Northeastern Naturalist Vol. 26, No. 1 E.J. Zawatski, D.A. Gross, and M.C. Brittingham 2019 31 2019 NORTHEASTERN NATURALIST 26(1):31–42 Habitat of Breeding Blackpoll Warblers at a Persistent Extralimital Breeding Site in Pennsylvania Eric J. Zawatski1, Douglas A. Gross2, and Margaret C. Brittingham1,* Abstract - Setophaga striata (Blackpoll Warbler) is a boreal forest breeder that inhabits an expansive breeding range, with its southern limit in the northeastern US. The Pennsylvania breeding population is small and isolated but has persisted since its discovery in 1993, with the nearest breeding population about 150 km northeast in the Catskill Mountains of New York. Little information is available on the structure and composition of the vegetation where Blackpoll Warblers establish territories and how these vegetative parameters compare with those present in the core of their range. In 2016, we quantified the understory and canopy structure and composition, as well as the groundcover of active Blackpoll Warbler breeding territories (n = 15). Blackpoll Warblers occupied areas that were dominated by spruce (Picea rubens [Red Spruce] and P. mariana [Black Spruce]; 75% canopy cover) with a relatively low mean canopy height (6 m) and a mean diameter at breast height of 13 cm. Overall, the structure and composition of the vegetation within territories of Blackpoll Warblers in the Pennsylvania population are similar to those found in core portions of their breeding range. While the Pennsylvania population has bred exclusively within 1 small, confined area, similar Red Spruce and Black Spruce communities can be found to the east in the Pocono Mountains of Pennsylvania, suggesting there may be an opportunity for range expansion. However, there are increasing concerns that populations of boreal species at the southern edge of their range are especially vulnerable to climate change as warming and weather extremes decrease the suitability of isolated locations. Introduction Setophaga striata (Forster) (Blackpoll Warbler) is a Neotropical migrant that winters in northern South America and breeds across the northern Canadian boreal forest west into Alaska, as well down into high-elevation “island” forests in the northeastern US (Deluca et al. 2013). It is considered a characteristic boreal forest bird, with over 80% of its breeding population in North America’s boreal forest (Wells 2011). Across Canada, this species is primarily a denizen of Picea mariana (Mill.) Britton, Sterns and Poggenb. (Black Spruce) and Larix laricina (Du Roi) K. Koch (Tamarack) swamps and forests, while in the northeastern US, the species inhabits montane forests consisting primarily of Abies balsamea (L.) Mill. (Balsam Fir), Picea rubens Sargent (Red Spruce), and Betula papyrifera Marshall (Paper Birch) (Deluca et al. 2013). Prior to 1993, there were no records of Blackpoll Warblers breeding in Pennsylvania, and the southern extent of the species’ breeding range was delimted by 1Department of Ecosystem Science and Management, The Pennsylvania State University, University Park, PA 16802. 2PA Game Commission, Endangered and Non-game Bird Section, 106 Winters Road, Orangeville, PA 17859. *Corresponding author - MXB21@psu.edu. Manuscript Editor: Daniel Keppie Northeastern Naturalist 32 E.J. Zawatski, D.A. Gross, and M.C. Brittingham 2019 Vol. 26, No. 1 isolated populations in the Catskill and Adirondack Mountains of New York and mountains of northeastern Massachusetts and Vermont (Deluca et al. 2013). A territorial male Blackpoll Warbler was discovered in Coalbed Swamp in Pennsylvania State Game Lands (SGL) 57, Wyoming County, PA, in July 1993 as part of the county’s Natural Area Inventory, with nesting confirmation occurring the following year (Gross 1994). Singing Blackpoll Warblers were discovered in Tamarack Swamp SGL 57 in 1995, and a second nest was located in Coalbed Swamp. Singing Blackpoll Warblers were also detected at Opossum Brook wetland, South Brook wetland, and the Somers Brook headwaters, all within SGL 57 in 2002 (Gross 2003). The species continues to breed within a few boreal conifer swamps exclusively within SGL 57 in Wyoming and Luzerne