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Eastern Phoebe Breeding-range Expansion into the Pee Dee Region of South Carolina
Douglas B. McNair

Southeastern Naturalist, Volume 16, Issue 4 (2017): 516–528

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Southeastern Naturalist D.B. McNair 2017 Vol. 16, No. 4 516 2017 SOUTHEASTERN NATURALIST 16(4):516–528 Eastern Phoebe Breeding-range Expansion into the Pee Dee Region of South Carolina Douglas B. McNair* Abstract - Sayornis phoebe (Eastern Phoebe) has expanded its breeding range southeasterly at water-based anthropogenic structures along forested streams in the Pee Dee region of South Carolina. Its current range is truncated in the Upper Coastal Plain, 31–35 km below the lower boundary of the Sandhills subregion (Orangeburg Scarp). The predominance in the Upper Coastal Plain of South Carolina of structures that are less suitable for nesting by Eastern Phoebes (e.g., small bridges without ledges, box culverts), in contrast to preferred structures (i.e., small bridges with ledges) available upriver in the Sandhills and eastern Piedmont of North Carolina, has not limited their colonization (~2 km y-1 since ca. 1990). This breeding-range expansion is consistent with a correlative ecological-niche model (ENM) prediction that the Eastern Phoebe is expanding its breeding range in the Pee Dee region across a broad front toward the SC coast. This breeding-range expansion, with a slight drop in latitude and elevation, has been slower than the contemporary expansion of Petrochelidon pyrrhonota (Cliff Swallow) in the same direction in the same region. Hirundo rustica (Barn Swallow) uses water-based anthropogenic structures, but has also widely colonized land-based structures during its faster past breeding range-expansion into the Pee Dee region. Differences in the number, size, and suitability of water-based anthropogenic structures used by these 3 species suggest additional constraints that influence the colonization rate and population size of Eastern Phoebes. Introduction Sayornis phoebe Latham (Eastern Phoebe, hereafter, Phoebe) has expanded its breeding range for over 40 years in southeastern North America, primarily nesting at water-based anthropogenic structures along streams within woodland corridors or forest (McNair 1984, 2016; Weeks 2011). Phoebes in south-central North Carolina utilize small bridges with ledges that support statant nests over water as their preferred nest-site type (McNair 1984, 2016). The breeding-range front of the Phoebe in adjoining South Carolina had reached the Sandhills subregion in the Upper Coastal Plain by the late 1980s (Cely 2003, McNair and Post 1993); several pairs nested near the southern boundary of the Sandhills subregion within the protruding uppermost alluvial plain of the Great Pee Dee River at Cheraw and Wallace, SC, where Phoebes have used other water-based anthropogenic nest-site types (McNair 1990). Over the last 25–30 years, the Phoebe breeding range has continued to slowly expand southeasterly along forested streams in the Pee Dee region of South Carolina and, recently, Phoebes began infrequently nesting at land-based anthropogenic structures in the Sandhills subregion of North Carolina (McNair and Campbell 2013). Phoebes have only rarely been reported nesting at *35 Rowell Road, Wellfleet, MA 02667; dbmcnair@gmail.com. Manuscript Editor: Jason Davis Southeastern Naturalist 517 D.B. McNair 2017 Vol. 16, No. 4 a few land-based sites below the southern boundary of the Sandhills subregion in the Upper Coastal Plain of South Carolina (hereafter, Upper Coastal Plain), with 1 outlier site in the Lower Coastal Plain (Horry County), but breeding at these sites has not persisted (see McNair and Campbell 2013). In the Pee Dee region of the extreme eastern Piedmont and Sandhills of southcentral North Carolina, McNair (1984) documented that the mean height above water at the center of small bridges with ledges (from the floor of structure) was significantly greater at bridges with nests in the Piedmont compared with those in the Sandhills. However, bridges with nests were still higher compared to bridges without nests in the Sandhills. The mean difference in