Short Communication

INTRODUCTION

Many species limit activity to defined spatial areas at points in the annual cycle. These areas can vary in size and spatial orientation, depending on habitat characteristics, the abundance and distribution of resources, and the density of conspecifics (Wiens et al. 1985). Areas used by individuals for food gathering, mating, and caring for offspring are typically referred to as home ranges (Burt 1943). Portions of the home range that are guarded or defended from conspecifics are called territories (Noble 1939, Odum and Kuenzler 1955). During the bird breeding season, territories are often established and defended using acoustic signals, such as songs or calls, or visual displays (Nice 1941). These behaviors aid in attracting and retaining mates, while reducing interspecific aggression (Odum and Kuenzler 1955).

Quantifying space use is important for understanding the ecology of species, such as important habitat features and breeding area requirements, critical for conservation and management efforts (Anich et al. 2009, Streby et al. 2012, Connare and Islam 2023). Defining territory boundaries has often been done using standard observational methods, such as spot-mapping, but this method can underestimate the full extent of space use (Streby et al. 2012). Additionally, home range estimates derived from radio telemetry locations are often substantially larger than territory boundaries derived by observers mapping song perches (visual observations hereafter; Anich et al. 2009, Streby et al. 2012, Connare and Islam 2023). Therefore, using visual observations alone can underestimate space use and provide an incomplete or incorrect assessment of habitat associations (Anich et al. 2009, Streby et al. 2012).

The Grasshopper Sparrow (Ammodramus savannarum) is a small, stocky, and generally inconspicuous inhabitant of grasslands and open habitats across North America (Vickery 2020). In most jurisdictions the species is in peril, with continental populations having declined by >60% in recent decades (Rosenberg et al. 2016). Declines in the eastern subspecies (A. s. pratensis) have led to this subspecies being listed as a species of special concern in Canada (COSEWIC 2013). Currently, there are no published territory or home range size estimates for this subspecies in Canada (COSEWIC 2013), and few contemporary estimates exist across the breeding range (Vickery 2020). Previous territory estimates, derived from mapping of singing locations, may underestimate space use given specific nuances of this species’ behavior. Within suitable habitat, breeding territories are often aggregated in clumps, even when areas of apparently suitable habitat remain unoccupied (Winnicki et al. 2020). This could make visual confirmation of territory boundaries difficult, especially when birds are not individually marked. Similarly, birds often forage close to the ground and are known to walk and run long distances (e.g., >100 m), making them difficult to observe when not singing (Vickery 2020).

The goal of our study was to document space use in a population of eastern Grasshopper Sparrows. First, we wanted to provide contemporary estimates of territory and home range sizes, since these metrics are important for conservation and management. Second, we wanted to compare space use estimates from visual observations with radio telemetry to understand how estimates of space use can differ depending on method. Specifically, we predicted that territory size, derived from visual observations, would underestimate home range size as in several previous studies (Anich et al. 2009, Streby et al. 2012, Connare and Islam 2023). Third, we wanted to determine if birds used space differently between our study sites and to identify potential causal explanations for the differences, if present.

METHODS

Study area

Our study sites were situated southwest of Ottawa, Ontario, Canada (Fig. 1). One study site was in Burnt Lands Provincial Park (Burnt Lands hereafter; 45.27° N, -76.19° W), a natural alvar, where thin soils and exposed limestone maintain a sparse cover of grasses with limited tree cover (Catling 2014). While Burnt Lands contains forested uplands and extensive shrubby areas, an area of continuous grassland habitat approximately 100 ha in size exists within the park. Grass cover primarily consists of native grasses, and it is ungrazed. The second study site on Panmure Road (Panmure hereafter; 45.29° N, -76.23° W), on private land, consists mainly of pasture containing a mix of native and non-native grass species. An area of continuous grassland habitat approximately 60 ha in size exists at this site and was lightly grazed by approximately 15 cattle throughout the duration of our study.