counties. Blackpoll Warblers are feeding generalists that consume a wide variety of insects and spiders (Deluca et al. 2013). The Pennsylvania population was discovered during an Ennomos subsignaria (Hubner) (Elm Spanworm) outbreak (Gross 1994). This outbreak began in 1991, in northcentral Pennsylvania, and peaked in 1993– 1994 (Haney 1999). At its height, the outbreak covered 15 counties and defoliated more than 0.5 million ha of forestland, including the area where Blackpoll Warblers were first discovered (Haney 1999, Hoover 2002). Although Blackpoll Warblers are not known as Ennomos spp. specialists, a super-abundant prey outbreak may have been the impetus for colonization of the area as was the case for a Choristoneura sp. (spruce budworm) outbreak (Bolgiano 2004). The Pennsylvania population, while small and isolated, has persisted over the past 22 y since its discovery, with 10–20 breeding pairs every year (Gross 2010). In 2005, after 10 y of confirmed breeding records—a requirement for listing as a state breeding species—the Pennsylvania Game Commission placed the Blackpoll Warbler on the list of Pennsylvania Endangered Species due to the scarcity of both the species and the boreal forest swamps that it occupies (Gross 2010). While the habitat that the Blackpoll Warbler occupies in Pennsylvania has been qualitatively described, there have been no quantitative studies on the structure and composition of the vegetation where they breed (Davis et al. 2001a, Gross 1994). Our objectives were to quantify the breeding habitat of Blackpoll Warblers in Pennsylvania and compare it to conditions in the core portions of the species’ range as well as to determine the possibility of expansion within the state, based on the availability of suitable vegetation types. Field Site We conducted surveys for Blackpoll Warblers in 5 boreal conifer swamps— Coalbed Swamp, Tamarack Swamp, Indefatigable Swamp, Boulder Run Swamp, and Opossum Brook Swamp—all within SGL 57 of the North Mountain area where Blackpoll Warblers had a history of breeding (≥22 years; Fig. 1). These swamps represent some of the largest and most important boreal swamps in Pennsylvania, varying in size from 30 ha to 150 ha and together forming the Dutch Mountains wetland complex (Davis et al. 2001b, Gross 1994). There is no current evidence of Blackpoll Warblers breeding elsewhere in the state despite 2 comprehensive Northeastern Naturalist Vol. 26, No. 1 E.J. Zawatski, D.A. Gross, and M.C. Brittingham 2019 33 Pennsylvania Breeding Bird Atlases (Wilson et al. 2013), searches of several forested conifer swamps across the state by D.A. Gross (unpubl. data), Natural Area Inventories in counties throughout the state (e.g., Davis et al. 2001a), and a Western Pennsylvania Conservancy study of the state’s avian peatland communities (D. Yeany, Western Pennsylvannia Conservancy, Pittsburgh, PA, pers. comm.). The dominant tree species of these swamps are Red Spruce, Black Spruce, Tamarack, and Acer rubrum L. (Red Maple) (Davis et al. 2001b). Canopy openings yield dense shrub and sapling understories consisting of Vaccinium corymbosum L. (Highbush Blueberry), Rhododendron viscosum (L.) Torr. (Swamp Azalea), Ilex mucronata (L.) M. Powell, Savol. and S. Andrews (Mountain Holly), Rhododendron groenlandicum (Oeder), Kron and Judd (Labrador Tea), and Kalmia angustifolia L. (Sheep Laurel). The sites include many herbaceous species of more northerly affinity including Vaccinium sp. (cranberry), Cornus canadensis L. (Bunchberry), Coptis trifolia Salisb. (Goldthread), Gaultheria hispidula (L.), Muhl. ex Bigelow (Creeping Snowberry), Osmundastrum cinnamomeum (L.) C. Presl (Cinnamon Fern), and Sarracenia purpurea L. (Purple Pitcher Plant) (Davis et al. 2001a, 2001b). The groundcover is often a dense mat of living Sphagnum (peat) mosses overlaying a saturated substrate of decomposing peat. These swamps are surrounded by extensive northern hardwood forests where Red Spruce occurs Figure 1. Location of swamps visited for bird surveys to locate breeding Blackpoll Warblers within Pennsylvania, 2016. All locations have a history of breeding Blackpoll Warblers, State Game Lands 57 shaded in gray. Northeastern