Population Status of the Eastern Phoebe in South-Central
North Carolina: Breeding Increase at Water-Based
Anthropogenic Sites Congruent with Breeding Bird Survey
(BBS) and Christmas Bird Count (CBC) Data
Southeastern Naturalist, Volume 15, Issue 2 (2016): 299–314
Full-text pdf (Accessible only to subscribers.To subscribe click here.)
22001166 SOUTHEASTERN NATURALIST 1V5o(2l.) :1259,9 N–3o1. 42
Population Status of the Eastern Phoebe in South-Central
North Carolina: Breeding Increase at Water-Based
Anthropogenic Sites Congruent with Breeding Bird Survey
(BBS) and Christmas Bird Count (CBC) Data
Douglas B. McNair*
Abstract - Few studies in southeastern North America have compared local data sets to locally
based results from 2 national surveys (North American breeding bird survey [BBS],
Christmas bird count [CBC]). In 2012, I reexamined nest-site type use and nest type of
Sayornis phoebe (Eastern Phoebe) at 109 water-based anthropogenic structures originally
studied in south-central North Carolina in 1981. In 2012, Eastern Phoebes still strongly
preferred breeding at small bridges with ledges, especially at the same structures where I
studied them in 1981, even though use of other nest-site types slightly increased except at
circular culverts, where no Eastern Phoebes nested. During the 31-year interval between
studies, ~1/3rd of the bridges (20 of 62; 32%) were replaced with structures less favorable
as nest-sites; thus, a lower number and proportion of small bridges with ledges were available
in 2012. Although breeding Eastern Phoebes are still slowly increasing in abundance
at water-based anthropogenic sites in south-central North Carolina, I project that this
population will reach zero growth in 2027 as replacement of small bridges with ledges by
other structures continues. The findings from this local water-based anthropogenic nest-site
survey in south-central North Carolina was congruent with results from 2 national surveys
(BBS, CBC) in a portion of the Pee Dee region in documenting a modest increase in the
number of Eastern Phoebes during a time span ranging over 30 years.
Nest-sites are the major limiting resource for Sayornis phoebe Latham (Eastern
Phoebe, hereafter, Phoebe) during the breeding season when breeding-site fidelity
is very high (Beheler et al. 2003, Weeks 2011). Phoebes prefer nest-site types
located over or near water, and breeding directly over water at water-based anthropogenic
sites (bridges, culverts) has been documented in many areas of central and
eastern North America (Coffey 1963, 1976; Faanes 1980; Jackson and Weeks 1976;
McWhorter 2001; Ware and Duncan 1989; Weeks 1979, 2011), including North
Carolina in 1981 (McNair 1984). In south-central North Carolina, centered on
Richmond County that borders South Carolina, Phoebes were uncommon in 1981,
and in the Sandhills they only nested along woodland or forest streams at waterbased
sites (McNair 1984). Since the turn of the 21st century, Phoebes have nested
at some land-based anthropogenic sites in the Sandhills (McNair and Campbell
2013), but they still primarily breed at water-based sites. Phoebe use of water-based
nest-site types has rarely been re-assessed within the same study area, and never in
*35 Rowell Road, Wellfleet, MA 02667; firstname.lastname@example.org.
Manuscript Editor: Michael Steinberg
2016 Vol. 15, No. 2
southeastern North America. In this study I compare nest-site type use, its availability,
and nest type of Phoebes at water-based sites in Richmond County, NC,
including adjacent areas of neighboring counties in North and South Carolina in
2012, thirty-one years after the original study.
Extensive rehabilitation and replacement of ~30% of bridges and culverts occurred
within my study area from 1981 to 2012 (NCDOT 2015, SCDOT 2015),
including changes in design and construction materials. In the mid-Atlantic region,
by the early 1970s, corrugated metal and concrete circular culverts were
being employed to replace small bridges with steel I-beams that contain ledges
(Whitaker 1974). Weeks (1984) stated that changing bridge and culvert design,
especially replacement of small wooden or concrete bridges and square concrete
culverts with unusable circular, corrugated metal or plastic culverts can significantly
reduce available nest-sites for Phoebes in Indiana. The availability of small
bridges with ledges, especially with steel I-beams that provide complete shelf
support for statant nests, was by far the most important factor that determined
if Eastern Phoebes nested at water-based sites in 1981 in south-central North
Carolina (McNair 1984). Phoebes were much less numerous at the other nest-site
types I documented: (1) box culverts (Phoebes uncommon), (2) small bridges
without ledges (Phoebes scarce), and (3) large bridges/railroad trestles/circular
culverts (Phoebes absent).