height between bridges with and without nests in both areas was 0.4–0.5 m. In addition, the covarying height from the bottom of the nest to water was significantly higher in the Piedmont compared to the Sandhills (mean difference = 0.5 m). Despite these differences in height between both areas, Phoebes nesting at small bridges with ledges were not proportionally more numerous in the Piedmont. This pattern suggests that among small bridges, higher bridges were not an advantage as long as bridges in both physiographic provinces were high enough to offset threats to nest success such as flooding, which may be a significant cause of nest failure in some years (Weeks 1979, 2011). Hydrogeological differences of streams and rivers in the Pee Dee region, such as the greater capacity to absorb rainfall in the Sandhills compared to the Piedmont (Feaster et al. 2009, Wachob et al. 2009, Weaver et al. 2009) can reduce susceptibility to flooding and thus buffer differences in mean height above water of anthropogenic structures and mean height of nests above water in these 2 physiographic provinces. Otherwise, nest-site availability and suitability may constrain the breeding distribution and abundance of Phoebes in forested riparian habitat (Hill and Gates 1988, Weeks 2011). In this study, I documented Eastern Phoebe breeding at water-based anthropogenic structures in the Upper Coastal Plain of the Pee Dee region of South Carolina below the southern boundary of the Sandhills subregion (delimited by the Orangeburg Scarp; see Fig. 1 in Rovere et al. 2015). I determined whether water-based anthropogenic structures used by Phoebes in the Upper Coastal Plain are different from those used in the adjacent Pee Dee region of south-central North Carolina, where breeding populations in 2 different physiographic provinces (Piedmont, Sandhills) are fully or almost fully saturated (McNair 1984, 2016). To reduce susceptibility of nests to flooding in the Upper Coastal Plain of South Carolina, I expected that height above water within each type of water-based anthropogenic structure (e.g., small bridge without ledges) that contained Phoebe nests would be ~0.4–0.5 m greater compared to structures without nests (McNair 1984). I also expected that most structures with nests would be located (1) closer to the forest edge compared to structures without nests because Phoebes prefer closed rather than more-open habitats (Schukman 1993; Schukman et al. 2011; Weeks 1984, 2011), (2) on intermediate Strahler stream orders (McNair 1984, 2016; Schukman 1993, Schukman et al. 2011), and (3) on older structures because weathered surfaces on Southeastern Naturalist D.B. McNair 2017 Vol. 16, No. 4 518 concrete provide better attachment areas for nests (McNair 2016, Weeks 2011). I also evaluated the prediction from an ENM, which omitted nest-site information, that the Phoebe breeding range should reach the South Carolina coast (Schukman et al. 2011). I compared and contrasted the contemporary breeding range expansion of the Phoebe to Petrochelidon pyrrhonota Vieillot (Cliff Swallow), which has wholly relied on water-based anthropogenic structures in the Pee Dee region of South Carolina (McNair 2013). This is the only study that has closely examined the concurrent breeding-range expansion of these 2 species from 1 region in southeastern North America. Finally, I also compared and contrasted both of these species’ ongoing breeding-range expansions into the Pee Dee region of South Carolina to the earlier breeding-range expansion in this same region of Hirundo rustica L. (Barn Swallow), the 3rd major species in southeastern North America (as in Indiana; Weeks 1984) that uses water-based anthropogenic structures, but which has also widely colonized land-based anthropogenic structures. Methods Field-site description The study area extended from the North Carolina state line west to the Lynches River in the Upper Coastal Plain of the Pee Dee region of South Carolina, south to Dillon and Florence counties, and comprised in part or in whole those 2 counties as well as 3 others (Chesterfield, Darlington, and Marlboro; Fig. 1). The study area is predominantly very rural in character, with the city of Florence the