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2009 SOUTHEASTERN NATURALIST 8(1):107–120
Avian Response to Microclimate in Canopy Gaps in a
Bottomland Hardwood Forest
Tracey B. Champlin1,2, John C. Kilgo3,*, Marcia L. Gumpertz4,
and Christopher E. Moorman1
Abstract - Microclimate may infl uence use of early successional habitat by birds.
We assessed the relationships between avian habitat use and microclimate (temperature,
light intensity, and relative humidity) in experimentally created canopy gaps
in a bottomland hardwood forest on the Savannah River Site, SC. Gaps were 2- to
3-year-old group-selection timber harvest openings of three sizes (0.13, 0.26, 0.50
ha). Our study was conducted from spring through fall, encompassing four bird-use
periods (spring migration, breeding, post-breeding, and fall migration), in 2002 and
2003. We used mist netting and simultaneously recorded microclimate variables
to determine the infl uence of microclimate on bird habitat use. Microclimate was
strongly affected by net location within canopy gaps in both years. Temperature
generally was higher on the west side of gaps, light intensity was greater in gap
centers, and relative humidity was higher on the east side of gaps. However, we
found few relationships between bird captures and the microclimate variables. Bird
captures were inversely correlated with temperature during the breeding and postbreeding
periods in 2002 and positively correlated with temperature during spring
2003. Captures were high where humidity was high during post-breeding 2002, and
captures were low where humidity was high during spring 2003. We conclude that
variations in the local microclimate had minor infl uence on avian habitat use within
gaps. Instead, habitat selection in relatively mild regions like the southeastern US is
based primarily on vegetation structure, while other factors, including microclimate,
are less important.
Birds are often more abundant in and adjacent to gaps in forest canopies,
especially larger gaps, than in adjacent mature forest (Kilgo et al. 1999,
Moorman and Guynn 2001). However, the relative use of gaps by birds
varies temporally from spring migration through fall migration (Bowen et
al. 2007). For example, post-fl edging dispersal of birds from natal areas in
mature forest with high canopy cover tends to be towards large openings or
patches of early successional habitat with little to no canopy and a dense understory
(Anders et al. 1998, Vega Rivera et al. 1998), thus yielding greater
abundance there during the post-breeding period than the breeding period.
Higher food abundance (Blake and Hoppes 1986, Levey 1988), increased
1Department of Forestry and Environmental Resources, North Carolina State University,
Campus Box 8003, Raleigh, NC 27695. 2Current address - 10 Lepire Avenue,
Westport, MA 02790. 3USDA Forest Service Southern Research Station, PO Box 700,
New Ellenton, SC 29809. 4Department of Statistics, North Carolina State University,
Campus Box 8203, Raleigh, NC 27695. *Corresponding author - firstname.lastname@example.org.
108 Southeastern Naturalist Vol. 8, No. 1
vegetative structure (Martin and Karr 1986), or higher temperature (Karr and
Freemark 1983) may explain the greater use of gaps by birds. However, few
studies have addressed the roles these habitat components play in temporal
or spatial changes in bird use of early successional habitat.
Karr and Freemark (1983) suggested the importance of microclimate
as a factor in determining avian “physiological comfort.” Extremes in microclimate
may have adverse effects on birds and their reproductive fitness
(Martin and Ghalambor 1999, Wachob 1996a, Walsberg 1985). Conversely,
selection for particular microclimatic conditions may minimize foraging
costs in relation to benefits. Feeding in open areas such as forest canopy
gaps where the incoming solar radiation and air temperature is greater than
the adjacent forest (Chen et al. 1993) energetically reduces foraging costs
(DeWoskin 1980). Most microclimate studies have focused on fixed points
in time or space, such as single seasons, nest sites, roost sites, or breeding
or wintering grounds (Calder 1973; Martin and Ghalambor 1999; Wachob
1996a, 1996b). Less is known about the effects of microclimatic factors on
diurnal activities of birds across a spatial and temporal gradient, although
microclimate studies have been conducted in clearcuts, where air temperature
dramatically increases during the day relative to forest interiors (Chen et
al. 1993). Birds may respond directly to microclimate changes or indirectly
to changes in food or cover resulting from microclimate changes. For example,
many arthropods, which constitute an important food resource for birds,
respond positively to light and negatively to shade (White 1984). Abundance
and diversity of certain arthropod orders may be high in gaps because of high
light intensity (Ulyshen et al. 2004).