In 1981, Phoebes reached the southeastern periphery of their breeding range
along and near the Fall Line in south-central North Carolina; their breeding limit
including adjacent South Carolina was still truncated along or near the Fall Line in
the late 1980s and early 1990s (Cely 2003, McNair 1990, McNair and Post 1993,
Post and Gauthreaux 1989). As documented by the North American breeding bird
survey (BBS) of the US Fish and Wildlife Service, the Phoebe’s breeding-range
front has slowly expanded from the Piedmont into the Coastal Plain (see summer
distribution maps in Sauer et al. 2014). This range expansion has been accompanied
by an increase in the relative abundance of Phoebes in most areas of the Piedmont
and Coastal Plain including the Carolinas (see trend maps in Sauer et al. 2014);
these trends were especially pronounced in North Carolina from 2003 through
2013. The relative abundance of Phoebes in the Coastal Plain of southeastern North
Carolina and South Carolina remains low, at 0.05 to 1 bird per BBS route, rising
to ~1–3 birds along the Fall Line in south-central North Carolina, and higher still
at 3–10 birds in the Piedmont of North Carolina (Sauer et al. 2014). Phoebes are
behaviorally plastic and adapted very early on to use of water-based anthropogenic
nest-sites (Weeks 2011), which may be an ecological driver that has permitted them
to expand their breeding-range front and increase in abundance in southeastern
North America. Alternative explanations for an increase in their abundance, such
as enhanced survivorship in North Carolina during winter over the past several decades
because of global warming (LeGrand 2015), is also possible. Both the BBS
and Christmas bird count (CBC) in south-central North Carolina and a portion of
the Pee Dee region are expected to document a recent population increase of the
Phoebe in comparison to historical data circa 1981. However, the BBS increase
2016 Vol. 15, No. 2
should be a closer fit to my population sample at water-based anthropogenic nestsites
compared to the CBC because the BBS samples a similar population during
the breeding season.
If Phoebes have indeed increased in abundance in south-central North Carolina
in recent decades (Sauer et al. 2014), and if nest-site type availability has remained
unchanged at the same structure, especially at small bridges with ledges, I predicted
that occupancy of water-based anthropogenic nest-site types Phoebes use would
also have increased over the same period. If preferred nest-site type availability
had decreased by 2012 because the structures surveyed in 1981 had been replaced
by any of the other nest-site types in 2012, especially circular culverts, then breeding
Phoebes would decrease at these different structures. The overall population
change of Eastern Phoebes breeding in south-central North Carolina would then
be determined by the relative contributions of the predicted increase at the same
structures versus the predicted decrease at different structures in 2012.
The study area comprised Richmond County, NC, and adjacent areas of 6
neighboring counties (North Carolina: eastern Anson, southern Montgomery,
southwestern Moore, western Scotland; South Carolina: northeastern Chesterfield,
northern Marlboro). The study area included the Piedmont, the Sandhills subregion
of the Coastal Plain, and the Inner Coastal Plain (McNair 1984).
Eastern Phoebe surveys
Nest-site types. I examined 163 water-based anthropogenic sites in 1981 (Mc-
Nair 1984). In late April through May of 2012, I re-examined 109 (67%) of these
sites for which I had complete documentation, 104 sites in North Carolina (59 in
Richmond County) and 5 sites in South Carolina (NCDOT 2015, SCDOT 2015).
Of these sites, 68, 39, and 2, respectively, were located in the Piedmont, Sandhills,
and Inner Coastal Plain (adjacent to the Fall Line). Of the 109 sites, 75 included the
same water-based anthropogenic structure in 1981 and 2012 (= same structure), and
34 sites included different structures in 1981 and 2012. After 1981, the different
structure was usually a replacement of the older structure, but occasionally represented
extensive rehabilitation of the existing structure. I defined small bridges as
those less than 6 m in height at their center above ground or water and with a span
usually of less than 30 m; most small bridges were less than 5 m in height at their center
(McNair 1984). I defined nest types based on the appearance of the nest (for an
alternative definition, see Schukman et al. 2011).
Breeding bird surveys. The roadside bias of the BBS, a large-scale, long-term
survey (Pardieck et al. 2015, Sauer et al. 2014) should not underestimate numbers
of Phoebes in south-central North Carolina during the breeding season because
nesting Phoebes are closely associated with anthropogenic structures along or near
roads. Twelve routes for the BBS recorded at least 1 Phoebe within or near a portion
of the Pee Dee region of the Carolinas (NC: 8 routes; SC: 4 routes). All routes were
2016 Vol. 15, No. 2
in predominantly rural areas and included 3 physiographic provinces, the Piedmont,
Sandhills, and Inner Coastal Plain, with some overlap between physiographic provinces
on one route (see results).