Radiotelemetry and visual observations

We captured a total of 12 breeding males, six at Burnt Lands and six at Panmure by luring them into mist nets placed between song posts frequented by individual birds. All captured birds were sexed as male, based on the presence of a pronounced cloacal protuberance. Birds were uniquely banded with a numbered aluminum band on the right leg and two darvic color bands on the left leg to distinguish individuals. We attached radio transmitters (BD-2 transmitter; 0.62 g; Holohil Systems Ltd., Carp, Ontario) to birds using a leg-loop harness (Rappole and Tipton 1991, Streby et al. 2015) with 0.7 mm elastic beading cord. To reduce weight, we removed 50 mm of the transmitter antenna (~0.08 g), with minimal apparent impact to the detection distance. We weighed each bird to within 0.01 g using an electronic scale and only attached transmitters on birds with a weight of ≥16.25 g (< 4% body mass).

We tracked each bird for 13–31 days (24.4 ± 6.7), between 0500 to 1900 hours EDT, from 31 May to 11 July 2016. We tracked tagged birds using a telemetry receiver (R-1000, Communications Specialists Inc., Orange, California) with attached three-element folding Yagi antenna using the homing method (White and Garrott 1990). We varied the order of sites and individuals tracked each day to minimize differences associated with time of day. Once an individual radio transmitter signal was detected, we circled the signal area and recorded three bearings for each bird. We used the antenna orientation where the audible signal strength peaked and recorded the compass direction and a waypoint using a handheld global positioning system (GPS) unit (e.g., telemetry locations). We recorded song post locations if an individual was detected in spontaneous song upon arrival, or if the bird emerged from cover, alighted on a perch, and began to sing (e.g., visual observations). Since male Grasshopper Sparrows often engage in extended agonistic interactions with neighboring males (e.g., counter-singing, physical interactions; Vickery 2020), collecting consecutive locations may not have been independent. Once a single point location was collected for each individual, we moved on to the next closest individual. We systematically cycled through all territories in this manner each day until the radio transmitter batteries failed or an individual left the study area.

We calculated triangulated points using at least three successive telemetry bearings using the radiotrack R package (Dang 2019) through the maximum likelihood estimator. If there were >3 iterations for estimating a triangulation point or if the maximum likelihood estimator failed to converge, we plotted the telemetry points with their bearings to check if a logical triangulation point was generated. If the calculated location appeared to be incorrect because a faulty bearing was taken, we manually plotted the location to where the two other bearings intersected.

Territory and home range size estimation

We used visual observations to define territories and a combination of visual and triangulation locations to define home ranges. Territory and home ranges were estimated using the 95% KDE and least-squares cross validation was used to estimate the bandwidth. Kernel density estimates represent a probability density function and are the preferred method for estimating space use in birds (Barg et al. 2005, Connare and Islam 2023). We generated estimates for all birds with ≥10 visual observation locations for territories and ≥15 locations for home ranges. We generated our estimates in R version 4.3.2 (R Core Team 2021) using the ks and eks packages (Duong 2022). Variation in estimates was examined using a jackknife analysis to understand the impact any single observation had on the home range estimates. We compared the overlap between individual territories and home ranges by calculating the proportional amount of overlap between each of the two-dimensional areas (ha). Similarly, we compared the amount of overlap between neighbors by calculating the area of an individual home range that was shared by one or more adjacent territories as a proportion of the total home range for that individual.

Statistical analysis

To explore the impact of sample size on our estimates of territory and home range we ran independent bootstrap analyses. For each bird, we sampled with replacement between 2 and 50 observations of the estimated territory and home range. We repeated this 1,000 times and then calculated the mean and standard error for a given sample size across all replicates and birds.

We used the Shapiro-Wilk test for normality before comparing our sample data. Since our data were not normally distributed, we tested whether individual home ranges were larger than territories using a one-sided Wilcoxon Signed Rank test. To compare home range sizes between study sites, we used a nonparametric Mann-Whitney U-test. All values represent mean ± standard error.

