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Stopover Duration and Habitat Use by Tennessee Warblers (Oreothlypis peregrina) at a High-Elevation Bald
Scott A. Rush, Eric C. Soehren, and Mary Miller

Southeastern Naturalist, Volume 13, Issue 1 (2014): 92–100

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Southeastern Naturalist S.A. Rush, E.C. Soehren, and M. Miller 2014 Vol. 13, No. 1 92 2014 SOUTHEASTERN NATURALIST 13(1):92–100 Stopover Duration and Habitat Use by Tennessee Warblers (Oreothlypis peregrina) at a High-Elevation Bald Scott A. Rush1,*, Eric C. Soehren2, and Mary Miller3 Abstract - Use of high-elevation balds by Nearctic-Neotropical passerines remains a poorly understood component of migration ecology. We used radio-telemetry to assess the habitat use of 20 Oreothlypis peregrina (Tennessee Warbler) at a high-elevation bald during September 2011. Our goals were to estimate the duration of stopover, examine variation in stopover length by age class, and determine patterns of habitat use during fall migration. For radio-tracked birds, the average length of time spent at the bald was 4.6 days (range: 1–12 days). The majority of tracked locations corresponded with heath habitat that surrounded the bald opening (79% of locations), relative to the more distal and less complex, hardwood forest. Collectively, these results provide information on the important, yet ephemeral role that high-elevation balds play in supporting the autumnal migration of eastern North American passerines. Introduction Nearctic-Neotropical songbirds may spend up to a third of a year in migration, and most spend the majority of that time at stopover sites (Mehlman et al. 2005). Stopover sites therefore play a critical role in providing migrating passerines some key resources needed to complete these epic journeys. As a consequence, understanding the impact of environmental changes such as habitat loss or fragmentation along their migratory routes is essential for the implementation of effective conservation schemes (Buler et al. 2007, Hutto 1998, Mehlman et al. 2005). Although migrating passerines use both coastal and inland routes, the majority of stopover studies have focused on coastal locations (e.g., Bonter et al. 2007, Moore and Kerlinger 1987). Focus on coastal stopover sites has left much of the stopover ecology at inland sites unexplored (Vogt et al. 2012). We have limited understanding of how songbirds use inland stopover sites, including the duration of time spent at these locations. For instance, relatively little is known about age-related differences in fat deposition and habitat selection by migrating passerines (Jones et al. 2002); a case especially true for inland stopover sites. Studies focused on the ecology of passerines migrating through inland sites are limited. Of the existing studies, several have identified differences in stopover length (Ellegren 1991, Jones et al. 2002) and pattern of mass deposition (Ellegren 1991) between young and adult migrant passerines (Wang et al. 1998). Several factors can contribute to age-related differences in mass gain among migrant 1Department of Wildlife, Fisheries and Aquaculture, 775 Stone Boulevard, Mississippi State University, Mississippi State, MS 39762. 2Elhew Field Station, Wehle Land Conservation Center, State Lands Division, Alabama Department of Conservation and Natural Resources, 4819 Pleasant Hill Road, Midway, AL 36053. 3United States Department of Agriculture, Forest Service, Cherokee National Forest, 2800 Ocoee Street North Cleveland, TN 37312. *Corresponding author - Manuscript Editor: Wylie Barrow Southeastern Naturalist 93 S.A. Rush, E.C. Soehren, and M. Miller 2014 Vol. 13, No. 1 passerines. Young birds may be less efficient fliers than adults, causing them to burn more energy during migration and arrive at stopover sites with more depleted energy reserves (Wang and Moore 1994). Further, upon arrival at stopover sites, young birds may be less efficient foragers than adults, choose less nutritious foods, or forage in sub-optimal habitats (Chernetsov 2006, Morris et al. 1996, Wunderle 1991). Patterns of habitat use therefore appear to be very important factors for determining the success of migration. Because the benefits of a stopover site can be determined by factors measureable at both localized and landscape scales, derived benefits can also vary along the migratory route (T ankersley and Orvis 2003). For Oreothlypis peregrina (Wilson) (Tennessee Warbler), the primary pathway for fall migration appears to be inland, along a route that includes the southern Appalachian Mountains (Rimmer and McFarland 1998). High-elevation grasslands or “balds”, an early successional habitat type unique to the southern Appalachian Mountains represent “habitat islands” that are used by passerines regularly during fall migration (Vogt et al. 2012). Thus, the inter-year recapture of 14 Tennessee Warblers (unprecedented for any migrant Parulidae) at one high-elevation bald, Whigg Meadow in eastern Tennessee, suggests that this location may be along a route consistently used during this species’ autumnal migration (Vogt et al. 2012). Regular use of Whigg Meadow by Tennessee Warblers during fall