largest urban area (1 July 2016 population estimate = 38,317; US Census Bureau 2017). The study area lies within a portion of the Pee Dee River Basin (Cooke 1936, Wachob et al. 2009) that contains portions of 3 sub-basins: 1 dominated by the Great Pee Dee River, and 2 smaller sub-basins dominated, respectively, by the Lynches River that also originates outside the coastal plain and by the Little Pee Dee River that originates within the Upper Coastal Plain in Marlboro County. The largest tributary of the Great Pee Dee River is Black Creek, which flows through the more urbanized part of this sub-basin in the cities of Darlington and Florence. Stream classification follows the Strahler stream order, which is used to define permanently flowing stream size based on a hierarchy of tributaries. Stream order increases when streams of the same order intersect. The index of a stream or river may range from 1 (a stream with no tributaries) to higher orders. I determined stream order at Phoebe breeding sites from county maps that illustrated extensive tributary networks (Puetz 1990a, 1990b). Surveys I surveyed all accessible water-based anthropogenic structures in the defined study area from 4–11 May 2016, ignoring circular culverts, which Phoebes do not use for breeding (McNair 2016). This survey included 20 of 29 water-based anthropogenic structures in Darlington, Dillon, and Marlboro counties that I had examined during December 2012. I also sampled areas ~20 km beyond the breedingrange front, including northern Marion County but excluded measurements from Southeastern Naturalist 519 D.B. McNair 2017 Vol. 16, No. 4 these water-based anthropogenic structures because I could not assume Phoebes had used them during the breeding season. I recorded structural type and 3 structural and nest-site characteristics as follows: (1) height above water of the floor of the structure at its center (m), (2) distance from bottom of nest to water (m), and (3) shortest perpendicular distance from structure to either side of forest edge (m). I used Google Earth 2015 (version 7.1.2) to measure the straight-line distance (km) of the farthest water-based anthropogenic structures where nesting occurred to the nearest point along the southern boundary of the Sandhills subregion (Orangeburg Scarp). Data analysis I used descriptive statistics to document all water-based anthropogenic structures with and without Phoebe nests below the Orangeburg Scarp in the Upper Coastal Plain of the Pee Dee region of South Carolina. I combined data from both years (2012, 2016), but eliminated duplicate data at the same structure Figure 1. Location of 13 confirmed breeding sites (2012, 2016) of the Eastern Phoebe in the Upper Coastal Plain of the Pee Dee region (4 counties) of South Carolina at and near the range-expansion front. Small filled squares represent small bridges without ledges in 1 or both years, filled diamonds represent box culverts in 2016, and the filled circle represents a large bridge in 2012. The hatched black segmented line (Orangeburg Scarp) represents the division between the lower boundary of the Sandhills subregion and the remainder of the Upper Coastal Plain. The thick, black solid line represents the Great Pee Dee River and the thinner black solid lines represent the Little Pee Dee and Lynches rivers. Southeastern Naturalist D.B. McNair 2017 Vol. 16, No. 4 520 from shared years; I used only 2012 data from NC (McNair 2016). For more advanced analyses, I eliminated 6 sites in South Carolina where Phoebes nested at a structure in 1 year but not the other because these data were not independent. Otherwise, all data were independent but did not always follow a normal distribution. Consequently, I used the Real Statistics resource pack for Excel (www. real-statistics.com) to perform 2-tailed non-parametric tests (Mann-Whitney U; Kruskal-Wallace H). I examined differences in height above water (from the floor of structure) at the center of the structure among the 3 physiographic provinces (Upper Coastal Plain, Sandhills, Piedmont) at structures where Phoebes did not breed in the Pee Dee region of North and South Carolina (Kruskal-Wallace Htest). I also examined differences in height at