Light, wind, temperature, radiation, humidity, and soil conditions define
specific microclimates (Chen et al. 1999, Phillips and Shure 1990,Walsberg
1985). Whether natural or man-made, changes in forest structure affect the
local microclimate (Minckler et al. 1973, Phillips and Shure 1990, Saunders
et al. 1991). The creation of a canopy gap results in spatial and temporal
differences in the microclimate (Chen et al. 1999, Phillips and Shure 1990,
Saunders et al.1991, Xu et al. 1997), in plant species composition and structure
(Phillips and Shure 1990), and hence in resource availability (Levey
1988). These effects of microclimate on components of habitat quality may
be important in infl uencing aspects of avian life history such as survival,
reproduction, timing of breeding seasons, species dispersal, and habitat selection
(McCollin 1998, Perrins 1965, Perry 1994).
Even within seemingly homogenous habitat patches, small-scale variability
in microclimate may be important in habitat selection by birds (Sisk
et al. 1997). Because microclimate metrics vary differentially with the type
and size or length of edge (Chen et al. 1999, Saunders et al. 1991), gap size
may similarly infl uence microclimate. For example, light intensity and air
temperature increase from mature to successional forests (Chen et al. 1995,
Sisk et al. 1997). Similarly, particular locations within a canopy gap likely
2009 T.B. Champlin, J.C. Kilgo, M.L. Gumpertz, and C.E. Moorman 109
have unique microclimates that may infl uence bird activity because of shading
or distance to forest.
The degree to which birds respond to microclimatic variation within
canopy gaps and among gaps of different size is unknown. We assessed the
relationships between avian use of gaps and microclimate measures (temperature,
light intensity, and relative humidity) in experimentally created forest
canopy gaps of three sizes in a bottomland hardwood forest on the Savannah
River Site in South Carolina. We hypothesized that mist-net captures of birds
would vary with temperature, light intensity, or relative humidity.
Study site and design
We conducted the study during 2002 and 2003 at the US Department
of Energy’s Savannah River Site, a 78,000-ha National Environmental Research
Park in the Upper Coastal Plain of South Carolina. The climate of
the area is humid subtropical, with a mean annual temperature of 18 ºC and
mean annual precipitation of 122.5 cm (Blake et al. 2005). During April–October
(our study period), long-term (1964–2001) mean monthly temperature
ranged from 18.4 oC (April) to 28.1 oC (July), and mean relative humidity
ranged from 56% (April) to 78% (August, September) (Blake et al. 2005).
Our 120-ha study site was located in a 70- to 100-year-old, seasonally
fl ooded bottomland hardwood forest. The forest canopy of the study site
included typical bottomland hardwood species: Quercus falcata var. pagodaefolia
Elliot (Cherrybark Oak), Q. laurifolia Michaux (Laurel Oak),
Q. phellos Linnaeus (Willow Oak), Q. lyrata Walter (Overcup Oak) Q. michauxii
Nuttall (Swamp Chestnut Oak), Liquidambar styracifl ua Linnaeus
(Sweetgum), and Pinus taeda Linnaeus (Loblolly Pine). The understory
was dominated by Sabal minor Persoon (Dwarf Palmetto) and Arundinaria
gigantea Muhlenberg (Giant Switchcane), and a generally sparse midstory
consisted primarily of Ilex opaca Aiton (American Holly), Morus rubra Linnaeus
(Red Mulberry), and Carpinus carolinianus Walter (Ironwood).