Christmas bird counts. The CBC is another large-scale, long-term survey, but
is primarily recreational (Carolina Bird Club 2015, National Audubon Society
2015), in contrast to the BBS that was explicitly created to detect population
trends (Dunn et al. 2005). Five CBCs have been conducted within or near the Pee
Dee region of North and South Carolina. They were carried out at Pee Dee National
Wildlife Refuge, Southern Pines, and Cumberland County (Fayetteville)
NC; and at Carolina Sandhills National Wildlife Refuge and the Pee Dee area
(Florence) in SC. All 3 physiographic provinces are represented. Count circles
within Cumberland County and the Pee Dee area include metropolitan statistical
areas, defined by the US Office of Management and Budget as a central urban
area or cluster; the Pee Dee area count circle is ~20% urban, 20% semi-rural,
and 60% rural (S. Smolen-Morton, Francis Marion University, Florence, SC,
SSmolenMorton@fmarion.edu, pers. comm.). The Southern Pines count circle
includes a portion of the Pinehurst-Southern Pines micropolitan statistical area,
defined as a smaller or less-dense central urban area or cluster; this count circle
is ~10% urban and 90% rural (S. Campbell, email@example.com, 910-949-3207,
pers. comm.). The remaining 2 count circles, Pee Dee NWR and Carolina Sandhills
NWR, are almost entirely (~95%) rural areas, the remainder (~5%) being
urban or semi-rural (N. Jordan, US Fish and Wildlife Service, nancy_jordan@
fws.gov, 843-335-6026, pers. comm.; D.B. McNair, pers. observ.).
Nest-site types. I divided the 109 water-based anthropogenic sites into 6 nestsite
types (small bridge with ledges, small bridge without ledges, large bridge,
box culvert, circular culvert, railroad trestle), grouped by whether Phoebes nested
at the same or a different structure after 1981. For all nest-site types combined, I
performed 2 x 2 chi-square tests with Yate’s correction to examine whether these
structures with or without active nests were proportionally more or less numerous
in either year (1981, 2012). I also examined whether each of the 6 nest-site types
with or without nests at the same structure in both years or at a different structure in
2012 were proportionally more or less numerous. I then performed 1 x 1 chi-square
tests with Yate’s correction to examine 4 outcomes: with and without nests at the
same structure in 1981 and 2012 (75 sites) and at different structures (34 sites); the
4 outcomes were reduced for both chi-square tests to 2 categories—unchanged and
changed outcomes. For nest types, I performed a 2 x 2 chi-square test with Yate’s
correction to examine if statant nests compared to all other nest types combined
were proportionally more or less numerous at the same structure in both years or at
a different structure in 2012. Otherwise, I present descriptive statistics.
Breeding bird surveys. I retrieved raw data collected by the BBS (Pardieck
et al. 2015) to calculate the relative abundance of Phoebes on individual routes
that were surveyed by 1–4 observers per route. The small bias of observer effects
2016 Vol. 15, No. 2
(5% underestimation of counts in the first year; Link et al. 2008) is expected to
be negligible for Phoebes. This species is generally solitary, easily detected by
sight and sound (not ventriloquial), and no more than 2 phoebes would usually
be expected per stop; during their first survey, any observer should easily distinguish
the 2 individuals. I calculated results using arithmetic and geometric means,
which were similar. I chose the arithmetic mean because results were slightly
more conservative (even though the use of the geometric mean for proportional
and ratio data which are used in this study can have advantages). Only 2 routes
had data from before the 1990s and preceding my first survey of nesting Eastern
Phoebes in south-central North Carolina in 1981. Therefore, for all routes I compared
the mean count of birds for 2 non-exclusive year-groups, the first group for
all years of data available on each route versus the second group of only 2009–
2014 (3 years before-and-after I collected data in 2012). For all years of data, I
also calculated Pearson’s product correlation coefficients (r) to examine which
routes had a significant increase in the number of Phoebes. I next calculated the
difference in mean counts between the 2 year-groups and rounded to the nearest
0.5 bird to examine how much Eastern Phoebes had increased or decreased on
each route. I used this conservative approach between the 2 year-groups (which
are not independent) to reduce the contrast in mean counts so any consistency in
results would be more robust, given potentially confounding factors such as differences
between physiographic provinces, the span of years, the number of years
run, and observer ability within and between the routes (Link et al. 2008). Finally,
I calculated the ratio (proportional change) of the mean count for 2009–2014
divided by the mean count of all years of data for each BBS route to compare to
ratios from my nest-site data and CBC results.
