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Bull. Texas Ornith. Soc. 54(1-2): 2021
INFLUENCE OF BRUSH COVER AND ARTHROPODS ON AVIAN
INSECTIVORES IN NATIVE RANGELANDS OF SOUTH TEXAS
Beatriz Martinez-Martin1, Janel L. Ortiz2,4, Andrea Montalvo3
1Department of Biology, University of San Diego, San Diego, CA 92110
2Center for Excellence in Mathematics and Science Teaching, California State Polytechnic
University-Pomona, Pomona, CA 91768
3East Foundation, San Antonio, TX 78216
ABSTRACT.—Bewick’s Wren (Thryomanes bewickii), Yellow-billed Cuckoo (Coccyzus
americanus), Golden-fronted Woodpecker (Melanerpes aurifrons), Ladder-backed Woodpecker
(Picoides scalaris), White-eyed Vireo (Vireo griseus), and Verdin (Auriparus flaviceps) are all
residents of the South Texas landscape. While the species are unique in appearance, they share
the diet of arthropods with each of these species categorized as an insectivore during the breeding
season. During this time, they need more nutrients to keep up with the pressures of finding a
suitable mate and maintaining their young. The objective of this study was to investigate the
influence of brush cover and arthropods on avian insectivores in native rangelands of South
Texas. It is important to look at relationships because it can help determine what is impacting bird
abundance if populations are low. We established 600 m x 100 m line transects in two levels of
brush cover (high [.50%] and low [,50%]). We placed pitfall traps at every 100 m along transects
to capture arthropods utilizing ground habitat. We also used beatsheets and branch clippings at
each pitfall site to sample arthropods living in shrubs and trees. Bird surveys were conducted once
a week on transects and the focal bird species were recorded by visual and auditory means. Our
results showed that there was a significant effect of brush cover level (high and low) and arthropod
diversity on the relative abundance of avian insectivores. We observed a higher relative abundance
of birds as arthropod diversity increased in high brush, and a lower relative abundance of birds as
arthropod diversity increased in low brush. There were no significant effects of brush cover level
and arthropod metrics on avian richness or diversity. Having a mosaic habitat with different canopy
heights and diverse vegetation communities can positively influence animal populations and is
important when considering land management strategies.
Avian insectivores consume arthropods as their
main diet source; they are very common and are
diverse in species which makes them essential
to any ecosystem (Powell et al. 2015). Bewick’s
Wren (Thryomanes bewickii), Yellow-billed
Cuckoo (Coccyzus americanus), Golden-fronted
Woodpecker (Melanerpes aurifrons), Ladder-
backed Woodpecker (Picoides scalaris), White-
eyed Vireo (Vireo griseus), and Verdin (Auriparus
flaviceps) are resident insectivores based on foraging
guilds described by Graaf et al. (1985). These birds
are classified as insectivores because their diet
consists of 20% insects, either year-round or during
the breeding period (Graaf et al. 1985). This has been
further supported with results from fecal samples
that have shown that Coleoptera, Hymenoptera,
Orthoptera, Formicidae, and Arachnids are
common prey for understory insectivores, making
up about 75% of their individual diet (S¸ekerciog¯ lu
et al. 2002). The substrate in which these birds hunt
arthropods differs from species to species as some
find their prey on the ground, shrub, bark, and/or
canopy.
The Golden-fronted Woodpecker, Ladder-
backed Woodpecker, and Bewick’s Wren remain
insectivorous year-round (Graaf et al.1985).
Schroeder et al. (2013) found the diet of Golden-
fronted and Ladder-backed Woodpeckers had a
high percentage of animal matter, consisting of
larvae and adult invertebrates. Ladder-backed
Woodpeckers brought 100% of the animal matter
to their nestlings with 99.5% being invertebrate
4 E-mail: ortizjanel@gmail.com
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Bull. Texas Ornith. Soc. 54(1-2): 2021
considering land management practices used in
ranching and wildlife conservation that may alter
vegetation patterns that will impact arthropod and
avian populations.