Naturalist 34 E.J. Zawatski, D.A. Gross, and M.C. Brittingham 2019 Vol. 26, No. 1 naturally (Gross 2010). These wetland communities are classified as either Red Spruce Palustrine Forest or Acidic Shrub Swamp Natural Community with a spruce component (Davis et al. 2001b, Davis 2011, Eichelberger 2011). This community type, while fairly prevalent in the North Mountain region, is not exclusive to that particular area within the state and also occurs in the Pocono Mountain region of northeastern Pennsylvania, as well as a few other scattered locations across the state. (Fig. 2; Davis 2011, Eichelberger 2011). These palustrine forests and woodlands are of mixed age with a variety of age and size classes. The thin soil in the glaciated upland conifer forest and perpetually wet peatland soils contributed to natural canopy openings from blowdowns and other natural perturbations. Standing dead trees, tip-ups, and downed logs are commonplace in these forested wetlands and are often components of the Blackpoll Warbler territories (Gross 2010, Deluca et al. 2013). The oldest living trees in Figure 2. Locations of the 5 peatland communities (black dots) in eastern and central Pennsylvania that often contain Red Spruce and Black Spruce (Black Spruce–Tamarack Peatland Forest, Red Spruce Palustrine Forest, Red Spruce–Mixed Hardwood Palustrine Forest, Black Spruce–Tamarack Palustrine Woodland, and Red Spruce–Mixed Hardwood Palustrine Woodland). Locations mapped by Pennsylvania Natural Heritage Program, 2017. Stars indicate sites where Blackpoll Warblers were found nesting, PA, 2016. The Southern star is Opossum Brook Swamp and the Northern star represents Coalbed and Tamarack Swamp, combined. Northeastern Naturalist Vol. 26, No. 1 E.J. Zawatski, D.A. Gross, and M.C. Brittingham 2019 35 Coalbed Swamp are about 120 y old; trees were last harvested there at the turn of the 20th century (Taber 1970). In contrast, the trees in Tamarack Swamp are much younger—perhaps 70 y old. The last logging railroad in Pennsylvania was constructed to log the virgin spruce of Tamarack Swamp in 1942 (Taber 1970). Historic timber information is not known for Opossum Brook Swamp, but the area would have been last harvested prior to 1942, as Tamarack Swamp was the last to be cut. Although the sites have not been significantly altered since their initial logging, spruce was as much as 5 times more common throughout northern Pennsylvania prior to logging (Gross 2010). Methods Bird surveys We conducted initial site surveys in early June 2016 by systematically walking through each swamp, ensuring we covered the entire area, while listening for singing Blackpoll Warblers. In addition, we made sure to target locations where Blackpoll Warblers had established territories in the past. Indefatigable Swamp had no singing Blackpoll Warblers and had never had any in the past, so we surveyed it only once. Boulder Run also did not have any singing Blackpoll Warblers, but we surveyed it 1 additional time in July because it had a history of Blackpoll Warblers. The other 3 sites had singing males on the initial survey, so we surveyed each of them 6 times, during the height of the breeding season in June and early July 2016, to map the locations of breeding males. When we located a bird, we used a GPS unit to mark the location of the individual, allowing us to conduct a digital form of spot mapping (Ralph et al. 1993, Robbins 1970). Owing to the Blackpoll Warbler’s persistent singing during the breeding months, playback was generally not needed and we used it only if we failed to hear or see Blackpoll Warblers during the first 5–10 min of our visits. We conducted surveys in the early morning hours to take advantage of when the Blackpoll Warblers were most actively singing and ended them by 1000–1100 EDT. We did not surevy on days with rain, strong wind, or other inclement weather. Once a territorial Blackpoll Warbler was located, we recorded the GPS coordinates of its location for future visits to assess the breeding activity, to delineate the center of the territory, and for post-breeding–season vegetation surveys. We determined the center of the territory through repeated visits (n = 6) and digital spot-mapping of the territorial males. Territories of Blackpoll Warblers are fairly small (0.4–1.85 ha; Deluca et al. 2013), so we were able to visually determine if observations were of 1 or multiple birds, since it is possible to detect more than 1 male at a time. Vegetation surveys We conducted vegetation measurements at each of the Blackpoll Warbler territories in August and September 2016, after the breeding season, using a modified BBIRD protocol (Martin et al. 1997). We established four 11.3-m–radius plots within each territory: 1 located at the center of the territory, and 3 others located 35 Northeastern Naturalist 36 E.J. Zawatski, D.A. Gross, and M.C. Brittingham 2019 Vol. 26, No. 1 m from the plot center at angles of 0°, 120°, and 240°. Using the same plot center, we also established a single 5-m–radius subplot within each of the 11.3-m plots. Within each 11.3-m–radius plot, we used a clinometer to measure canopy height and a diameter at breast height (dbh) tape to determine the dbh of all canopy trees. We estimated canopy coverage visually and identified all tree species and estimated their relative percentage in the canopy. We averaged the values for the 4 individual plots to estimate overall territory parameters. We used the territory as the sample unit to calculate the metrics for the overall state-wide population. Within the 5-m–radius subplots, we estimated groundcover (% cover) and understory cover and height. We visually estimated the groundcover, defined as the percent of ground covered by both living and non-living material that was less than 50 cm tall, including grasses and sedges, moss, woody debris, water, rock, and litter/duff. We measured understory cover, defined as shrubs and trees less than 3 m tall, via ocular estimates, and recorded percent cover by species and overall height. Results We identified 15 different territories distributed over 3 swamps: Coalbed (n = 7), Tamarack (n = 7), and Opossum Brook (n = 1) (Fig. 3). Blackpoll Warbler territories varied in elevation from 627 m to 686 m. Overall, Blackpoll Warbler territories had Figure 3. Distribution of 15 Blackpoll Warbler territories (white dots). (A) Opossum Brook Swamp and (B) Coalbed Swamp (south) and Tamarack Swamp (north), PA, 2016. Northeastern Naturalist Vol. 26, No. 1 E.J. Zawatski, D.A. Gross, and M.C. Brittingham 2019 37 relatively open canopies (mean = 42%) and were dominated by conifers with 75% of the forest canopy composed of spruce (Table 1). Red Maple was the most common deciduous tree (Table 1). Tree species distribution and canopy metrics were not uniform across the 3 swamps (Table 1). For example, Black Spruce comprised 65% of the canopy within Tamarack Swamp, but much less (19%) in Coalbed Swamp. Red Spruce formed the majority of the canopy within Coalbed (52%), but only 17% of the Tamarack Swamp canopy. Trees on Blackpoll Warbler territories were relatively small, with a mean dbh of 13 cm (Table 1). The dbh varied greatly among territories; Coalbed Swamp had the largest trees (Table 1). Mean canopy height was 6 m and varied among the 3 swamps from 5 m to 8 m (Table 1). Most territories also possessed a few trees and snags that rose well above the main canopy (Fig. 4). The understory was thick and densely vegetated (Table 2). There were differences among the sites. Coalbed Swamp had markedly lower understory coverage, a much higher spruce-sapling component, and a higher understory-height than the other 2 swamps (Table 2). Tamarack Swamp had the densest understory that consisted primarily of Mountain Holly and Highbush Blueberry, with very little spruce-sapling cover (Table 2). Peat moss was the primary groundcover, with an overall mean coverage of 68%. Table 1. Characteristics (mean ± SE) of canopy trees measured within 15 Blackpoll Warbler territories in Pennsylvania 2016. n = number of territories. Values for each territory were calculated as the mean of four 11.3-m–radius plots within each territory. Coalbed Opossum Brook Tamarack Overall Variable (n = 7) (n = 1) (n = 7) (n = 15) Canopy cover (%) Overall 55 ± 2.3 35 ± 2.0 30 ± 2.0 42 ± 2.1 Black Spruce 19 ± 2.8 0.0 ± 0.0 65 ± 4.2 39 ± 4.0 Red