As part of a larger study examining bird-arthropod relations in canopy
gaps (Champlin 2007), we created 12 experimental canopy openings (hereafter
gaps) via group-selection timber harvest in August 2000. Four replicates
of three sizes (0.13, 0.26, and 0.50 ha) were harvested, with the boundary
of each gap at least 150 m from the nearest adjacent gap. Because arthropod
abundance was manipulated in six of the gaps, herein we consider only data
from the six unmanipulated control gaps (two of each size). Gaps were circular
in shape and were defined to include all of the cleared area within the
circumference delineated by the boles of trees left standing at the gap perimeter.
During our study, which occurred during the second and third growing
seasons post-harvest, gaps mainly contained early pioneering species such
as grasses (Poaceae), sedges (Cyperaceae), Eupatorium capillifolium (Lam.)
110 Southeastern Naturalist Vol. 8, No. 1
Small (Dogfennel), Dwarf Palmetto, Giant Switchcane, and some woody
stump sprout regenerative growth.
We determined the relationship between bird use of various sized gaps
and their microenvironments during four periods: spring migration (1
Apr–12 May), breeding (13 May–7 Jul), post-breeding (8 Jul–31 Aug), and
fall migration (1 Sep–18 Oct). Dates for these periods were based on previous
research in the area (Kilgo et. al 1999, Moorman and Guynn 2001).
Mist-netting and microclimate measurements (see below) were conducted
simultaneously, 5 days per week during all periods. Because vegetation
structure was generally similar among our gaps and was <3 m tall (the height
of our mist nets), we felt that biases typically associated with mist-net sampling
were minimized (Remsen and Good 1996). Five nets (4-panel, 30-mm
mesh) were deployed in each gap: one at center, one each on and perpendicular
to the north and south edges, and two halfway to the east and west
gap edges. Nets were operated in three gaps (one of each size) each day from
daylight until approximately 3–5 hrs post sunrise, depending on weather
conditions. Captured birds were identified to species, banded with a metal
federal band, aged, sexed, measured, and released (Pyle 1997). Numbers of
captures were standardized to captures per 100 net hours. Our sample size
was too small to conduct analyses at the species level.
Vertical vegetation structure was recorded at each mist net during July
and early August in 2002 and 2003, using a modification of the techniques of
Karr (1971) and Schemske and Brokaw (1981). Because most gap vegetation
occurred within 3 m of the ground, we sampled only the lowest 3 m of vegetation.
Two 12-m transects were established parallel to and 2 m distant from
each side of each mist net. At 10 sampling points (1.2-m intervals) along
each transect, the presence or absence of vegetation in each of 7 height intervals
was recorded for a total of 20 points per height interval per net. Height
intervals were 0–0.25, 0.25–0.50, 0.50–0.75, 0.75–1.00, 1.0–1.5, 1.5–2, and
2–3 m. Vegetation touches were recorded along a 2-cm x 2-m vertical pole
at each sampling point and were tallied as grass/sedge, herb/forb, woody, or
vine. For the 2–3-m height interval, we sighted along the pole and recorded
the presence or absence of vegetation. We grouped all height intervals to
calculate % cover for each of the four vegetation types (GRASS, FORB,
WOODY, and VINE), and a gap’s total % cover (COVER) was calculated as
the sum of % cover for the four vegetation types.
Microclimate data collection
We measured temperature, light intensity, and relative humidity. We
selected these variables because they were easily measured and have been
used commonly in other studies of bird response to microclimate. We could
2009 T.B. Champlin, J.C. Kilgo, M.L. Gumpertz, and C.E. Moorman 111
not measure the effects of wind because we could not sample bird response
in windy conditions with mist nets. We used Four-channel HOBO® H-8 data
loggers (Onset Computer Corporation, Bourne, MA) that had an accuracy
and collection range for temperature, light intensity, and relative humidity of
±0.70 °C over -20–70 °C, ±21.5 lumens/m2 (±20% of reading) over 22–6458
lumens/m2, and ±5% over 5–50 °C, respectively.