METHODS
Study Area
Our study occurred on the East Foundation’s San
Antonio Viejo Ranch (SAVR) from July to August
2019. The 60,000 ha ranch is in the southern plains
of Texas in Jim Hogg and Starr counties located W
of Hebbronville and N of Guerra. The SAVR is one
of the 6 ranches operated by the East Foundation
to promote land stewardship through ranching,
science, and education. The ecological region of
the area is characterized by coastal sand plain and
Tamaulipan thornscrub with the general vegetation
cover of honey mesquite (Prosopis glandulosa),
grasslands, and shrubs (Omernik 1987). The
average temperature at SAVR during the study was
33°C but had highs of 38°C and winds that ranged
from 3-4 (6-20km/h) on the Beaufort scale.
Sampling Design
We sampled 8, 600 x 100 m transects located in
the central part of the SAVR (Fig. 1). There were
four transects located in each of two brush levels:
1) low brush cover (Fig. 2) and 2) high brush cover
(Fig. 3) that were randomly assigned in ArcGIS 10.4
using a vegetation cover layer from the Texas Parks
and Wildlife Landscape Ecology Program. We
completed this by calculating the percent of brush
in the area of the transects based on the vegetation
layer. Brush cover for this study is defined as cacti,
shrubs, and trees. Low brush transects had ,50%
of brush cover, whereas high brush transects had
.50% brush cover.
Bird Surveys
Bird surveys were conducted twice per week with
each transect surveyed once per week to ensure that
all transects were completed before the heat of the
day (approximately 1200 CST) when bird activity
decreases. Transect visits were rotated and two
transects per brush level were surveyed each day.
On survey days, the vehicle was parked at least 50 m
away from the starting point to avoid disturbing
birds that may be inhabiting the location. At the
start of the survey, environmental variables such
as temperature, cloud coverage, and wind speed
(Beaufort) were recorded using a Kestrel 2000.
larvae and only 0.5% being adult invertebrates.
Golden-fronted Woodpeckers brought back 77.5%,
yet with more invertebrate adults (56.3%), and
fewer invertebrate larvae (21.2%) compared to
the Ladder-backed Woodpecker, as well as 20.1%
vegetation (Schroeder et al. 2013). The insects that
the woodpeckers brought back were reflective of
their foraging behavior (i.e., excavating and prying
into bark). Yard et al. (2004) took stomach content
samples from Bewick’s Wrens which resulted in a
variety of arthropods including Araneae, Hemiptera,
Homoptera, Coleoptera, Diptera, Hymenoptera,
Lepidoptera larvae, and other smaller samples with
a higher proportion of Araneae. These orders are
all reflective of an insectivore’s diet and can all be
found in the native rangelands of South Texas.
Arthropods are the most diverse group of
animals and have the largest number of species in
the world (Misof et al. 2014). The origin of insects
has been dated back to derive from the Early
Ordovician period, nearly 479 million years ago
(Misof et al. 2014). Insects occur in almost any
possible environment and play a crucial part of our
ecosystem since they fulfill many roles ranging
from decomposing organic matter to serving as
food for fish and wildlife (Rosenberg et al. 1986).
Due to the large abundance of insects practically
everywhere in the world, it makes it rather easy to
sample them and see the impact they make in both
aquatic and terrestrial ecosystems as predators, prey,
parasites, herbivores, among others (Rosenberg et
al. 1986). Serving as prey, insects play a crucial
role in the life cycles of many South Texas avian
species, specifically insectivores that need protein
year-round as well as during the breeding season for
themselves and nestlings (Dhondt and Hochachka
2001).
Habitat and prey relationships of avian
insectivores are important to their conservation
and management, yet have not been fully explored
in South Texas. The objective of our study was to
determine the influence of brush cover level and
arthropods on avian insectivores. We hypothesized
that higher brush cover and higher relative
abundance and diversity metrics of arthropods
would yield a higher abundance and diversity
metrics of birds. This was hypothesized because
higher brush cover offers more protection and
more resources benefitting both arthropods and
insectivores. Having this information can provide
further support in the precautions taken when
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Bull. Texas Ornith. Soc. 54(1-2): 2021
Figure 1. East Foundation’s San Antonio Viejo Ranch in Jim Hogg and Starr counties, TX, USA. High (blue) and low (red) brush
transects marked with circles indicating beginning and end of transects sampled from July-August 2019.