structures where Phoebes nested although small samples required pooling data for the Sandhills and Piedmont physiographic provinces (Mann-Whitney U-tests). Within the Upper Coastal Plain of South Carolina, I also used Mann Whitney U-tests to examine differences in height above water (from the floor of structure) at the center of the structure and differences in shortest perpendicular distance from structure to either side of forest edge at structures with and without Phoebe nests. Results Nest-site types In 2016, all Phoebe nests at water-based anthropogenic structures in the Upper Coastal Plain of the Pee Dee region of South Carolina were built on concrete, either at small bridges without ledges (n = 6 of 61; 9.8%) or within box culverts (n = 2 of 14; 14.3%), for a total occupation of 8 of 80 (10%) available water-based anthropogenic structures. Phoebes did not nest on large bridges (n = 4) or a small bridge with ledges (n = 1). In terms of structure age, of the 52 structures built before 1979, seven (13.5%) were used by Phoebes for breeding; of the 26 structures built after 1979, one (3.8%)—a bridge constructed in 2000—was used by Phoebes for breeding . One Phoebe nest was built atop a Barn Swallow nest, and this was the only Phoebe nest that was placed over ground; nest reciprocity was otherwise not observed. Only small bridges without ledges with or without nests were numerous enough to examine differences in height above water at center of structure between the 3 physiographic provinces. At sites where Phoebes did not breed, the median height at center of bridges without ledges (from the floor of structure) was significantly lower in the Upper Coastal Plain compared to the Sandhills or Piedmont physiographic provinces (Kruskal-Wallis H-test = 9.24, df = 2, P = 0.01; Table 1). Similarly, at sites where Phoebes did breed, the median height at center of bridges without ledges was also significantly lower in the Upper Coastal Plain compared to the Sandhills and Piedmont physiographic provinces combined (Mann-Whitney U-test = 16, P = 0.03; Table 1). However, my analyses detected no significant differences between the median height at center of bridges without ledges (from the floor of structure) in the Upper Coastal Plain between structures where Phoebes did or did not breed (Mann-Whitney U-test = 327, P = 0.96). The difference in the Southeastern Naturalist 521 D.B. McNair 2017 Vol. 16, No. 4 proportion of breeding sites at small bridges without ledges in the Upper Coastal Plain compared to the Sandhills and Piedmont provinces combined was not significant (χ2 = 0.56, df = 1, P = 0.28), although the proportion was lower in the Upper Coastal Plain (13.2% vs. 21.1%). The difference in the distance from bottom of nest to water between the Upper Coastal Plain and the Sandhills and Piedmont of North Carolina was not significant (Mann-Whitney U-test = 10.5, P = 0.07), although the median distance in the Upper Coastal Plain was considerably lower (1.5 m, min– max = 0.9–3.6 m; n = 7) compared to the Sandhills and Piedmont (2.7 m, min–max = 1.45–4.1 m; n = 7). Within the Upper Coastal Plain, the median perpendicular distance from structure to either side of forest edge was significantly smaller for structures with Phoebe nests compared to structures without Phoebe nests (with nests: median = 8 m, min–max = 4–10 m; n = 9; without nests: median = 10 m, min–max = 4–30 m; n = 60; Mann-Whitney U-test = 119.5, P = 0.005). Creeks and rivers The 8 breeding sites in 2016 were located along 5 of 40 (12.5%) creeks and rivers that contained anthropogenic structures examined for nesting Phoebes. These 5 creeks and rivers were Beaverdam Creek (at McNairs Mill Pond; n = 1), Panther Creek (n = 1), and Three Creek (n = 2) in Marlboro County, Little Pee Dee River (n = 3) in Dillon County, and High Hill Creek (n = 1) in Florence and Darlington counties. Thus, 24 water-based anthropogenic structures along these 5 creeks and rivers contained 8 nests (33%), in contrast to no nests at the other 56 anthropogenic structures along the other 35 creeks and rivers. Seven of the 8 breeding sites in 2016 occurred along secondary roads (n = 51; 13.7%), whereas 1 breeding record occurred along a