Acorn woodpecker (Melanerpes formicivorus) behavior varies in response to intra- and interspecific calls - UC Natural Reserve System
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Acorn woodpecker (Melanerpes formicivorus) behavior varies in
response to intra- and interspecific calls
Josephine Collier1, Skye Hoolihan2, Ethan Jakob3, Hannah St. John4
1
University of California, San Diego; 2University of California, Santa Cruz; 3
University of California, Santa Barbara; 4University of California, Davis
Acorn woodpeckers are a community-oriented species that, like other birds, uses and
responds to vocalizations as a form of communication. The calls of certain competitors,
predators, and intraspecific individuals change the behavior of a listener in different ways.
The behaviors of three acorn woodpecker colonies were observed in response to four
recordings: Steller’s jay, sharp-shinned hawk, an acorn woodpecker distress call, and a
control. An ethogram was recorded for pre-call and post-call behavior of each colony.
Behavioral response varied between colonies, possibly due to environmental factors and
developed behaviors, leading to inconclusive results.
Keywords: acorn woodpecker (Melanerpes formicivorus), focal sampling, playback,
auditory signaling, interspecies relationships
INTRODUCTION Cheney 2003). Birds are heavily reliant on
auditory signals due to the vast number of
Organisms that are at risk for predation or species-specific songs and calls that provide
resource theft must be aware of the them with information about other birds
potential interspecific threats to their around them. Birds will use auditory stimuli
survival. Staying keyed into the warnings of to identify other birds and mark their
predatory or competitive behavior gives an territories, both of which are crucial in
advantage to the listener and allows them to discerning friend from foe (Sibley et al. 2001).
take preventative measures. Animals One species that heavily relies on auditory
communicate using primarily auditory, signals is the acorn woodpecker (Melanerpes
olfactory, or visual signals that inform their formicivorus); their cooperative tendencies
behavior (Lakin & Hendrie 2020). Auditory and strict territoriality result in frequent and
signals function well for terrestrial necessary communication. Acorn woodpeckers
organisms that may live in habitats with live in social groups averaging five or six
limited sightlines, as visual signals do not individuals and each colony defends a
travel as effectively through these areas. territory averaging six hectares (MacRoberts
Auditory signals allow an individual to & MacRoberts 1976). These territories
spread its message effectively, but an contain all necessary resources and do not
unintended listener, such as a predator, may overlap with other colonies (MacRoberts
catch on and harm the signaler (Seyfarth & 1970). To survive with decreased food
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sources during the winter, acorn intruder species. We hypothesized that a
woodpeckers have evolved an extremely competitor’s call, when compared to a
unique behavior: the creation and use of designated control call, would cause acorn
granaries, trees with holes drilled into them woodpeckers to group up for territory
for the purpose of nut storage for the winter, defense, therefore increasing their activity
most notably acorns (Sibley et al. 2001). and time spent visible. Alternatively, we
These granaries are fiercely defended and theorized that a call indicating a predator in
are important markers of the acorn the area (a predator or distress call) would
woodpecker’s territory (Drake et al 2020). To cause the woodpeckers to hide, thus
defend these territories, acorn woodpeckers lowering their activity and time in sight more
will “supplant” intruders, meaning that they than a control call.
will make aerial passes at the intruder to
persuade them to leave or face pursuit METHODS
(MacRoberts 1970).
There are many species of intruders, both We conducted our research at the
competitors and predators, that are known University of California’s James San Jacinto
to invade granaries and are often challenged Mountains Reserve (33.8081° N, 116.7769°
by defensive acorn woodpeckers. Acorn W) in Riverside County from May 5 to 9,
woodpeckers will alert each other of 2021. At 1,623 to 1,692 m in elevation, the
intruder activity when territories are reserve experiences hot and dry summers,
invaded (Doherty et al. 1996). This list of and cool and snowy winters, which allows
intruders includes the Steller’s jay for the success of the mixed coniferous
(Cyanocitta stelleri), a generalist competitor, forest that the reserve is situated in (James
who has been known to steal acorns from San Jacinto Mountains Reserve 2021). This
granaries (Mumme 1985). The sharp- mixed coniferous forest includes pines
shinned hawk (Accipiter striatus) is a (Pinus) and oaks (Quercus), both highly important
significant predator of acorn woodpeckers, genera of trees for acorn woodpeckers.