RESULTS

Radiotelemetry and visual observations

In 2016, we collected 405 point locations for 12 male Grasshopper Sparrows. We collected a mean of 33.8 locations per bird (range 15–46; Table 1). We collected 190 locations based on visual observations of singing birds (average per bird = 15.8 ± 2.0) and 215 locations from telemetry (average per bird = 17.9 ± 2.4).

In mid-July, after transmitters stopped responding, we recaptured 5 tagged males (41.7%) and removed their radio transmitters. Of the 12 males banded in this study, 4 (33.3%) returned the following year. In 2017, one male returned to our study site at Burnt Lands (1 out of 6; 16.7%) and 3 males returned to our study site at Panmure (3 out of 6; 50%).

Territory and home range size estimation

Our bootstrap analysis indicated that 95% KDE territory and home range estimates increased with increasing sample size (Fig. 2). Based on the average sample size of locations for both territory and home range estimates, each were approaching an asymptote suggesting our values were reasonable. Our 95% KDE territory estimates ranged from 0.52 to 2.21 ha, whereas home range estimates ranged from 0.86 to 5.79 ha. Home ranges (average KDE = 2.24 ± 0.51 ha, n = 10) were significantly larger than territories (average KDE = 1.05 ± 0.16 ha, n = 10; Wilcoxon Signed Rank z = -2.80, P = 0.003; Fig. 3). On average, territories overlapped home ranges by 54.7 ± 6.9% (range 25.0–95.7). Home ranges at Burnt Lands (average KDE = 4.06 ± 0.54 ha, n = 6) were significantly larger than home ranges at Panmure (average KDE = 1.32 ± 0.15 ha, n = 6; Mann-Whitney U = 0, n1 = n2 = 6, P = 0.005, two-tailed). Individual birds had greater territory overlap with neighbors at Panmure (29.9 ± 12.3%) than at Burnt Lands (3.3 ± 5.8%; Fig. 4). Similarly, the home ranges of adjacent birds had greater overlap with neighbors at Panmure (50.1 ± 13.1%) than at Burnt Lands (8.1 ± 5.3%).

DISCUSSION

Our study provides contemporary estimates of space use, specifically territory and home ranges, for the eastern Grasshopper Sparrow. Few published estimates of the home range exist for this species and we are aware of none for this subspecies. Our territory estimates are similar to those in the published literature, averaging slightly larger than other parts of the breeding range (Vickery 2020). The average territory size in our study was slightly larger than in Pennsylvania (0.8 ha; Smith 1968), Connecticut (0.66–0.78 ha; Crossman 1989), Wisconsin (0.85 ha; Wiens 1969), and West Virginia (0.32 ha; Wray, II, 1979), but averaged smaller than in Michigan (1.4 ha; Smith 1968). Variation in the methods employed to estimate space use in this species makes meaningful comparisons difficult, especially in relation to our study. Given our study took place in two distinct habitat types at the northern limit of the breeding range, our results may not be representative of space use elsewhere. Therefore, additional studies of space use employing kernel density estimates elsewhere in the breeding range of this subspecies would be helpful in this regard.

Territories in our study were about half the size of home ranges (e.g., ~55%), meaning that visual observations of singing birds captured only half of the space used by individual males. Few studies report differences between territory and home range size, but in several North American studies, covering a broad range of bird species, territories averaged about 70% of home ranges (Anich et al. 2009, Tomasevic and Marzluff 2018, and Connare and Islam 2023). In several European studies, territories averaged much smaller (e.g., <50%) than home ranges (Ferry et al. 1981, Maciejok et al. 1995, and Naguib et al. 2001), highlighting the importance of telemetry when estimating space use in breeding birds.