primary road (n = 29; 3.4%). All breeding sites were in rural areas, except for a secondary road that crossed High Hill Creek in an exurb (i.e., a semirural area not highly developed lying just beyond the suburbs of a city). Supplemental 2012 data Three of the 8 structures that contained nests in South Carolina in 2016 also contained nests in 2012. Five structures not used in 2016 were used in 2012. Thus, a total of 13 structures were used over the 2 years (Fig. 1). The 4 additional creeks where Phoebes nested in 2012 were Black Creek (n = 2) and Hurricane Branch (n = 1) in Darlington County, Gum Swamp Creek (headwaters of the Little Pee Dee River; Table 1. Height at center (m) of small bridges without ledges (from the roof of structure) with and without Eastern Phoebe nests in the Pee Dee region of North and South Carolina. Median Interquartile Min–max Physiographic province State n height (m) height (m) height (m) Without nests Upper Coastal Plain SC 66 2.10 1.60–2.90 0.9–5.0 Sandhills SC, NC 18 2.70 2.40–3.20 1.4–3.9 Piedmont NC 12 2.93 2.35–3.85 1.6–6.0 With nests Upper Coastal Plain SC 10 2.00 1.60–2.50 1.4–4.5 Sandhills and Piedmont NC 8 3.35 2.73–3.95 2.4–5.0 Southeastern Naturalist D.B. McNair 2017 Vol. 16, No. 4 522 n = 1) in Marlboro County, and Shoehill Creek (n = 1) in Dillon County. One of the structures in an urbanized area along Black Creek was a large bridge (built in 1938), not used in 2016. The 4 other structures were small bridges without ledges in rural areas; 3 of them were built after 1979 (in the early 1990s). Over both sample years combined, Phoebes nested at 13 of 89 (14.6%) structures in the Upper Coastal Plain of the Pee Dee region of South Carolina, including 6 sites in the larger Great Pee Dee River sub-basin and 7 sites in the Little Pee Dee River sub-basin (Fig. 1). I discovered no nests within the defined study area in the Lynches River sub-basin. Strahler stream order at the 13 Phoebe breeding sites varied from 2 to 5 (mode = 4; n = 7). Distance to breeding-range front The 2 farthest nest sites, along High Hill Creek and the Little Pee Dee River in the Upper Coastal Plain of South Carolina at the breeding-range front were located 31–35 km away from the lower boundary of the Sandhills subregion (Orangeburg Scarp; Fig. 1). The distance between these 2 sites was 51 km, spanning both sides of the Great Pee Dee River. Thus, the mean colonization rate since ca. 1990 has been ~2 km yr-1. Discussion Breeding-range expansion As expected, during my study, Phoebes in the Upper Coastal Plain of the Pee Dee region of South Carolina generally bred in rural areas along narrower secondary roads closer to forest edges at older, small bridges (without ledges) and box culverts along larger tributaries. As in Kansas (Schukman 1993, Schukman et al. 2011), suitable water-based anthropogenic structures along woodland corridors were scarcer in upper reaches of drainage basins. Compared to the adjacent Piedmont and Sandhills provinces of NC, Phoebes nested at lower bridges and built nests at lower heights above water in the Upper Coastal Plain of SC, but contrary to expectations, I detected no differences in height above water of the floor of the structure at its center for bridges with and without nests. The median height of end walls above water or ground of small bridges without ledges with or without nests was 1.8 m (D.B. McNair, pers. observ.), 0.86–0.90% of their median height at the center of the bridge (from the floor of structure). This proportion is much higher than that at sites in the Sandhills and Piedmont (1.5-m height at end walls) where topographic relief along channels at most stream crossings is steeper and more incised. Thus, more water typically flows underneath structures in proportion to their length in the Upper Coastal Plain versus the Sandhills and Piedmont. This topographic effect provides more choices for Phoebe nest placement because the birds rarely nest over ground at water-based anthropogenic structures (D.B. McNair, pers. observ.; Weeks 2011). Phoebes in the coastal plain are adapting to these lower structures, which often have a higher proportion of water underneath them, but the structures still have to have