Spruce 52 ± 4.9 50 ± 3.5 17 ± 4 36 ± 3.6 Tamarack 12 ± 3.0 38± 3.2 0 ± 0.0 8 ± 1.9 Red Maple 17 ± 1.3 13 ± 2.5 18 ± 1.5 17 ± 1.0 DBH (cm) 16 ± 0.2 10 ± 0.4 9 ± 0.2 13 ± 0.2 Canopy height (m) 8 ± 0.3 7 ± 0.4 5 ± 0.2 6 ± 0.2 Table 2. Characteristics (mean ± SE) of understory vegetation measured within 15 Blackpoll Warbler territories in Pennsylvania 2016. n = number of territories. Values for each territory were calculated as the mean of four 11.3-m–radius plots within each territory. Coalbed Opossum Brook Tamarack Overall Variable (n = 7) (n = 1) (n = 7) (n = 15) Understory cover (%) Overall 63 ± 3.3 78 ± 3.2 95 ± 1.5 78 ± 2.5 Mountain Holly 26 ± 3.4 49 ± 5.5 45 ± 2.9 36 ± 2.5 Highbush Blueberry 28 ± 2.7 31 ± 3.1 34 ± 2.2 31 ± 1.7 Spruce saplings 39 ± 3.1 13 ± 4.3 3 ± 1.2 21 ± 2.7 Understory height (cm) 210 ± 10 170 ± 10 180 ± 10 190 ± 10 Northeastern Naturalist 38 E.J. Zawatski, D.A. Gross, and M.C. Brittingham 2019 Vol. 26, No. 1 Discussion Breeding Blackpoll Warblers in Pennsylvania have occurred most consistently in Tamarack Swamp and Coalbed Swamp, with Coalbed Swamp having breeders annually since first discovered (Gross 2010; D.A. Gross, unpubl. data). These peatlands are among the largest boreal swamps in Pennsylvania and contain not only rare tree species, but additional northern herbaceous species that are rare in the state (Davis et al. 2001b). In addition to Blackpoll Warblers, these swamps support other state-rare bird species with northern affinities, such as Empidonax flaviventris (Baird) (Yellow-bellied Flycatcher), Parkesia noveboracensis (Gmelin) (Northern Waterthrush), Cardellina canadensis (L.) (Canada Warbler), Zonotrichia albicollis (Gmelin) (White-throated Sparrow), Aegolius acadicus (Gmelin) (Northern Sawwhet Owl), Catharus ustulatus (Nuttall) (Swainson’s Thrush), and Oreothlypis ruficapilla (Wilson) (Nashville Warbler) (Gross 1994, 2010). The persistence and occupancy of this community of northern bird species on this SGL over an extended period of time highlights the importance of this unique ecosystem. The habitat occupied by this disjunct breeding population of Blackpoll Warblers was similar to habitat in the core portions of the Blackpoll Warbler range; it was quantitatively similar to the subalpine populations of the northeastern US and similar in species composition to the boreal forest populations across northern Canada (Deluca et al. 2013). For example, similar to territories in Pennsylvania, stands Figure 4. Image of typical Blackpoll Warbler habitat in Pennsylvania at Tamarack Swamp, PA. July 2016. Northeastern Naturalist Vol. 26, No. 1 E.J. Zawatski, D.A. Gross, and M.C. Brittingham 2019 39 occupied by Blackpoll Warblers on Mt. Moosilauke in New Hampshire averaged 6 m in height and had 57% overall canopy cover, the majority of which was small conifers (9 cm mean dbh) (Sabo 1980). However, a major contrast between Pennsylvania Blackpoll Warbler breeding territories and others in the northeastern US and southeastern Canada is the almost complete absence of Balsam Fir at Pennsylvania locales. In New Brunswick, Balsam Fir comprised 63% of all trees in Blackpoll Warbler territories; in the White Mountains of New Hampshire it made up 60–97% of all trees in territories above 950 m (Deluca et al. 2013, Morse 1979). The vegetative type occupied by Blackpoll Warblers in Pennsylvania more closely resembles their habitat throughout boreal Canada—Black Spruce and Tamarack forests and swamps—than it does the montane Balsam Fir, birch, and Prunus pensylvanica L.f. (Pin Cherry) forests it occupies in the northeastern US and southeastern Canada (Deluca et al. 2013). The combination of a few tall trees extending above the main canopy and a dense understory appear to be important components of Blackpoll Warbler territories. We located territories by mapping singing males and noted, similar to other studies, that males often sang and foraged on the taller, older trees that towered above the canopy (Deluca et al. 2013). Females were generally more difficult to observe, as they stayed hidden in the dense understory that is also used for nesting. For example, Blackpoll Warbler nests discovered in Pennsylvania tend