and when they are detected in woodpecker We observed acorn woodpeckers at three
territory, woodpeckers will hide in their granaries (Fig. 1). Site A (33.81032,
roost until it is safe to carry on with daily -116.77486) had an elevation of 1,657 m;
activities (MacRoberts 1976). In cases where The primary granary was a dead pine tree, a
acorn woodpeckers are caught by predators, snag, with two adjacent snags at the bottom
they will send out a distress call. There are of a slope. At the slope’s base was a campsite
two main hypotheses on the purpose of this and a trail with minimal foot traffic. There
call: that the scream’s purpose is to convene were few oak trees in the area compared to
other woodpeckers to protect the attacked the abundant pines. Site B (33.80878,
individual, and that the call attracts secondary -116.77730) was at an elevation of 1,655 m
predators to cause the original predator to and had the most open space. Site C
abandon the prey (Koenig et al. 1991). (33.80632, -116.77463) was at a higher
Given the need to defend their territory, elevation than the other sites at 1,707 m and
we wanted to know how acorn woodpeckers was the most heavily forested with an even
would respond to the calls of various mix of oak and pine.
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the call, we observed our individuals for 15
minutes to analyze their post-call behavior.
We rotated and observed each site in the
morning and the evening for four days, for a
total of 12 different observation periods
(four at each site). We created an ethogram
to study the distribution of acorn
woodpecker activity before and after each
different type of call, over our observed
period. The ethogram shows the behaviors
organized into three categories: active,
stagnant, and out of view.
Table 1. Description of the different Acorn
Woodpecker behaviors recorded during observation
Figure 1. Satellite map of UC James San Jacinto
periods.
Mountains Reserve, Lake Fulmor, Idyllwild,
California. There are three experimental sites within
the territory of three different Acorn Woodpecker
colonies on the reserve. Sites are labelled A (red), B
(green), C (blue). The elevation of each site is 1657 m
(A), 1655 m (B), and 1707 m (C). Site A is near a
campsite and a footpath, site B is next to a dirt road,
and site C is adjacent to a trail heading upslope. The
points were marked and the map was made using
Google Earth and Gaia GPS.
Each morning, from about 9:00 am to
10:30 am, and evening, from about 4:00 pm
to 5:30 pm, we would go to one site and give
the woodpeckers a 10-minute acclimation We specifically looked at stagnant
period to allow them to get used to our behavior, which includes perching, pecking,
presence. We then selected two individuals and preening, and compared it against active
from each group and recorded their actions behavior, which includes all other visible
for 30 minutes (Table 1), from about 10 to 30 actions: interactions, calling, drumming,
m away. If an acorn woodpecker flew out of moving, climbing, and flying. We also
sight, we would choose a new individual. compared proportion of time woodpeckers
After those 30 minutes, we would play a were not visible and time they were visible
randomly selected 45-second call of either a before and after each call. We took written
Steller’s jay (Keller 1989), a sharp-shinned notes during each observation to characterize
hawk (Budney 1993), an acorn woodpecker acorn woodpecker activity as the call was
distress call, or a chorus of Pacific tree frogs played. For another measure for activity
(Pseudacris regilla). The call was played level, we also calculated the rate of change
using a phone connected to a JBL Clip 3 of activity before and after each call was
Bluetooth speaker at full volume. Following played. The rate of change of activity was
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CEC Research https://doi.org/10.21973/N3C963 Spring 2021 Vol. 5 Issue 4 3/8calculated in actions per minute and
represents these rates before and after each
call was played. This was conducted to see if
there was a significant difference in changes
in activity rates based on the different calls
we played.
We used JMP statistical software v15 (SAS
Institute 2019) for all statistical tests. We ran
an ANOVA to compare the rate of change of
activity before and after each call at each
site. To compare certain behaviors before
and after each call at each site to see if there Figure 2. Ethogram of acorn woodpecker behavior.
was a significant relationship between the Acorn woodpecker pre-call behaviors were recorded
behavior before and after calls were played, for 30 minutes and compared to the 15-minute post-
we ran 24 different Pairwise Fisher’s call observations for four different recorded calls:
Steller’s jay, sharp-shinned hawk, acorn woodpecker
Exact Tests.
distress call, and Pacific tree frog control call. The
recorded behavior categories were active, stagnant,
RESULTS and when the woodpeckers were out of view.
Ethogram shows that primarily more time is spent out
3.1 Statistical of view, except for the distress call, where the
majority of their behavior was stagnant (N = 12).