enough clearance and be of a certain height. With the exception of nests 0.3 m and 0.6 m above water at the same small bridge site during Southeastern Naturalist 523 D.B. McNair 2017 Vol. 16, No. 4 2 consecutive years (McNair 1990), I found no other nests at a small bridge without ledges in the Upper Coastal Plain of SC less than 1.4 m above the water. The Phoebe breeding population in the Pee Dee region of the Upper Coastal Plain of SC is slowly expanding along a limited number of creeks and rivers, regardless of the virtual absence of preferred nest-sites (i.e., small bridges with ledges; D.B. McNair, pers. observ.). Nest reuse or appropriation of Barn Swallow nests by Phoebes was rare, a negligible factor in their range expansion. Phoebes failed to use many structures that cross larger tributaries in the Upper Coastal Plain that appeared to be suitable for nesting—high enough above water and close to the forest edge (Weeks 1984). The small breeding population is probably unsaturated, and future surveys within the current range can document annual population fluctuations. Regardless, within ~20 km beyond the breeding-range front in the Pee Dee region, especially along and near the Little Pee Dee River where the range expansion is most concentrated, I expect in future that Phoebes will colonize suitable small bridges without ledges (and box culverts). Similar to Cliff Swallows in southeastern North America (McNair 2013), available evidence suggests that the dispersal route of breeding Phoebes has consisted primarily of an interconnecting network of river basins. Creeks and rivers function as habitat corridors (= networks; Saura et al. 2014), connecting nodes of stable water-based anthropogenic structures (except when structures are replaced). The smooth shape of the local breeding-range front in the Pee Dee region of South Carolina is similar to the smooth shape of the statewide breeding-range front of ~25 years ago (Cely 2003). The smooth shape, slow rate of range expansion, and some site fidelity in the Pee Dee region generally support the stepwise model of range expansion for this population of Phoebes (Fortin et al. 2005, Saura et al. 2014), not a leapfrogging pattern (see McNair 2013). Stepwise dispersal generates a fragmented patchwork of high- and low-frequency areas (Ibrahim et al. 1996) compared to the leapfrogging pattern, where dispersal in many species of plants and animals is leptokurtic with more pronounced patchiness. The local pattern of Phoebe occurrence within the study area, despite some apparent sequential linear movements especially along and near the Little Pee Dee River, contains some large gaps between confirmed breeding sites. I expect that there will be some back-filling in the future (see above paragraph). I also predict that Phoebes will slowly continue their southeasterly range expansion; the range edge is ~30 km away from the Surry Scarp, the last significant wave-cut scarp which separates the Middle Coastal Plain from the Lower Coastal Plain (Wachob et al. 2009). Although the Lower Coastal Plain is heavily forested, the decrease in elevation in this area where topographic relief is nearly flat to flat results in increasingly limited choices of suitable waterbased anthropogenic structures towards the Atlantic Coast, with greater distances expected between colonized sites. Ecological niche model The correlative ENM developed with genetic algorithm for rule-set prediction (GARP; Schukman et al. 2011; also see Peterson et al. 2011), applied at the coarse Southeastern Naturalist D.B. McNair 2017 Vol. 16, No. 4 524 continental-scale, uses 2 sets of predictor variables (vegetation indices and climatic variables). This tool predicts high suitability for Phoebe breeding across the southeast Atlantic coastal plain on a broad front including South Carolina (Schukman et al. 2011). Results from this study (also see McNair 2016) are consistent with the ENM prediction that the Eastern Phoebe is expanding its breeding range in the Pee Dee region across a broad front toward the SC coast. Applying the mean rate of expansion (~2 km y-1) in the Pee Dee region, Phoebes would expand their range ~115 km from the current breeding-range front to the