migration highlights the importance of conserving this site. It may also indicate the additional need for early successional habitats, and maintenance of other high-elevation balds within the southern Appalachians; especially those habitats used by other species of conservation need including Vermivora chrysoptera (L.) (Golden-winged Warbler) and Sylvilagus obscurus Chapman, Cramer, Deppenaar, and Robinson (Appalachian Cottontail). Our objective was to identify habitat use by Tennessee Warblers at Whigg Meadow during fall migration. Our specific goals were to determine patterns of habitat use, estimate the duration of stopover periods, and examine variation in stopover length between younger and older Tennessee Warblers using this high-elevation bald. Study Site This project was carried out at Whigg Meadow, a high-elevation site in eastern Tennessee (35°189'N, 84°029'W; elevation: 1490 m; Monroe County; Fig. 1). Whigg Meadow is located in the Tellico Ranger District of the Cherokee National Forest and is comprised of 2 forest stands, reflecting a high-elevation bald with an opening approximately 3 ha in size. The dominant habitat type at Whigg Meadow is a forest-edge ecotone that transitions between open grassland and northern hardwood forest. A shrub-dominated assemblage surrounds the herbaceous bald with a dense thicket of Rubus canadensis L. (Smooth Blackberry) and scattered Vaccinium corymbosum L. (Highbush Blueberry), encroaching upon the open herbaceous area. A forest dominated by second-growth or stunted Fagus grandifolia Ehrh. (American Beech) and Betula alleghaniensis Britton (Yellow Birch) can be found north of the bald opening, while Acer saccharum Marshall (Sugar Maple), Quercus rubra L. (Northern Red Oak), and Crataegus crus-galli L. (Cockspur Hawthorn) become more dominant south of the opening (TDEC 1999). Southeastern Naturalist S.A. Rush, E.C. Soehren, and M. Miller 2014 Vol. 13, No. 1 94 Methods Radio telemetry Fall mist-netting and banding efforts have been performed annually at Whigg Meadow since 1998 (Vogt et al. 2012), and fieldwork for this project was carried out during September 2011. Data collected from migration monitoring at Whigg Meadow indicates that the duration of Tennessee Warblers’ migration stopovers at this site differs by age class (D.F. Vogt et al., Cherokee National Forest, Monroe County, TN, unpubl. data). Figure 1. Location of Whigg Meadow in eastern Tennessee. The lowest panel, reflecting the highest level of resolution, shows the distribution of telemetered Tennessee Warblers (black dots) at Whigg Meadow during September 2011. The darker shaded polygon reflects edge/heath habitat that encircles the grassy bald opening (lighter shaded polygon). Southeastern Naturalist 95 S.A. Rush, E.C. Soehren, and M. Miller 2014 Vol. 13, No. 1 However, several factors such as age-related recapture probability can influence this estimate. Clarification of this potentially confounded result could be achieved by radio-tracking both hatch-year (HY) and after-hatch-year (AHY) birds and evaluating differences in stopover duration. Therefore, a goal of the project was to outfit 10 HY and 10 AHY birds with radio-transmitters. Transmitters used for this study (Model A2412, ATS, Isanti, Minnesota) weighed 0.28 g (≈3% of adult body weight) and had a battery life of 23 days, a longevity that exceeded the estimated maximum stopover duration of Tennessee Warblers (14 days) calculated from banding data (analyzed using mark-recapture models) collected during 2001–2008 (D.F. Vogt, unpubl. data). Beginning 1 September 2011, we captured Tennessee Warblers incidentally using mist nets (see Vogt et al. 2012 for description of methods). We banded all captured birds using USGS aluminum bands and aged them following Pyle (1997). We examined each bird in hand to determine the amount of subcutaneous fat visible in the furculum of the clavicle (tracheal pit) and assigned a “fat” score ranging from 0–5, where 0 = no visible fat and 5 = fat overflowing the tracheal pit (Krementz and Pendleton 1990). Of the Tennessee Warblers captured, 10 HY and 10 AHY birds were selected for this radio-telemetry study on the basis of “first-come, first-served”. We attached radio-transmitters to each bird using a small square of chiffon, which was affixed to each transmitter and attached to the interscapular region of each bird using cyanoacrylate glue (Johnson et al. 1991). We used a Biotracker telemetry receiver (ATS, Isanti, MN) with a three-element Yagi antenna to relocate radio-marked individuals. We determined the estimated detection range for these transmitters in this study area to be 0.5–1.5 km. During this study, we located radio-marked Tennessee Warblers three times a day by homing in on their location (White and Garrott 1990). Successive tracking sessions for each bird were separated by at least 3 hrs. During tracking events, we confirmed the location of each bird visually or by tracking to the location where the strongest radio signal was identified and species-specific vocalizations were heard. We recorded each location using a handheld GPS device (Garmin GPS 60, Garmin, Olathe, KS; average measured error ± 3.7 m). During radio-tracking activities, we considered