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2017 Vol. 16, No. 4
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.
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; email@example.com.
Manuscript Editor: Jason Davis
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
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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.
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).
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
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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
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.
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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.
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
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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)
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
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
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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.
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
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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
2017 Vol. 16, No. 4
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
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.
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).
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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.
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
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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.
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