Christmas bird counts. The Phoebe is generally a solitary species that is easily
located and identified, and rarely occurs at feeders (see Dunn et al. 2005). In my
study area, Phoebes are more widely distributed in winter than during the breeding
season; thus, CBC data should be useful for the study of this species in the Carolinas
(McNair 1987; cf., Arenaria interpres L. [Ruddy Turnstone] in California,
Pandolfino and Helmericks 2005; Calidris alpina L. [Dunlin] in North America, Xu
et al. 2015). The number of birds counted on CBCs can be confounded by variation
in effort (Butcher and McCulloch 1990, Link et al. 2008). Large CBC data sets have
generally followed an over-dispersed Poisson distribution with complex adjustments
made for variation in effort (Dunn et al. 2005, Link and Sauer 2007, Link et
al. 2008, Sauer and Link 2002; though see Xu et al. 2015). For my small CBC data
set, I selected 3 winters before and after each of the 2 years I sampled breeding
Phoebes for a total of 6 years for each year-group (historical: 1978–1979 to 1983–
1984; recent: 2009–2010 to 2014–2015) to ensure an adequate and representative
sample for each CBC. The total number of birds and party-hours for each year from
all CBCs (n = 44), as well as CBCs partitioned into historical (n = 17) and recent
(n = 27) year-groups, each approximated a normal distribution at an alpha value of
0.05 (six Shapiro-Wilk tests: W = 0.93–0.95 [range], P = 0.06–0.41 [range]), which
I confirmed by examination of Q-Q plots. Pearson’s product correlation coefficient
2016 Vol. 15, No. 2
between the total number of birds and party-hours for each year from all CBCs was
significant (r = 0.37, P = 0.01), indicating that additional effort influenced counts
and that the relationship was approximately linear. The relationship between the
number of birds and party-hours remained approximately linear for the recent yeargroup
(r = 0.40, P = 0.04), but was non-linear for the historical year-group (r = 0.01,
P = 0.96) even though the mean number of party-hours from all counts within each
year-group was similar (historical year-group: 42.01; recent year-group: 43.65), unlike
most CBCs where the mean count effort has increased over a long time interval
(Butcher and McCulloch 1990). This disparity between the 2 year-groups indicates
that observers had improved in their ability to efficiently detect Phoebes on recent
CBCs. Dunn et al. (2005) cautioned that the relationship between the number of
birds and effort can vary among CBC circles. Examination of CBC circles in the
historical year-group revealed that 1 (Carolina Sandhills NWR) approximated a
linear relationship whereas the other 2 did not. Nonetheless, I did not make any
additional effort adjustment to the number of party-hours for the 2 historical CBCs
that were non-linear, given the small sample size and linear distribution of the other
historical and all recent CBCs, but this shortcoming should be recognized.
Consequently, I calculated the number of Phoebes per party-hour, a simple
index of relative abundance, for each year of the 2 year-groups for each of the 5
CBCs. I also compared these results after converting counts to a common scale
corresponding to the mean level of effort for each of the 2 year-groups, a simple
additional effort adjustment (Blem 1995, Butcher and McCulloch 1990, Link
et al. 2008). The mean effort between the 2 year-groups was very similar (see
above), effort on counts was generally low (maximum of 86 party-hours on 1
count), and the range of effort was low on all 5 count circles, which should reduce
concerns about the scaling effects of effort when effort is high (Link et al. 2008,
Sauer and Link 2002). Regardless, results from this comparison using unadjusted
versus scaled party-hours were virtually identical, so I chose the simpler method
of using unadjusted party-hours. After also examining the distribution of the
number of birds and party-hours for each year on each CBC, I rejected 3 outlier
values on 2 count circles from the recent year-group that were associated with an
abnormally low number of party-hours which distorted results. I then calculated
the mean number of birds per party-hour over all years within each year-group for
each CBC, which reduced annual variability in counts and count effort, including
any potential weather effects (Link and Sauer 2007, Link et al. 2008). I also converted
these counts to represent an 8-h party-hour/day and then calculated the difference
between the mean counts of the 2 year-groups, rounded off to the nearest
0.5 bird, for each CBC. Finally, I calculated the ratio (proportional change) of the
mean number of birds per party-hour between the recent year-group divided by
the mean number of the historical year-group for each CBC. Using only 5 count
circles and the availability of only incomplete data for several of them weaken
this examination of CBC data, but consistent results here may strengthen interpretation
of results from the other 2 data sources (mine at nest-sites, BBS).
2016 Vol. 15, No. 2
Of the 218 structures representing 6 nest-site types used over both years (1981,
2012), 124 (57%) were small bridges with ledges which contained 80 of the 97 active
Phoebe nests (82%) at all structures (Table 1), much greater occupancy than
at any of the other 5 nest-site types. In 1981, 45 structures contained active nests
and 64 structures did not, whereas in 2012, 52 structures contained active nests and
53 did not; the difference in proportion of nests between the 2 years for all nestsite
types combined was not significant (χ2 = 1.15, df = 1, P > 0.2). In contrast, the
difference in the proportion of nests at the same structure in both years compared
to different structures in 2012 was highly significant (χ2 = 15.4, df = 1, P < 0.001).
All 6 nest-site types at a different structure in 2012 contained proportionally fewer
nests, including small bridges with ledges (Table 1).
There were 62 individual small bridges with ledges, but only 8 of these were
built after 1981 (13%). Only 2 (3%) small bridges with ledges were extensively
rehabilitated or replaced with another small bridge with ledges after 1981, whereas
20 (32%) small bridges with ledges were replaced by other nest-site types by 2012.
Structural materials that included steel alone or in combination with other materials
(wood, concrete, stone) comprised 60 of the 62 small bridges with ledges (97%);
steel and wood was the most common combination. In contrast, there were 36 individual
small bridges without ledges, and 12 of these were built after 1981 (33%),
a much higher number and proportion than small bridges with ledges. Structural
materials of small bridges without ledges that included steel alone or in combination
with other materials (concrete, wood), comprised only 5 of 36 (14%) of these
bridges and none contained steel beams.