Although birds were present for the duration of our study, some males sang infrequently, resulting in fewer visual observations for some individuals. As a result, we set a relatively low threshold of samples to define territories (e.g. ≥ 10 locations), which has provided accurate estimates in past studies (Börger et al. 2006). Others have suggested using >15 samples to create reasonable KDEs (Saïd et al. 2005, Connare and Islam 2023), but given varying sensitivity to sample size in other studies, this number is not considered a requirement (Laver and Kelly 2008). Anich et al. (2009) found little effect of increased sample size above 10 locations on the home range size in Swainson’s Warblers (Limnothlypis swainsonii), so we considered this as a minimum number for estimating territory size. Using a minimum sample size of 10 visual observation locations may have underestimated territory size for some individuals. However, based on the average number of visual observations per bird, our territory estimates were approaching an asymptote. Similarly, a minimum sample size of ≥15 locations for home ranges may have underestimated space use for a few individuals, but our average estimates also approached an asymptote. Based on our bootstrap analysis, using a minimum sample of 20 locations for territory size estimation and 30 locations for home range size estimation is recommended.

We detected differences in the space used by individuals between study sites with home ranges at Burnt Lands triple the size of those at Panmure, and a similar number of breeding Grasshopper Sparrows at each site despite the grasslands at Burnt Lands being about 40 ha larger than Panmure. While we were unable to quantify differences in vegetation cover at each study site, Burnt Lands contained more areas of non-habitat, consisting mainly of exposed limestone with no vegetation (Catling 2014). Habitat at Panmure was more consistently vegetated, though some areas were intensively grazed by cattle. Home range size can often vary in response to habitat quality, whereby birds in poorer quality habitats use larger areas (Anich et al. 2009, Tingley et al. 2014). We also found less spatial overlap and greater spacing between adjacent territories and home ranges at Burnt Lands (Fig. 3), suggesting that space use at this site was more discrete. While we documented some nesting at both sites, we were unable to measure differences in habitat quality directly. Breeding site fidelity, although only an indirect measure of habitat quality (Hannah et al. 2008), was higher at Panmure than Burnt Lands, suggesting that habitat quality may have been higher at this site. Further research could link space use to both habitat quantity and quality, especially in the eastern subspecies of Grasshopper Sparrow.

Most information on the abundance and distribution of birds comes from acoustic field surveys (e.g., points counts, spot-mapping) and obtaining accurate counts is important, especially for species of conservation concern (Hochachka et al. 2009, Wood et al. 2017). However, individual marking (e.g. color bands) may be necessary to differentiate individuals to obtain accurate counts in species with highly aggregated or overlapping territories, such as the Grasshopper Sparrow (Winnicki et al. 2020). Given the relatively high rates of territory overlap observed in our study, obtaining accurate counts using visual surveys seems unlikely. Autonomous recording units (ARUs) might be advantageous when surveying Grasshopper Sparrows, since birds sing individually distinct songs and can be differentiated on spectrograms, or by ear with training (Lohr et al. 2013). A study comparing audible surveys with those obtained by ARUs would be beneficial in answering this question.

Quantifying space use is important for improving our understanding of the basic ecology of species and differences in the various approaches can have important management and conservation implications. For instance, differences in the types and amount of habitat used by species is often underestimated or missed when relying solely on territory estimates derived from visual observations (Anich et al. 2009, Streby et al. 2012, Connare and Islam 2023). Specifically, the majority of male Grasshopper Sparrows in our study were detected visually when using elevated song perches which were often limited within territories and distributed non-randomly, thereby limiting interpretations of space use. Males often moved great distances on the ground and were silent during most of these forays, especially when approaching or overlapping nearby territories. Since some of our individual home range estimates were quite large (>5 ha in size), well above other space use estimates in the literature (Vickery 2020), care should be taken when defining protected area boundaries or identifying critical habitat for this species without using radio telemetry. This species has been described as area-sensitive (Davis 2004, Johnson and Igl 2001), requiring habitat patches >6 ha in size (Jobin and Falardeau 2010). Based on home range estimates in our study, we recommend the conservation of larger patches of suitable habitat to ensure adequate space for the full range of breeding season activities.

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