a bird to have left the stopover site if no signal was received for at least three consecutive days throughout the bald, with departure time defined as night following the last confirmed observation. Habitat measurements We created a land-cover map as a GIS layer in ArcMap 9.3 (ESRI, Redlands, CA) from aerial photographic material completed with field verifications. Based on direct observations, we considered three different habitats as dominating the study area: grassy meadow habitat (primarily Smooth Blackberry with scattered Highbush Blueberry), heath or forest-edge ecotone (transition between open grassland and northern hardwood forest), and hardwood forest. We plotted the locations of radio-tracked Tennessee Warblers on these maps to determine habitat use within each bird’s range of movements, as defined by radio-telemetry locations. Southeastern Naturalist S.A. Rush, E.C. Soehren, and M. Miller 2014 Vol. 13, No. 1 96 We assessed structural differences in habitat by overlaying a grid of 50-m x 50-m squares on top of aerial imagery of the study site. We classified habitat intersecting each corner of the overlain grid by identifying the dominant vegetation class at each location. Additional assessments collected for the habitat at each sampling point included canopy cover, canopy height, density of vegetation <1 m and 1–2 m above ground, and the distribution of tree sizes (diameter at breast height: DBH). Following methods detailed in James and Shugart (1970), we centered a 5-mradius circular plot on the location corresponding to each grid intersection. At each plot center, we measured the DBH (1.4 m above the ground surface) of all trees >2 m tall with a dbh >5 cm. At each of the four cardinal directions 5 m from the center of the plot, we measured the average height of the vegetation using a clinometer (Forestry Suppliers, Inc. Jackson, MS). We measured the vertical distribution of foliage in each cardinal direction by placing a 2-m graduated pole, marked at decimeter intervals vertically in the vegetation, and recording the number of times foliage hit the pole within each increment. We also calculated canopy cover at 5 m from plot center in each cardinal direction using a concave spherical densitometer (Forestry Suppliers, Inc. Jackson, MS). We averaged repeated measurements obtained for each plot prior to further analysis. Statistical analysis We performed all statistical analyses using the statistical package R (Version 2.14.2; R Development Core Team 2012). Prior to analysis, we tested all data for normality and homogeneity of variance using probability plots. We assessed mean differences in the duration of time spent at Whigg Meadow between Tennessee Warbler age classes using t-tests, with the degrees of freedom conditioned for unequal variance (i.e., Welch’s t-tests; Ruxton 2006). We explored relationships between the fat index of Tennessee Warblers and time spent at Whigg Meadow using linear models. We used multiple analysis of variance (MANOVA) to test for the effect of site (heath vs. forest habitat) on habitat measurements, and then applied analysis of variance to specify significant dif ferences found in the MANOVA. Results During 1–29 September 2011, we captured 20 Tennessee Warblers and outfitted them with radio-transmitters. For these birds, the average length of time spent at Whigg Meadow was 4.6 days (min, max: 1–12 days), with a maximum length of stay similar to that identified through banding efforts (D.F. Vogt, unpubl. data). On average, older Tennessee Warblers tended to stay longer at Whigg Meadow than younger birds (AHY mean: 5.7 days; HY mean: 3.4 days), but this difference was not statistically significant (t = 1.34, df = 17.9, P = 0.20). There was no difference in the time Tennessee Warblers spent at Whigg Meadow relative to the bird’s fat index for either young (HY: F1,8 = 0.04, P = 0.85) or older individuals (AHY: F1,8 = 1.46, P = 0.26). Multiple analysis of variance of habitat metrics revealed significant differences between heath and forest habitats (Wilk’s l = 0.32, df = 1, 6, P = 0.01; Table 1). Southeastern Naturalist 97 S.A. Rush, E.C. Soehren, and M. Miller 2014 Vol. 13, No. 1 Habitat differences between heath and forest locations were less canopy cover (F1,18 = 6.81, P = 0.02) and shorter canopy height (F1,18=27.95, P < 0.01) at heath locations. Heath habitat was also characterized by more dense vegetation less than 1 m above ground (F1,18 = 12.86, P < 0.01) and 1–2 m above ground (F1,18 =16.82, P less than 0.01). There was no difference in basal area measured in heath or forest habitats (mean DBH per ha heath = 567.91 m2, mean DBH per ha forest = 658.81 m2; t = -0.66, df = 15.52, P = 0.52). There was, however, a greater proportion of smaller trees (those with DBH ≤ 20 cm) in heath relative to forest habitat (Fig. 2), but a greater proportion of larger trees (with DBH > 20 cm) in the forest habitat. The majority of locations identified for the radio-tracked birds (n = 164 locations for 20 individual Tennessee Warblers) corresponded with the heath habitat (79% of locations), with considerably fewer locations in the hardwood forest habitat (21%) (Fig. 1). There was no apparent directionality relative to the