Table 1. Eastern Phoebe water-based anthropogenic nest-site types with and without nests at the same
structure in both years (1981, 2012; n = 75) or at a different structure in 2012 (n = 34) in south-central
North Carolina. Total = totals for each type over both years (i.e., each structure that was available in
both years counts twice).
All structures Same structure Different structure
With Without With Without With Without
Nest-site type Total nest nest nest nest nest nest
Small bridge with ledgesA 124 80 41 67 28 13 13
Small bridge without ledges 51 9 42 7 23 2 19
Large bridgeB 19 2 16 2 11 0 5
Box culvert 13 5 8 3 3 2 5
Circular culvert 8 0 8 0 0 0 8
Railroad trestle 2 1 1 1 1 0 0
No structureC 1 0 1 0 0 0 1
Total 218 97 117 80 66 17 51
AThree outcomes in 2012 were undetermined.
BOne outcome in 2012 was undetermined.
CBox culvert along seasonal feeder rivulet into Hitchcock Creek was removed after 1981, road bed
was built up, and area was largely filled in, supporting only a very short, small square concrete cap
that allows water to pass through.
2016 Vol. 15, No. 2
Small bridges with ledges comprised 49 of the 75 (65%) sites with the same
structure in both years (Table 2). I excluded 4 sites that had undetermined outcomes
in 2012; 49 of 71 (69%) outcomes at the same structure remained unchanged
(Table 2), whereas 22 outcomes (31%) changed (χ2 = 9.52, df = 1, P < 0.005). Of
the 22 outcomes that changed, 18 structures (82%) without a nest in 1981 contained
a nest in 2012, representing 5 nest-site types, especially small bridges with ledges
(Table 2). In contrast, only 4 of 22 (18%) changes were of structures that contained
a nest in 1981 but were without one in 2012. The net-gain differential was greatest
for small bridges with ledges (Table 2). Thus, 32 of 71 sites (45%) contained nests
in 1981, whereas 46 of 71 sites (65%) contained nests in 2012. This net gain of 14
nests at the same structures between the 2 years over a time interval of 31 years
represents a net percentage gain of 20% (or 0.65%/year).
Small bridges with ledges were present in only 4 of the 34 (12%) sites with a
different structure in both years (Table 3). Twenty-one (21) outcomes at 34 sites
(62%) with a different structure remained unchanged (Table 3), whereas 13 outcomes
(38%) changed (χ2 = 1.44, df = 1, P > 0.2). Of the 13 outcomes that changed,
4 structures (31%) without a nest in 1981 contained a nest in 2012. In contrast, 9 of
13 (69%) changed outcomes represented structures without a nest in 2012 that contained
a nest in 1981 (Table 3). Thus, 11 of 34 sites (32%) contained nests in 1981,
whereas 6 of 34 sites (18%) contained nests in 2012. This net loss of 5 nests at different
structures between the 2 years over a time interval of 31 years represents a
net percentage loss of 14% (or 0.45%/year). Therefore, adding together the net difference
for sites that changed structures and those that remained the same, Phoebes
in south-central North Carolina centered on Richmond County had a net gain of 9
nests at all sites between the 2 years over a time interval of 31 years, which represents
a net percentage gain at all water-based anthropogenic nest-sites of 6% (or
0.2%/year). The ratio (proportional change) of the number of all nest-sites with
nests in 2012 divided by the number of nests in 1981 is 1.21, and for only sites
Table 2. Four outcomes, with and without nests in 1981 and 2012, at 75 sites with the same structure
for 5 water-based anthropogenic nest-site types in south-central North Carolina.
No nest With nest in
in 1981, 1981, and
Total Nests in No nests and with nest no nest
Nest-site type sites both years in both years in 2012 in 2012
Small bridge with ledgesA 49 26 7 10 3
Small bridge without ledges 15 1 9 4 1
Large bridgeB 7 4 2
Box culvert 3 1 1 1 0
Railroad trestle 1 0 0 1 0
Total 75 28 21 18 4
ATwo small bridges with ledges contained nests in 1981, and one did not, but all three outcomes in
2012 were undetermined.
BOne large bridge without a nest in 1981 had an undetermined outcome in 2012.
2016 Vol. 15, No. 2
with the same structures in both years, a better measure of unhindered population
increase of Phoebes, is 1.44.
Only statant nests were present at 79 of 93 structures (85%). The remaining nest
types were 2 different types in the same year, statant and adherent nests (plastered to
a rough, irregular vertical surface) on the same structure (n = 2), semi-statant (incomplete
shelf support; n = 2), adherent (n = 7), and undetermined (n = 3). The proportion
of statant nests compared to all other nest types combined at the same structure
in both years or at a different structure in 2012 was similar (χ2 = 0.88, df = 1, P >
0.2). All statant nests were on ledges, whereas support for 11 non-statant nests was
Crabronidae (organ-pipe mud-dauber wasp) nests (n = 6), single screw-bolts (n = 3),
1 weathered protuberance from an old outer pillar of a railroad trestle, and 1 on top of
a foundation of a Hirundo rustica L. (Barn Swallow) nest that was adherent.