SC coast in ~57 years (2073). If suitable water-based anthropogenic structures for nesting Phoebes are not available, particularly in the Lower Coastal Plain, Phoebes may not reach the coast even if environmental conditions are suitable. The basis for this ENM prediction relies on accurate Breeding Bird Survey (BBS) data from the southeast Atlantic coastal plain including South Carolina. Analyses using BBS data were biased toward positive breeding (summer) outcomes, especially for species at the edge of their range where positive effects of presence (one bird on a route in any year; Peterson 2001, Schukman et al. 2011) would be magnified. This would constitute a commission error, i.e., including areas actually uninhabited by breeding Phoebes that would lead to over-prediction of their breeding range if the environmental conditions are actually unsuitable, which has been a recurring weakness using GARP for ENMs (Peterson et al. 2007). Below the Sandhills subregion, only 6 of 21 BBS routes in the South Carolina coastal plain have documented presence of Phoebes (Sauer et al. 2015; D.B. McNair, pers. observ.). Results from 2 of these routes, located in the Upper Coastal Plain of the Pee Dee region (McNair 2016), are consistent with results in this study. The other 4 BBS routes, all outside the Pee Dee region, are located in the Lower Coastal Plain. These early BBS data from South Carolina were used by Schukman et al. (2011:374, fig. 1), but the data are suspect and were not recognized as representing valid breeding records for the Eastern Phoebe (Cely 2003, McNair and Post 1993). Thus, my current local independent data set (see Peterjohn 2001) from the Pee Dee region, the earlier regional independent data set (South Carolina breeding bird atlas; Cely 2003), and my critique herein casts doubt upon the ENM prediction that conditions are suitable for breeding Phoebes to reach the South Carolina coast. A re-analysis of an ENM with corrected BBS data in South Carolina (and perhaps other states in the southeast Atlantic coastal plain) should reduce the size of geographic units (50 km2) in accordance with the slow range expansion (even for a species with a large range such as Eastern Phoebe) to capture and improve precision of the predicted distribution. The re-analysis can then examine a fine local-scale or regional-scale rather than coarse continental-scale prediction that breeding Phoebes will reach the South Carolina coast. Compared to cooler temperatures in other geographic regions where Eastern Phoebes breed, temperatures are generally warmer in southeastern North America where water-based anthropogenic sites such as small bridges provide cool microclimates (protection from direct sun rays, lower temperatures along creeks). Refinement from coarse-scale to local-scale or Southeastern Naturalist 525 D.B. McNair 2017 Vol. 16, No. 4 regional-scale niche conditions in the coastal plain should incorporate 2 more sets of predictor variables (both abiotic) on (1) the availability and suitability of waterbased and land-based anthropogenic structures for breeding and (2) microclimatic variables (e.g., ambient temperature, wet-bulb temperature) at these anthropogenic structures, if this can be achieved using ENMs. This proposed refinement, which incorporates new conditions in ecological space to build a more realistic estimate of this species’ niche and corresponding geographic distribution (cf. Soley-Guardia et al. 2014), can probe for any climatic conditions behind the general scarcity of breeding at buildings in the coastal plain where Phoebes are expanding their breeding range (McNair 2016 and references cited therein), in contrast to ready use of buildings in upland habitats throughout most of their breeding range (Weeks 2011). Fine local-scale or regional-scale re-analysis may reveal that as breeding Phoebes approach the South Carolina coast, predicted climatic conditions from an ENM may transition from high suitability to low suitability. Range-expansion comparison Phoebes have slowly expanded their breeding range southeasterly into the Upper Coastal Plain of the Pee Dee region of South Carolina, but unlike Cliff Swallows, they are solitary and nest in small