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Bull. Texas Ornith. Soc. 54(1-2): 2021
Figure 2. Example of vegetation found in a low brush (,50% brush cover) transect on the San Antonio Viejo Ranch, Jim Hogg and
Starr counties, TX, USA in July-August 2019.
Figure 3. Example of vegetation found in a high brush (.50% brush cover) transect on the San Antonio Viejo Ranch, Jim Hogg and
Starr counties, TX, USA in July-August 2019.
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Bull. Texas Ornith. Soc. 54(1-2): 2021
2. Beatsheets were sampled at the nearest shrub
to the pitfall trap and within 50 m of the
transect and sampled twice throughout the
study period.
3. Branch clippings taken from three different
trees near each pitfall trap, each from a
different height of the canopy (low canopy,
center canopy, upper canopy) within 50 m
of the transect and were sampled twice
throughout the study period.
For pitfall traps, we used 16 garden staples, a
plastic cup (9 oz), plastic plate, three nails, 50%
propylene glycol (0.5 oz), and PVC flashing as
walls in an X-shape following the recommendations
of Koivula et al. (2003) (Fig. 4). The traps were
alternated in which each transect was sampled
twice for a week throughout the 6 weeks of the
study. To sample arthropods in the trees and shrubs
we used branch clippers and a white bed sheet.
Three branches were clipped from three different
trees per pitfall location at three different heights
Surveys were performed by a single observer and
all surveys began at sunrise with transects walked
at the same pace while listening and observing for
the focal species. The six focal species were year-
round insectivores based on Graaf et al. (1985) and
since we were at the end of the breeding season we
also included breeding period insectivores. Focal
species included: Bewick’s Wren, Golden-fronted
Woodpecker, Verdin, Ladder-backed Woodpecker,
Yellow-billed Cuckoo, and White-eyed Vireo. No
surveys were conducted with rainfall or winds with
consistent $4 on the Beaufort scale.
Arthropod Sampling
We used three methods, accounting for the
locations in which birds forage, to estimate
arthropod populations:
1. Pitfall traps were set up at every 100 m along
the transect (6 per transect) and sampled
twice throughout the study period for one
week.
Figure 4. Pitfall trap design with X-shaped guidance barriers used on the San Antonio Viejo Ranch, Jim Hogg and Starr counties,
TX, USA in July-August 2019.
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Bull. Texas Ornith. Soc. 54(1-2): 2021
different species represented in your sample, N
5 total number of individual organisms in your
sample) was used to measure species richness in the
area for both. Data for bird and arthropod metrics
were log transformed to meet assumptions and are
interpreted on the log scale.
RESULTS
We documented 407 avian insectivores and
sampled 2587 arthropods. Bewick’s Wren was the
most abundant species in both the high and the low
brush areas (Fig. 5). The Yellow-billed Cuckoo
was the least abundant with none found in the low
brush area and only one in the high brush. The
Ladder-backed Woodpecker was not included in
data analysis as there were birds in the area but they
were not within the transect perimeter. There were
more arthropod orders found in the high brush than
the low brush with Hymenoptera being the most
abundant order in both high and low brush areas
(Fig. 6). Odonata, Siphonaptera, Trombidiformes,
and Myriapoda were the least common orders
found. Siphonaptera and Trombidiformes were
found only in the high brush.
Avian diversity and avian richness were not
significantly influenced by brush cover level
or arthropod metrics (Table 1) but brush cover
level and arthropod diversity had a significant
influence on avian abundance (P , 0.003). Figure 7
demonstrates the interaction between arthropod
diversity and brush cover. The effect of high brush
(lower canopy, center canopy, upper canopy),
we visually analyzed the clipping for arthropods
then beat branches onto the sheet to account for
any remaining arthropods. We then placed a sheet
directly under a shrub within 5 m of each pitfall
trap and used a PVC pole to hit the shrub for 20-30
seconds to knock all arthropods onto the sheet. The
sheet was dusted off each time and between each
method to prevent double counting. Arthropods
were counted and classified based on their order
after each method.