Breeding bird surveys
Phoebes significantly increased over time spans ranging from 17 to 46 years on
4 of 12 BBS routes, and there was a difference in the mean count between the 2
year-groups (all years, 2009–2014) of at least an increase of 1 bird for these 4 routes
(Table 4). Phoebes on Biscoe and the other 7 routes with smaller numbers did not
increase. Two counts showed an increase of 0.5 bird and there were no differences
in relative abundance on the other 6 routes (Table 4). The highest mean counts for
both year-groups were on 2 of the 3 Piedmont routes, where the greatest increase
Table 3. Four outcomes, with and without nests in 1981 and 2012 at 34 sites with a different structure
(of a same or different type as was used at that site in 1981) in 2012 for four 1981 water-based anthropogenic
nest-site types in south-central North Carolina.
No nests No nest in With nest in
1981 nest-site type Total Nests in in 1981, and with 1981, and
2012 nest-site type sites both years both years nest in 2012 no nest in 2012
Small bridge with ledges
small bridge with ledges 2 1 1
small bridge without ledges 9 1 4 4
large bridge 1 1
box culvert 3 1 2
circular culvert 7 2 5
Small bridge without ledges
small bridge with ledges 2 2
small bridge without ledges 3 2 1
box culvert 3 3
circular culvert 1 1
large bridge 2 2
no structure 1 1
Total 34 2 19 4 9
2016 Vol. 15, No. 2
also occurred. The general pattern within and between year-groups was for mean
counts and differences in mean counts highest in the Piedmont, followed by the
Sandhills, and lowest in the Inner Coastal Plain, where the total number of birds
on each of the 4 routes was less than 10, regardless of differences in the span of years, the
number of years run, and observer ability. Nine of 12 ratios (proportional change)
of the mean count for 2009–2014 divided by the mean count for all years on BBS
routes ranged from 0.96 to 1.73 (Table 4). The other 3 routes represented outlier
values (0 on 2 routes and 1 ratio of 2.63 on the Sandhills route), all 3 of which had
a total number of birds less than 10 each. The largest valid ratio of 1.73 had the longest span
Christmas bird counts
The mean number of Phoebes per party-hour in the recent year-group (2009–
2010 to 2014–2015) was 2–3x higher on the single CBC circle in the Piedmont
(0.98) than any of the very similar results on the 4 count circles in the Sandhills
or Inner Coastal Plain (range = 0.36–0.43) within or near a portion of the Pee
Dee region of North and South Carolina (Table 5). The lowest mean number of
Phoebes per party-hour in the historic year-group (1978–1979 to 1983–1984) was
in the Sandhills (range = 0.09–0.15) compared to the Inner Coastal Plain (0.34),
although historic data were not collected on 2 of the 5 CBCs. When converted to
Table 4. The total number of birds, mean count for 2 groups of years (all years, 2009–2014), difference
in mean count between the 2 groups of years, and the ratio of the mean count between both groups
of years for the Eastern Phoebe on 12 Breeding Bird Survey (BBS) routes within or near the Pee Dee
region of North and South Carolina. Physiographic province (Prov.): P = Piedmont, S = Sandhills, and
I = Inner Coastal Plain. Probability for Pearson’s correlation coefficient: * = 0.01–0.05; ** = less than 0.01. Differences
in mean count (Diff.) are rounded to the nearest 0.5 bird. Ratio = Ratio 2009–2014/all years.
BSS route Years Total All 2009–
Prov. Name number State Years run birds years 2014 Diff. Ratio
P Wilgrove 63216 NC 1995–2012 16 94 5.88* 8.25A 2.5 1.40
P Oakboro 63217 NC 1969–2014 19 106 5.58* 9.67 4.0 1.73
P Biscoe 63215 NC 1990–2011 21 55 2.62 2.67B 0.0 1.02
S Lake Surf 63314 NC 1998–2014 13 42 3.23* 5.33 2.0 1.65
S Hamlet 63207 NC 1995–2014 19 33 1.74** 2.83 1.0 1.63
S Raeford 63900 NC 1990–2014 21 28 1.33 1.50 0.0 1.13
S Sandhills 80900 SC 1994–2014 21 8 0.38 1.00 0.5 2.63
S/I Mount Pisgah 80053 SC 1999–2010 7 11 1.57 1.50C 0.0 0.96
I Bethel 63315 NC 2002–2013 9 4 0.44 0.75D 0.5 1.70
I Rowland 63106 NC 1990–2013 22 3 0.14 0.00E 0.0 0.00
I Bennettsville 80054 SC 1999–2010 10 9 0.90 1.00C 0.0 1.11
I Dillon 80010 SC 1970–2010 28 1 0.04 0.00C 0.0 0.00
2016 Vol. 15, No. 2
an 8-h party-hour/day, the difference in mean counts between the 2 year-groups
was greatest in the Sandhills (2–2.5 birds) compared to the Inner Coastal Plain (0.5
bird). Phoebes increased on all 3 CBCs with data for both year groups, where the
ratio (proportional change) of the mean number of Phoebes per party-hour between
recent and historic year-groups was greatest in the Sandhills (2.6, 4.33) compared
to the Inner Coastal Plain (1.26).