Data loggers were placed each day at fixed positions at the midpoint of
each mist net (see Avian Response below), 1 m from the cleared net lane
and 1 m above the ground (Sisk et al. 1997). North and south nets measured
conditions at the gap edges, though data loggers were placed within the gap.
East, west, and center nets measured conditions in the gap. Loggers were affixed to the top of a 1-m cane pole with Velcro® to minimize contact surface
area that might infl uence the environmental readings. HOBO® logger data
collection intervals were set to once every 12 sec, for a maximum collection
time of 6 hr, 37 min (1985 data points per logger). We assumed that birds
in the net at the time of extraction were caught within the climate recording
hour interval (synonymous with mist-net checks every hour). Loggers
were not deployed when conditions were foggy (i.e., visible water vapor in
the air) to prevent damage to the humidity sensor and were removed if rain
was threatening. We calculated the mean for each microclimate variable by
averaging the values from points taken within the net-check hour each day
for each logger.
Only those mist-net captures that had corresponding microclimate data
were used in analyses. For example, if a data logger was removed early
because of the threat of rain, any subsequent captures were not included in
the analyses. Because of substantial differences in rainfall between sampling
years, with drought in 2002 and the presence of surface water in spring 2003,
we analyzed years separately. All data were tested for normality.
We used a linear mixed model (PROC MIXED, SAS Institute 2001) to
analyze the effects of gap size and net location on vegetation. We considered
gaps as the replicate whole plot units, gap size as the whole plot factor, and
net location as the subplot factor. Because vegetation data were collected
once per year, this model did not include period.
We used a linear mixed model (PROC MIXED, SAS Institute 2001) to
compare mean temperature, light intensity, and relative humidity among
periods, gap sizes, and net locations in each year. All microclimate measures
were highly correlated (r > 0.9), so each microclimate parameter was
analyzed in a separate model. We considered gaps as the replicate whole
plot units, gap size and period as the whole plot factors, and net as the subplot
We analyzed bird response to microclimate separately for each bird-use
period. We used a linear mixed model (PROC MIXED; SAS institute 2001)
112 Southeastern Naturalist Vol. 8, No. 1
to test avian response to the effects of temperature (°C), light intensity (lumens/
m2), and relative humidity (%). We considered gaps as the replicate
whole plot units, gap size as the whole plot factor, and net location as the
subplot factor. Birds considered winter residents (Hamel 1992), present only
in early spring or late fall, and hummingbirds were not included in analyses.
Only initial captures were used in analyses to minimize effects of autocorrelation.
We could not test for daily response because the number of bird
captures each day was small.
In 2002, we operated mist nests for 3842 net hours, during which time
we obtained microclimate data and captured 263 individual birds (excluding
recaptures) representing 41 species. In 2003, we operated mist nets for 4107
net hours and captured 282 individuals representing 33 species. However,
six species comprised 70% of the captures: Baeolophus bicolor Linnaeus
(Tufted Titmouse, 6%); Thryothorus ludovicianus Latham (Carolina Wren,
17%); Parula americana Linnaeus (Northern Parula, 7%), Geothlypis trichas
Linnaeus (Common Yellowthroat, 22%), Cardinalis cardinalis Linnaeus
(Northern Cardinal, 6%), and Passerina cyanea Linnaeus (Indigo
Total % cover of vegetation (COVER) and the four categories of vegetation
cover (GRASS, FORB, VINE, WOODY) did not differ among gap sizes
(Table 1). In both years, total % cover (COVER) and % cover of FORB and
GRASS were highest at center nets and lowest at the north and south edge
nets (Table 1).