Statistical Analysis
Data were analyzed using a multivariate multiple
regression in RStudio using the stats package
(RStudio Team 2020) to assess the influence of brush
level and arthropod metrics on avian insectivore
metrics. Predictor variables included brush cover
levels and arthropod metrics. Response variables
included avian insectivore metrics. Metrics for birds
and arthropods were relative abundance, diversity,
and richness. Relative abundances of birds and
arthropods were calculated using the ratio of the
total number of individuals counted by the length
of the transect by week and transect. The Shannon-
Weiner Diversity Index (H 5 2Σ Pi(lnPi);H 5
diversity, Pi 5 number of individuals of species i/
total number of samples) was calculated to measure
the species diversity in each transect for birds and
arthropods (Ali et al. 2016). The Menhinick’s Index
(D 5 s N; D 5 species richness, s 5 number of
Figure 5. Relative abundance of avian insectivores in high and low brush areas on the San Antonio Viejo Ranch, Jim Hogg and
Starr counties, TX, USA in July-August 2019.
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Bull. Texas Ornith. Soc. 54(1-2): 2021
arthropod abundance and avian insectivore
abundance (P , 0.003, Table 1) this was not as
ecologically meaningful since the most abundant
arthropod order (Hymenoptera) was primarily ants.
cover on avian insectivores becomes more positive
as arthropod diversity increases. The effect of
low brush cover on avian insectivores become
more negative as arthropod diversity increases.
While there was a significant relationship between
Table 1. Multivariate multiple regression results with log-transformed data for the effect of brush level cover and arthropod
metrics on avian insectivore metrics on the San Antonio Viejo Ranch, Jim Hogg and Starr counties, TX, USA in July-
August 2019.
Predictor Response
Avian Relative Abundance Avian Richness Avian Diversity
Estimate SE t P Estimate SE t P Estimate SE t P
Brush Level
(Low, High)
0.080 0.120 0.664 0.515 20.148 0.165 21.651 0.197 0.082 0.151 0.547 0.712
Arthropod
Relative
Abundance
0.479 0.142 3.368 0.003** 20.261 0.131 22.00 0.061 0.042 0.119 0.354 0.728
Arthropod
Richness
0.410 0.473 0.868 0.397 20.350 0.434 20.808 0.430 0.209 0.397 0.527 0.605
Arthropod
Diversity
1.11 0.450 2.476 0.023* 20.145 0.413 20.351 0.730 0.108 0.378 0.286 0.778
Brush Level
x Arthropod
Richness
0.992 0.489 2.027 0.058 20.256 0.449 20.570 0.576 0.193 0.411 0.471 0.643
Brush Level
x Arthropod
Diversity
-2.81 0.831 -3.392 0.003** 1.02 0.762 1.34 0.199 -0.526 0.696 -0.775 0.466
Adjusted R20.6438 -0.06169 0.1428
F-Statistic 8.23 0.7676 1.666
* P , 0.05; ** P , 0.01; *** P , 0.00
Figure 6. Relative abundance of arthropods in high and low brush areas on the San Antonio Viejo Ranch, Jim Hogg and Starr
counties, TX, USA in July-August 2019.
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Bull. Texas Ornith. Soc. 54(1-2): 2021
this should lead to less diversity in bird species as
there is potentially more competition between avian
insectivore species if they are specialists that seek
out specific arthropod types.
Many environmental variables can affect the
presence of birds. The Yellow-billed Cuckoo
was found only in the high brush area which was
expected since they are upper canopy gleaners
meaning they spend an abundant amount of the
time in trees (Graaf et al. 1985). More Verdins were
found in the high brush area which is expected since
they are lower canopy and shrub gleaners, and the
high brush area is dense with vegetation. A higher
number of Golden-fronted Woodpeckers were
found in the low brush area, yet they are typically
found in dense vegetation areas (Schroeder et al.