As expected, small bridges with ledges that contained statant nests remained
the preferred nest-site type and nest type of Phoebes in forested habitats centered
on Richmond County, NC, as is true for the mountains of Virginia (Clapp 1993).
Adherent nests are still rather scarce in south-central North Carolina (McNair
1984, this study), unlike western Alabama and the northern Piedmont of Virginia
where many nests under bridges were attached to mud-dauber wasp nests or old
Barn Swallow nests (Clapp 1993, Jackson and Weeks 1976). Nonetheless, Phoebes
have again demonstrated their adaptability (Weeks 2011) in south-central
North Carolina by using at least some alternative water-based anthropogenic
nest-site types, such as larger concrete-box culverts. However, replacement of
small bridges with ledges by less-suitable or entirely unsuitable alternative nestsite
types, especially circular culverts, is inappropriate for the conservation of
Phoebes unless improvements such as inverted-T wood structures that support
Table 5. The total number of birds, mean number per party-hour, and the ratio of the mean number
between 2 groups of 6 years each (1978–1983 and 2009–2014) for the Eastern Phoebe on 5 Christmas
Bird Counts (CBC) within or near the Pee Dee region of North and South Carolina. nd = no data
(counts not conducted) and na = not applicable. Ratio = Ratio 2009–2014/1978–1983.
Physiographic Total Mean number/
CBC province birds party-hour Ratio
Pee Dee NWR, NC Piedmont na
1978–1983 nd nd
2009–2014A 100 0.98
Southern Pines, NC Sandhills 4.33
1978–1983 28 0.09
2009–2014 174 0.39
Carolina Sandhills NWR, SC Sandhills 2.60
1979–1983B 15 0.15
2009–2014C 52 0.39
Cumberland County [Fayetteville], NC Sandhills/Inner Coastal Plain na
1978–1983 nd nd
2009–2014 85 0.36
Pee Dee Area [Florence], SC Inner Coastal Plain 1.26
1978–1983 58 0.34
2009–2014 114 0.43
AOne outlier value removed (2010–2011 winter).
BOne count not conducted (1978–1979 winter).
CTwo outlier values removed (2011–2012 and 2014–2015 winters).
2016 Vol. 15, No. 2
statant nests are installed within larger circular culverts with a diameter of at least
1.2 m (Weeks 1984, Whitaker 1974). The decks, superstructures, and usually substructures
of most small bridges are now primarily built of reinforced concrete
with no ledges, i.e., slab bridges (McNair 1990, Weeks 1984; this study). Similar
structural improvements for nesting Phoebes could also be installed within slab
bridges and concrete-box culverts. North Carolina has a backlog of ~40% of
bridges and culverts that require replacement or extensive rehabilitation due to inadequate
funding (NCDOT 2015), although the proportion of deficient bridges in
Richmond County (17%) is much lower than the statewide average. Construction
and installation costs for structures that support nesting Phoebes would add to the
funding deficit for replacement bridges and culverts in North Carolina, a problem
reported from many other states such as Iowa (TRIP 2015). If the projected
decline of Phoebes becomes too sharp at some time in the future (see below), the
NCDOT should revisit the issue of providing artificial platforms in structures that
are otherwise unsuitable for nesting Phoebes (and several other species that nest
at water-based anthropogenic sites; cf., Indiana; Weeks 1984).
Even though Phoebes are highly susceptible to severe winter weather, such as
the region experienced in 1976–1977 (Robbins et al. 1986), breeding Phoebes in
south-central North Carolina have slowly increased in abundance (this study), a
trend which has not yet been affected by a sharp reduction in small bridges with
ledges over the 31-y time interval (1981–2012). At the 109 water-based anthropogenic
sites within the study area, 34 changes (31%) to a different structure over 31
years represents ~1 replaced or extensively rehabilitated structure every year. As
such, assuming the number of Phoebes breeding at these water-based anthropogenic
sites is a reliable population sample, my projections suggest that in south-central
North Carolina they may reach zero population growth in 2027. Based on my analyses,
by 2027, the number of sites with the same structures will decline to 56 and
those with different structures will increase to 48 if conditions otherwise remain
unchanged. Breeding Phoebes have been scarce in the extreme eastern Piedmont at
land-based anthropogenic sites within the study area (cf. Coffey 1963; D. McNair,
unpubl. data) and have only had a limited recent spread to land-based anthropogenic
sites in the Sandhills, including Richmond County (McNair and Campbell 2013).