We deployed Hobo® data loggers 108 days in 2002 and 119 days in 2003,
resulting in 7949 logger-hours. This produced 2,384,700 12-sec readings
(7949 logger-hours x 60 min/hr x 5 readings/min) for each microclimate
variable over the 2-yr study. During daylight hours (06:00–13:00), mean air
temperature in canopy gaps averaged 22.56 ± 1.98 °C in 2002 and 22.08 ±
2.76 °C in 2003. Mean light intensity averaged 5884 ± 1173 lumens/m2 in
2002 and 5679 ± 1157 lumens/m2 in 2003. Mean relative humidity averaged
81.99 ± 6.94% in 2002 and 76.90 ± 8.71% in 2003.
We detected few effects of gap size on microclimate. Relative humidity
was highest in 0.13-ha gaps during the breeding period in 2003, and light
intensity was highest in 0.50-ha gaps during the breeding, post-breeding, and
fall migration periods in 2003 (Table 2).
Temperature and relative humidity differed among net locations in all
seasons of both years except during the 2002 post-breeding period (Table 3).
2009 T.B. Champlin, J.C. Kilgo, M.L. Gumpertz, and C.E. Moorman 113
Light intensity differed among net locations in every period of 2002 except
for fall migration, but differed among net locations only during the breeding
period and fall migration in 2003 (Table 3).
Avian response to microclimate
Bird captures decreased with temperature increases during the breeding
and post-breeding periods in 2002 and increased with temperature increases
during spring 2003 (Table 4). Bird captures were lowest where light intensity
was highest during the breeding period in 2002 (Table 4). Bird captures and
relative humidity were positively correlated during the post-breeding period
in 2002 and negatively correlated during spring migration in 2003 (Table 4).
During the fall migration period, no relationships between captures and microclimate
were detected (Table 4).
Table 1. Percent vegetation cover in three gap sizes (ha) and at five net locations in canopy gaps
at the Savannah River Site, SC (2002–2003).
a. Gap size
Variable 0.13 ha 0.26 ha 0.50 ha SE F2,3 P
2002 45.93 46.64 43.07 9.73 0.08 0.929
2003 53.79 48.14 58.79 11.23 0.45 0.675
2002 13.64 18.43 13.21 6.65 0.38 0.714
2003 12.50 12.21 12.57 3.42 0.01 0.994
2002 22.57 24.79 19.57 5.89 0.40 0.704
2003 23.21 27.71 20.14 4.49 1.44 0.365
2002 4.93 1.07 3.57 3.21 0.74 0.547
2003 7.79 3.00 8.57 6.04 0.50 0.650
2002 4.79 6.07 6.71 3.06 1.02 0.459
2003 10.29 5.21 17.50 7.53 1.35 0.383
b. Net location
Variable North South East West Center SE F4,12 P
2002 22.50 23.21 61.07 54.88 64.40 7.52 15.16 0.001
2003 24.17 34.29 65.12 64.40 79.88 5.99 30.33 <0.001
2002 2.50 1.55 27.02 17.74 26.67 5.64 9.82 0.001
2003 3.57 2.14 19.88 13.57 22.98 3.37 15.55 0.001
2002 15.00 11.79 23.57 30.48 30.71 5.62 4.79 0.015
2003 11.90 16.19 24.40 32.50 33.45 5.49 6.11 0.006
2002 0.60 4.05 6.19 2.38 2.74 2.32 1.61 0.236
2003 1.79 7.98 9.05 6.67 6.79 3.40 1.34 0.311
2002 4.40 5.83 4.29 4.29 4.29 2.11 0.21 0.929
2003 6.90 7.98 11.79 11.67 16.67 5.11 1.13 0.388
114 Southeastern Naturalist Vol. 8, No. 1
Microclimate varied much more within gaps than among gaps of different