2013). Within the United States, Golden-fronted
Woodpeckers are closely associated with mesquite
brushlands and riparian corridors (Husak and
Maxwell 2000), however in this study they were
primarily found within grassland areas which may
be due to food availability and vegetation condition.
The number of Bewick’s Wrens was relatively
similar in both high and low brush which is likely
due to the fact that they are a generalist species and
can be found almost anywhere (Kroodsma 1985).
For both high and low brush, there were instances
where no birds were recorded, which could have
DISCUSSION
There was a higher total number of insectivorous
birds found in the high brush than in the low
brush, which was expected as birds look for places
that provide the best coverage and have space to
reproduce (Krausman 1999). There were 16 orders
of arthropods identified, with the most abundant
being Hymenoptera. Arthropod diversity and
brush cover as an interaction had an effect on the
relative abundance of birds which could be a result
of birds having a broader diet. Since there was a
higher diversity of arthropods in high brush, birds
may not have to compete for one type of arthropod
which may lead to a higher abundance of avian
insectivores. Brush cover and arthropod metrics did
not influence avian diversity or avian richness. This
could have occurred due to that area already being
high in avian diversity and richness therefore brush
cover and arthropod metrics did not matter.
The relative abundance of arthropods was
relatively equal which was expected since
arthropods can live anywhere in the world
and occur in almost any possible environment
(Rosenberg et al. 1986). Fewer orders were found
in the low brush than the high brush area which can
be because the high vegetation provides more area
for shelter and food (Rosenberg et al. 1986). Since
there are fewer arthropod orders in the low brush,
Figure 7. The effect of brush cover level (high and low) and arthropod diversity on the relative abundance of avian insectivores on
the San Antonio Viejo Ranch, Jim Hogg and Starr counties, TX, USA in July-August 2019.
17
Bull. Texas Ornith. Soc. 54(1-2): 2021
Office of Undergraduate Research, and Biology
Department at the University of San Diego for
financial support. This is manuscript number 062 of
the East Foundation.
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resulted for two reasons. There could have been
possible human disturbance by the vehicle, and
while it was parked at least 50 m from the start
of the transect it could have possibly flushed any
birds in the area. At times, birds were heard but they
were not within the transect limits and therefore not
recorded. Overall, there were also environmental
factors that could have hindered the study. The
temperature changed throughout the course of the
five-week period. Temperatures rose from the start
to the end of the study from 32° C to 38° C. At the
beginning of the study, there were many more birds
recorded as the temperature was at an ideal range
for them to be active. As temperatures rose, less
birds were recorded as birds are generally not very
active in hot temperatures and it alters their activity
(Vafidis et al. 2019). There are some birds, like the
Yellow-billed Cuckoo, that were not vocal which
could have reduced our opportunity of recording
them during surveys. The Yellow-billed Cuckoo
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of the study which can be a result of the species
being active on the nest rather than vocalizing or
foraging.
Further research is required to determine
associations between arthropod and avian
insectivore abundance. Having a longer study
period or more transects would improve this study
as it will give a better representation of the birds and
arthropods in the area. Having additional observers
in the study would also help as this would allow for
multiple transects to be surveyed at the same time
while covering more ground but will have to come
with more rigorous analysis of observer reliability.
Implementing these changes will yield improved
information which will give a better representation
of what is happening with our avian populations on
South Texas ranches. Having information regarding
the importance of brush cover will help ranchers
make important land management decisions that
help the wildlife that resides there.
ACKNOWLEDGMENTS
The authors would like to thank the East
Foundation for access, lodging, and support at
the SAVR, Angelica Ocaña for her assistance in
fieldwork, Masi Mejia-Serna, and Tina Buford for
their support of the project. Thank you to Mikaela
Wells, Bailey Stotler, and Norma Martinez-Martin
for their assistance in compiling and analyzing data.
We recognize and thank the Associated Students,
18
Bull. Texas Ornith. Soc. 54(1-2): 2021
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