Despite this scarcity, the recent occupancy of Phoebes at land-based anthropogenic
sites suggests that saturation of suitable water-based nest-sites may be occurring
even though Phoebes are still relatively uncommon in south-central North Carolina
compared to many other areas of the central and eastern US such as Appalachia
(Hill and Gates 1988, Sauer et al. 2014, Weeks 2011). This local-scale study in
south-central North Carolina where water-based anthropogenic nest-sites form a
preponderance of all nest-sites can help inform solutions for Phoebe breeding populations
stressed or potentially stressed by a sharp reduction of suitable nest-sites in
other areas of North America.
Counts made by the BBS and the CBC surveys in the Pee Dee region likely
under-represent bird-population numbers because of low route densities and low
number of count circles. The results of these surveys are also weakened by missing
2016 Vol. 15, No. 2
data for some years, inconsistent route coverage because of difficulty of observer
recruitment, and inconsistencies in effort efficiency on several count circles. Sauer
et al. (2013) also flagged very small samples of BBS routes (less than 14), as in this study,
as a cause of possibly unreliable data. Nonetheless, despite differences between
some routes and count circles in their degree of urbanization, increases in relative
abundance of Phoebes on half of the BBS routes, with no decreases, and increases
on all 3 CBC circles is likely not due to chance alone but represent a true increase
in Phoebes. The local increase of breeding Phoebes at water-based anthropogenic
nest-sites over a span of 31 years in south-central North Carolina is one factor that
is responsible for increases documented on BBS and CBC data sets from a portion
of the Pee Dee region used herein. Peterjohn and Sauer (1995) documented
increasing complexity in the temporal patterns of population trends of Progne
subis L. (Purple Martin) as their analyses focused on national, to regional, and then
to state levels (and Bird Conservation Regions within states; Sauer et al. 2013).
They did not examine whether many population increases or declines were locally
distributed (e.g., of a small region or area within a state), although Peterjohn
and Sauer (1995) stated that Purple Martins exhibited a wide variety of temporal
patterns in their trends in the southern portion of their breeding range. The study
reported here is the first in the Carolinas and one of the few in southeastern North
America, that documents that a local independent data set that measured the temporal
pattern of a population increase is congruent with BBS and CBC data even
though my analyses of the latter 2 data sets are primitive (cf., Link and Sauer 2007,
Link et al. 2008, Sauer et al. 2013, Wilson et al. 2013). This robust result strongly
suggests that additional analyses should be performed on a variety of permanent
resident and temperate migrant species in North America, using other local data
sets in conjunction with BBS and CBC data (Wetzel and Krupa 2013), to determine
local factors responsible for temporal population changes in the Carolinas or
elsewhere in southeastern North America.
The 2 national surveys, using data only from a portion of the Pee Dee region,
were also consistent in their general documentation of larger numbers of Phoebes
in the Piedmont compared to the Sandhills and Inner Coastal Plain, which
is in agreement with BBS results on the relative abundance of Phoebes in these
physiographic provinces along the Atlantic slope of southeastern North America
(Sauer et al. 2014; see Introduction, this study). The mean counts in the Inner
Coastal Plain compared to the Sandhills were lower in the BBS surveys but comparable
in the CBCs, especially recently, which suggests proportionally more
Phoebes are temperate migrants wintering in the Inner Coastal Plain compared
to a higher proportion of permanent residents in the Sandhills. In addition, ignoring
1 BBS route with a very low number of birds, BBS ratios of mean counts
that measure proportional change in all 3 physiographic provinces aligned themselves
more closely with my nest-site ratios, compared to much higher ratios of
mean counts in the Sandhills on 2 CBCs. These results suggest that BBS data fit
my data more closely than CBC data, as expected, but differences in methodology
between the 2 national surveys and my local survey and differences in the
2016 Vol. 15, No. 2
time span of my nest-site data and most BBS routes prevent a more precise analysis.
Firmer evidence is needed from marked birds to authoritatively determine
the precise population status of Phoebes in each of the 3 physiographic provinces
in the Pee Dee region of the Carolinas.
BBS and CBC data were provided by, respectively, the Patuxent Wildlife Research Center
of the US Fish and Wildlife Service and the National Audubon Society and through the
generous efforts of volunteers in the Carolinas. I thank B. Bockhahn, S. Campbell,
N. Jordan, S. Smolen-Morton, and J. Walker, and R.J. Davis, respectively, who supplied me
additional information on their CBC circles and on 4 BBS routes in North Carolina; D.B.
Cook, R.L. Floyd, and J.M. Tucker at the SCDOT who supplied me with bridge information
(Pee Dee Region bridges-responsedoc.xls) through a Freedom of Information Act request;
J.C. Kilgo, M.T. Stanback, and H. Weeks Jr. for their reviews of a penultimate version of the
manuscript; and 2 anonymous individuals for their reviews of the submitted version of
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