sizes. This within-gap variation was likely a result of differential shading
or distance to mature trees. The progression of light entering a gap as the sun
rose infl uenced the microclimate gradients in the gap. For example, the west
nets were first to receive light as the sun penetrated the interior of gaps,
thus beginning to warm the gaps from the west and progressing east. As
the sun’s angle increased, the temperature within a gap increased from west
to east. Temperature generally corresponded to light intensity with a slight
time lag, because temperature usually increases as light intensity increases
(Ritter 2006). As gaps warmed, relative humidity changed inversely with air
temperature. Relative humidity, the ratio of the amount of water vapor in the
Table 2. Microclimate variation by gap size (ha) in six canopy gaps at the Savannah River Site,
Year Period/variable 0.13 ha 0.26 ha 0.50 ha SE F2,3 P
Temp oC 20.74 22.57 21.92 1.11 0.69 0.566
LightA 5656 6398 6802 475 1.50 0.354
HumidityB 78.68 76.34 71.52 2.72 1.80 0.306
Temp oC 21.17 21.03 21.67 1.06 0.10 0.907
Light 5169 5696 6458 464 1.95 0.286
Humidity 85.00 82.50 80.58 3.00 0.55 0.627
Temp oC 23.78 23.92 24.39 0.62 0.27 0.780
Light 4663 5437 6316 636 1.69 0.323
Humidity 89.53 89.85 87.75 1.62 0.49 0.655
Temp oC 22.57 23.04 23.92 0.82 0.70 0.564
Light 5229 5838 6555 829 0.64 0.586
Humidity 82.64 81.39 78.04 1.93 1.52 0.350
Temp oC 20.92 21.79 20.56 1.29 0.24 0.801
Light 5500 6488 6498 257 4.96 0.112
Humidity 82.84 76.21 74.78 4.46 0.93 0.485
Temp oC 20.95 22.03 22.23 0.54 1.65 0.329
Light 4778 5768 6406 262 9.81 0.048
Humidity 85.25 82.58 76.55 1.10 16.29 0.025
Temp oC 24.28 25.29 26.42 0.41 6.96 0.075
Light 4567 6009 6653 266 16.07 0.025
Humidity 67.76 66.98 66.52 2.27 7.78 0.065
Temp oC 18.19 20.32 22.04 0.78 6.09 0.088
Light 4158 5479 5848 233 14.50 0.029
Humidity 67.76 66.98 66.52 2.27 0.08 0.929
ALight = lumens/m2.
BHumidity = relative humidity (%).
2009 T.B. Champlin, J.C. Kilgo, M.L. Gumpertz, and C.E. Moorman 115
air to the amount of water vapor the air can hold for a given temperature,
is dependent on air temperature (Critchfield 1983). The east side of gaps
generally had cooler temperatures and thus experienced higher relative humidity.
When the sun reached its zenith, the temperature differential across
a gap abated. Variations in microclimate within gaps were fairly consistent.
In nearly every season in both years, temperature was higher at the west
nets, light intensity was greater at the center nets, and relative humidity was
higher at the east nets.
We found few relationships between bird captures and microclimate
variables. Birds apparently were attracted to lower temperatures and higher
humidity during the warmer seasons (breeding and post-breeding periods)
of 2002, and they tracked higher temperatures and lower humidity during
Table 3. Microclimate variation by net location in six canopy gaps at the Savannah River Site,
SC (2002–2003). N = north, S = south, E = east, W = west, and C = center.