to be well concealed in thickets of Mountain Holly, blueberry, and spruce. Several locations in New York also reported that Blackpoll Warbler territories were associated with dense, low vegetation with sparse trees (Peterson 1988). Blackpoll Warbler sites in Pennsylvania were at lower elevations than the nearest breeding populations in the Catskill Mountains of New York, where Blackpoll Warblers are found above 1000 m (Bull 1974, McFarland and Rimmer 2008). The New York state spruce and fir forests that support Blackpoll Warblers are usually on tabletop mountains. In contrast, the Pennsylvania sites are headwater swamps in basins that have a cool microclimate from cold air drainage and dense conifer cover, and perhaps, from the peatland soil. However, Blackpoll Warblers in New York are not restricted to high-elevation sites; breeding birds in the more northerly Adirondacks have been reported at elevations as low as 305 m (Peterson 1988). Despite the restricted breeding location of Blackpoll Warblers in Pennsylvania, the vegetation data suggest that there are potential breeding sites for Blackpoll Warblers in other areas of Pennsylvania, including the Pocono Mountain region (Fig. 2). The Poconos contain a number of quality Black Spruce, Red Spruce, and Tamarack swamp and forest communities that are similar in species composition to the currently occupied ones within SGL 57 (Davis 2011, Davis et al. 2001a). There also are a limited number of Balsam Fir stands with which Blackpoll Warblers are associated in other parts of their range. However, a possible deterrent to Blackpoll Warbler occupancy in the Poconos may be higher levels of forest fragmentation, as the Pocono Plateau forest is dissected by housing development and roads to a much higher degree than the North Mountain region (Gross 2010). There have been reports of Blackpoll Warblers Northeastern Naturalist 40 E.J. Zawatski, D.A. Gross, and M.C. Brittingham 2019 Vol. 26, No. 1 singing in the Pocono Mountains, but breeding activity has not been noted in these instances. These individuals may be late-migrants, especially since Blackpoll Warblers are among the latest spring migrants, or floating males that were unable to attract a mate (Deluca et al. 2013, McWilliams and Brauning 2000). Although the Pennsylvania population of Blackpoll Warblers has remained relatively stable since its discovery, it has also remained restricted to a relatively limited breeding area. Future studies should examine what habitat attributes are different between the North Mountain Region and the Poconos to better understand why Blackpoll Warblers are restricted to the North Mountain region. In addition, it is important to conserve and enhance the boreal swamps they are currently occupying as well as the areas they may possibly colonize in order to ensure that Blackpoll Warblers and other boreal conifer-forest birds continue as breeders in Pennsylvania. The Pennsylvania population represents the southernmost breeding location for Blackpoll Warblers; thus, this population is at risk for extirpation in response to climate change (Hampe and Petit 2005, Ralston and Kirchman 2013). Acknowledgments Funding for this project was provided by the Pennsylvania Game Commission, the Pennsylvania State University College of Agricultural Sciences Undergraduate Research Program, and the USDA National Institute of Food and Agriculture Federal Appropriations under Project PEN04563 and Accession number 1002595. The findings and conclusions of this study do not necessarily reflect the views of the funding agencies. We thank Tony Davis, Ephraim Zimmerman, David Yeany, and Christopher Tracey of the Western Pennsylvania Conservancy for the distribution map showing peatland communities in Pennsylvania (Fig. 2). Literature Cited Bolgiano, N.C. 2004. Cause and effect: Changes in boreal bird irruptions in Eastern North America relative to the 1970s Spruce Budworm infestation. American Birds 58:26–33. Bull, J. 1974. Birds of New York State. Doubleday/Natural History Press, Garden City, NY. 655 pp. Davis, A.F., J.A. Lundgren, B. Barton, J.R. Belfonti, J.L. Farber, and J.R. 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