, Amos W. Butler Audubon Society, and Ball State University's ASPiRE grant. Lastly, we would like to thank Alexander Sharp, Lisa Brouellette, Elsa Chen, Kristin Attinger, Tina Arthur, and Gillian Martin for their tireless work in the field.
Any use of trade, product, website, or firm names in this publication is for descriptive purposes only and does not imply endorsement by the U.S. Government.
References
The findings and conclusions in this article are those of the author(s) and do not necessarily represent the views of the U.S. Fish and Wildlife Service.
Author notes
Citation: Connare B, Islam K. 2023. Advancing our understanding of cerulean warbler space use through radio telemetry. Journal of Fish and Wildlife Management 14(1):75–89; e1944-687X. https://doi.org/10.3996/JFWM-21-100