Year Period/variable N S E W C SE F4,12 P
Temp oC 21.93 20.04 19.90 24.20 22.65 0.68 52.59 <0.001
LightA 5404 5789 6385 6949 6900 305 20.58 <0.001
HumidityB 74.64 79.48 80.99 68.73 73.73 1.69 50.03 <0.001
Temp oC 20.52 20.84 20.50 22.91 21.69 0.66 15.20 <0.001
Light 4534 5286 6083 6289 6681 314 22.13 <0.001
Humidity 84.10 83.12 86.29 77.77 82.20 1.91 11.97 0.001
Temp oC 23.93 23.81 23.37 24.94 24.10 0.47 2.97 0.064
Light 4857 5734 5102 5510 6157 439 3.68 0.035
Humidity 88.28 89.26 91.53 86.23 89.92 1.40 2.82 0.074
Temp oC 24.18 21.73 22.43 23.82 23.72 0.56 9.66 0.001
Light 5798 5752 5780 5998 6041 536 0.25 0.904
Humidity 77.15 83.95 83.94 78.49 79.91 1.51 7.57 0.003
Temp oC 21.40 19.39 19.42 22.49 22.74 0.80 25.23 <0.001
Light 5420 5844 6205 6407 6934 332 2.97 0.064
Humidity 77.60 81.76 83.16 73.13 74.07 2.69 26.96 <0.001
Temp oC 21.13 21.07 20.62 23.13 22.71 0.38 21.34 <0.001
Light 4564 6004 5742 5739 6203 338 3.56 0.039
Humidity 82.34 82.33 84.59 77.98 80.07 0.92 11.67 <0.001
Temp oC 24.92 24.56 24.36 26.77 26.04 0.36 11.74 0.001
Light 5028 5510 6021 6224 5932 301 2.72 0.081
Humidity 65.04 68.50 75.46 62.10 64.34 2.18 5.34 0.011
Temp oC 20.91 18.41 18.81 21.75 21.05 0.61 10.53 0.001
Light 4725 4639 5113 5967 5364 301 3.18 0.053
Humidity 65.04 68.50 75.46 62.10 64.34 2.18 7.22 0.003
ALight = lumens/m2.
BHumidity = relative humidity (%).
116 Southeastern Naturalist Vol. 8, No. 1
the cooler weeks of spring migration 2003. However, in most seasons,
relationships were not significant. Birds avoided areas of the greatest
light intensity during the breeding period in 2002, but nets with the highest
light intensity probably were most visible to birds. Also, mist-net captures
were lowest later in the morning when light intensity was highest. In general,
relationships between avian habitat use and microclimate were weak.
Two factors that may have limited our ability to detect avian-microclimate
relationships include the necessity of pooling all species and the scale
of investigation. Pooling species may have obscured any species-specific
responses. However, because our sample was dominated by shrub-scrub
Table 4. Slopes of the lines describing the relationships between microclimate variables and
mist-net captures in six canopy gaps at the Savannah River Site, SC (2002–2003). Also presented
are the effects of gap size and net location, included in the ANOVA model to control
variance contributed by these effects.
Captures Gap size Net
Year Period/variable Slope SE F1,11 P F2,3 P F4,11 P
Temp ºC -0.0619 0.208 0.09 0.771 2.03 0.277 0.78 0.560
LightA -0.0059 0.004 1.87 0.199 3.86 0.148 1.24 0.351
HumidityB 0.1327 0.068 3.78 0.078 5.23 0.105 1.66 0.228
Temp ºC -0.3207 0.131 5.98 0.033 3.73 0.153 2.90 0.073
Light -0.0070 0.003 7.18 0.021 8.13 0.062 2.74 0.083
Humidity 0.1017 0.054 3.57 0.085 3.12 0.185 2.94 0.070
Temp ºC -0.5613 0.246 5.20 0.044 0.96 0.477 2.84 0.077
Light -0.0041 0.003 1.57 0.237 1.20 0.414 1.71 0.218
Humidity 0.1729 0.075 5.28 0.042 0.67 0.575 2.62 0.093
Temp ºC -0.1566 0.153 1.05 0.328 22.37 0.016 3.69 0.038
Light -0.0013 0.002 0.57 0.467 20.67 0.018 3.72 0.038
Humidity 0.0384 0.058 0.44 0.520 18.24 0.021 3.03 0.066
Temp ºC 0.4391 0.142 9.62 0.010 3.04 0.190 5.38 0.012
Light -0.0044 0.003 2.68 0.130 0.31 0.754 5.93 0.009
Humidity -0.1393 0.047 8.85 0.013 2.30 0.248 5.15 0.014
Temp ºC 0.2938 0.263 1.25 0.288 2.76 0.209 0.88 0.509
Light -0.0019 0.003 0.32 0.582 3.27 0.176 1.88 0.184
Humidity -0.0215 0.118 0.03 0.859 0.52 0.638 1.05 0.424
Temp ºC -0.4362 0.331 1.74 0.214 2.85 0.202 1.02 0.441
Light -0.0061 0.004 2.45 0.146 4.15 0.137 1.27 0.341
Humidity 0.1176 0.055 4.61 0.055 6.30 0.084 1.63 0.236
Temp ºC 0.0451 0.178 0.06 0.805 0.42 0.693 0.38 0.821
Light -0.0004 0.004 0.01 0.971 0.24 0.802 0.33 0.854
Humidity 0.0031 0.050 0.00 0.951 0.77 0.539 0.37 0.823
ALight = lumens/m2.