time of the day when the birds were tracked (Fig. 1). Discussion Results from the present study support earlier findings (Vogt et al. 2012) that Tennessee Warblers use the early successional heath/edge habitat at Whigg Meadow during fall migration. This heath habitat is characterized by low canopy cover, shorter canopy height, and greater vegetation structure within 2 m of the ground. The majority of locations identified for radio-tracked Tennessee Warblers occurred within this heath habitat, whereas considerably fewer locations were within the surrounding forest. Furthermore, for the 20 Tennessee Warblers that were radiotracked during 2011, significantly more locations were determined to occur in the southern portion of Whigg Meadow. Little information exists on the directionality of diurnal movements and habitat use by migrating songbirds during stopover events. For instance, Chernetsov (2011) found no observable pattern in the directionality of movements among several species at stopover locations on the Baltic coast. Although our present study is focused on one location and one migratory period, it is of interest whether the behaviors observed for Tennessee Warblers are relative to resources at this site alone or reflect more general behaviors similar among other passerine species at other southern Appalachian balds. Table 1. Habitat characteristics evaluated at sample locations within heath or forest habitat at Whigg Meadow during 2011. Measurements are provided as mean (minimum–maximum). Heath Forest Canopy cover (%) 83.84 (51.04–95.84) 95.19 (92.20–96.62) Canopy height (m) 10.7 (2.5–20.0) 23.8 (17.0–29.0) Density of vegetation less than 1 m above ground (# of foliage hits/m) 2.90 (1.00–6.45) 1.24 (0.75–1.75) Density of vegetation 1 m to 2 m above ground (# of foliage hits/m) 1.41 (0.50–2.75) 0.37 (0.00–0.75) Basal area (m2/ha) 567.92 (132.1–1159.4) 658.81 (230.0–1032.1) Southeastern Naturalist S.A. Rush, E.C. Soehren, and M. Miller 2014 Vol. 13, No. 1 98 In general, high-elevation balds have received little attention, yet these ecosystems are used regularly by transient Tennessee Warblers and other species of migratory passerines (Vogt et al. 2012). Anticipated habitat change at Whigg Meadow, whether managed or not, will determine the suitability of these locations for future migrations. For instance, high-elevation balds such as Whigg Meadow are experiencing a shift in the forest boundary between heath and the forest community that is threatening these isolated communities (White and Sutter 1998). Without active management, it is anticipated that woody encroachment will reduce the herbaceous community and eventually the extent of heath at these sites. Although the experimental design employed in this study does not provide a clear assessment of whether increased availability of heath habitat would Figure 2. Tree distribution by DBH for heath (top graph) and forest (bottom graph) habitats at Whigg Meadow during 2011. Percentages reflect the proportion of basal area in each DBH category multiplied by 100, while the y-axis, “Frequency”, denotes the cumulative distribution of each size class. Southeastern Naturalist 99 S.A. Rush, E.C. Soehren, and M. Miller 2014 Vol. 13, No. 1 definitively benefit Tennessee Warblers, results do show that heath habitat is the dominant habitat type used by this species during autumn migration. The present study suggests that habitat management undertaken at Whigg Meadow with the aim of aiding species such as the Tennessee Warbler should focus on expanding and conserving the availability of heath/edge habitat. Coupling this management goal with those aimed at supporting Golden-winged Warblers and Appalachian Cottontail will likely afford protection to an array of species requiring this early successional habitat type. This study, and the findings of others (Vogt et al. 2012), highlight the importance of Whigg Meadow and the southern Appalachian Mountains as migratory stopover sites used by Tennessee Warblers. Additional research addressing the role of specific southern Appalachian habitats, such as high-elevation balds and adjacent forest, in supporting songbird migration is needed. For instance, inference drawn from the current study relates to one bald only. Further studies focused on habitat use at multiple balds could provide greater depth to understanding and developing best management practices. The placement of additional migratory monitoring stations, coupled with passive monitoring techniques such as acoustic monitoring arrays (Blumstein et al. 2011) could provide additional data concerning the importance of these stopover locations and important information key to the conservation of this unique ecological community. Acknowledgments This research was made possible through the tireless efforts of numerous volunteers, specifically Joe Carter, Nelson Edwards, Laura Lewis, John Trent, David Vogt, and Hayden Wilson. Funding and logistical support was provided by the USDA Forest Service, Cherokee National Forest and the Department of Wildlife, Fisheries and Aquaculture at Mississippi State University. We are indebted to John Trent for graphical assistance. 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