numbers (10–15 pairs) across a broad front, primarily at small, water-based anthropogenic structures (Table 2). In contrast, the Cliff Swallow’s faster contemporary breeding-range expansion along the same direction into the Middle Coastal Plain over approximately the same period has been in larger numbers (1 order of magnitude: 120–135 pairs; McNair 2013, unpubl. data) over a narrow front at large, water-based anthropogenic structures; its expansion has been restricted to the environs of the Great Pee Dee River. The Barn Swallow is the 3rd major species that breeds at water-based anthropogenic structures in southeastern North America. The earlier and complete range expansion of Barn Swallows to the coast (where a separate breeding population was already established; McNair and Post 1993) ended at roughly the same time as the range expansions of the other 2 species into the interior coastal plain began (Table 2). The earlier and faster range expansion of Barn Swallows (at least 3–5 times faster than the other 2 species) was also in much larger numbers (at least one order of magnitude larger than Cliff Swallow; Sauer et al. 2015) across an even broader front in upland and wetland habitats at numerous small to large land-based and water-based structures. Although their range expansion has ended, Barn Swallow numbers in most of the coastal plain have continued to increase at an annual rate of >1.5% (BBS trend map and BBS data for individual long-term routes including Coward and Dillon counties in the Pee Dee region; Sauer et al. 2015). Aside from land-based structures used by Barn Swallows, the number, size, and suitability of water-based anthropogenic structures that are available to each of the 3 species has strongly influenced the timing, rates, and paths of these species’ range expansions into southeastern North America, including the Pee Dee region of South Carolina (this study, McNair 2013). Southeastern Naturalist D.B. McNair 2017 Vol. 16, No. 4 526 Table 2. Summary of current (Eastern Phoebe, Cliff Swallow) and past (Barn Swallow) breeding-range expansion characteristics from the Fall Line into the interior of the Coastal Plain of the Pee Dee region of South Carolina. Uses Reciprocal land-based Water-based structures nest Colony Breeding-range Range expansion (km y-1) Species structures Size Number appropriation size front Timing Rate References Eastern Phoebe Rare Small Limited Yes Solitary Upper coastal 1988–2016 ~2 km y-1 McNair 1990, 2016; plain, broad this study Cliff Swallow Never Medium, Limited No Medium, Middle coastal 1995–2016 ~3.25 km y-1 McNair 2013 large large plain, narrow Barn Swallow Common Small, Extensive Yes Solitary, Complete coastal Mid-1960s to ~10 km y-1 McNair and Post medium, medium, plain, broad mid-1980s 1993, BBS data large large Southeastern Naturalist 527 D.B. McNair 2017 Vol. 16, No. 4 Acknowledgments I thank D.B. Cook, R.L. Floyd, and J.M. Tucker at the South Carolina Department of Transportation who supplied me with bridge information (Pee Dee region bridges responsedoc. xls), T.D. Feaster at the US Geological Survey South Carolina Water-Science Center for advice, and C.J. Randel for preparing Figure 1. I am grateful to B.G. Peterjohn, A.T. Peterson, J.M. Schukman, M.T. Stanback, and H. Weeks Jr. for their generous reviews of a penultimate version of the manuscript and 2 anonymous individuals for their comments on the final versions of the manuscript. Literature Cited Cely, J.E. 2003. The South Carolina breeding bird atlas 1988–1995. South Carolina Department of Natural Resources, Columbia, SC. 305 pp. Cooke, C.W. 1936. Geology of the Coastal Plain of South Carolina. Geological Survey Bulletin 867. US Government Printing Office, Washington, DC. 196 pp. Feaster, T.D., A.J. Gotvald, and J.C. Weaver. 2009. Magnitude and frequency of rural floods in the southeastern United States, 2006: Volume 3, South Carolina. US Geological Survey Scientific Investigations Report 2009-5156. US Geological Survey, Reston, VA. 226 pp. Available online at https://pubs.usgs.gov/sir/2009/5156/pdf/sir2009-5156.pdf. Accessed October 2016. Fortin, M.J., T.H. Keitt, B.A. Maurer, M.L. Taper, D.M. 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