BHumidity = relative humidity (%).
2009 T.B. Champlin, J.C. Kilgo, M.L. Gumpertz, and C.E. Moorman 117
species, we suspect that these species likely had similar tolerances for
the microclimatic conditions of the early successional habitat in our gaps,
and thus might have responded in similar ways. Likewise, the fact that we
sampled only in gaps, and not in adjacent mature forest, prevented us from
detecting birds that may have left gaps completely for potentially more desirable
microclimates in nearby forested areas. Although we did not sample
forested areas because our focus was on within-gap habitat use, we may have
detected more consistent patterns in avian habitat use had we encompassed
a broader range of microclimatic conditions.
Although microclimate does not appear to be a primary determinant of
habitat use in our study, it may be important in some seasons or conditions.
During some warmer seasons, captures were greater in areas of lower temperatures
and higher relative humidity, whereas during some cooler seasons,
captures were greater in areas of higher temperature and lower relative
humidity. These within-patch shifts in avian habitat use during various seasons
suggest that birds occasionally tracked microclimate for physiologic
comfort (Karr and Freemark 1983), even if these patterns were not consistent
throughout the study. Similarly, Phainopepla nitens Swainson (Phainopepla)
in the southwestern US shifted to cooler microclimates to balance thermoregulatory
demands during summer (Walsberg 1985), and Poecile gambeli
Ridgway (Mountain Chickadee) in Wyoming preferentially selected warmer
foraging sites during winter (Wachob 1996b). Both of these studies were
conducted in more extreme environments than ours. The infl uence of microclimate
on avian habitat selection may be less important at our study site
because of the relatively mild climate there. Despite the occurrence of both
high and low rainfall conditions between the two years of our study, the microclimate
variables we measured were within long-term norms for the area
(Blake et al. 2005). The Southeast generally lacks environmental extremes,
and this milder climate may favor selection of habitats by birds via factors
other than microclimate, except perhaps on the coldest and warmest days.
The lack of consistent relationships between bird captures and microclimate
may be evidence that vegetation structure has more infl uence on avian
use of canopy gaps than microclimate. Bowen (2004) and Champlin (2007)
concluded that avian use of forest canopy gaps was in response to the complex
vegetation structure there rather than arthropod prey availability. Birds
may select the dense vegetation in forest canopy gaps during migration and
post-breeding dispersal, periods of increased mobility and vulnerability to
predators. During the post-breeding period, molting adults and newly fl edged
young may seek out gaps for the protective cover they offer (Anders et al.
1998, Vega Rivera et al. 1998). We conclude that microclimate variability is
not the primary determinant of gap habitat selection by birds in bottomland
forests of the relatively mild southeastern US. Rather, habitat selection is
most likely based primarily on vegetation structure, with microclimate being
118 Southeastern Naturalist Vol. 8, No. 1
The US Department of Energy provided funding for this study through the USDA
Forest Service-Savannah River and the USDA Forest Service Southern Research
Station under Interagency Agreement (DE-AI09-00SR22188). Thanks also to the
USDA Forest Service-Savannah River for logistical support, particularly J. Blake,
K. Wright, and E. Olson. North Carolina State University provided logistical and
financial assistance. We thank D. Jones, S. Junker, F. Spilker, and M. Olsen for help
with fieldwork, S. Donaghy and K. Gross for statistical assistance, and C. Sorenson
for reviewing the manuscript.
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