Acorn woodpeckers spent the most
Table 2. Acorn woodpecker stagnant vs active (α =
amount of time out of view, followed by
0.004; N = 12); asterisks indicate values of
perching in all cases (Fig. 2). There was no significance. Yellow highlight indicates an increase.
difference in the rate at which the Blue highlight indicates a decrease. These were 12
woodpeckers performed activities between different Pairwise Fisher’s Exact Tests that
the pre-call period and the post-call period demonstrated how proportion of stagnant behavior
across all call types (N=12, F=2.8, p=0.13). (behavior of woodpeckers not in movement) to active
behavior (behavior of woodpeckers in movement)
For the twelve Pairwise Fisher’s Exact Tests, changes from before to after each call at each location.
looking at the proportion of time
woodpeckers spent being “stagnant,” we
found that nine showed a difference
between the pre- and post-call periods
(Table 2). When it came to the proportion of
time in view versus out of view, eight out of
the twelve tests displayed a relationship
between woodpecker absence and a call
being played (Table 3). Although our results
from most of our Pairwise Fisher’s Exact Test
were significant, the amount by which they
differed and the direction in which the
results shifted varied greatly.
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CEC Research https://doi.org/10.21973/N3C963 Spring 2021 Vol. 5 Issue 4 4/8Table 3. Acorn woodpecker presence vs absence (α = site C, and the woodpecker returned a flight
0.004; N = 12); asterisks indicate values of display, supplanting the jay from the area.
significance. Yellow highlight indicates an increase.
Blue highlight indicates a decrease. These were 12
When the sharp-shinned hawk call was
different Pairwise Fisher’s Exact Tests that played, we observed avoidance behavior by
demonstrated how time out of sight (time we were the woodpecker colonies at all three sites.
unable to see the woodpecker) to proportion of time They would fly to further trees, especially
in eyesight (time we were actually able to see the where they were less visible, such as in
woodpecker) changes from before to after each call
at each location.
densely foliated pine trees. We also noted
that they called less frequently in the post-
call periods. When we played the recorded
distress call, we noticed that multiple
individuals would fly to a tree closest to the
viewing area and appear to look for the
source of the noise. In two of the three
events, these individuals would give off their
own call during and directly after the
playback. We also observed the
woodpeckers flying away after their lookout
3.2 Observational
period, eventually returning later during the
We observed varying responses to the four post-call period. Finally, for the Pacific tree
different calls that were not consistent frog call we observed no shared reactions
across observation events. The three sites between playback events.
had very similar colony sizes: site A had six to
DISCUSSION
eight woodpeckers, site B had approximately
four woodpeckers, and site C had eight While we expected to see no reaction to
woodpeckers. During the Steller’s jay call we the control call, the mixed response we
observed two sites where multiple observed may have been because it was a
woodpeckers came together on a central loud and unexpected noise. The inconsistent
tree, close to the viewing area, and looked results may also point towards natural and
for the source of the call while also unrelated shifts in acorn woodpecker
vocalizing. We did not observe this grouping behavior during our selected time scales.
behavior at site B and the woodpeckers were In response to the Steller’s jay call, we
much more silent in the post-call period. expected acorn woodpeckers to increase
During one observation period we observed their activity and the rate at which they
up to seven Steller’s jays came to the nearby would change behaviors, as well as spend
trees, even landing on the central tree. more time in view. Contrary to what we
Rarely would other birds land on the trees thought, the woodpeckers seemed to stay in
the woodpeckers primarily perched on. view roughly the same amount of time
During a separate observation period, a before and after the call, seeming to not
Steller’s jay was observed making an mind the potential presence of a Steller's jay
aggressive flight display at a woodpecker at nearby. Jays are known to steal from acorn
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CEC Research https://doi.org/10.21973/N3C963 Spring 2021 Vol. 5 Issue 4 5/8storages, and woodpeckers will supplant the likely a preventative measure, the response
jays (MacRoberts 1970). We had personally might be stronger with a visual cue.
observed a Steller’s jay take an acorn from Predators such as hawks have been known
an active granary prior to our research, and to elicit prey birds' alarm responses in
we had expected more instances of this studies where visual cues were used, such as
while we recorded data. Jays were present bird models or video representation (Butler
at our three sites, but rarely interacted with et al. 2017).
the woodpeckers or the granaries. One Acorn woodpeckers have been known to
reason that could explain the lack of jay support the “predator attraction
interactions is that the granaries have fewer hypothesis,” where a predator catches a
acorn stores at this time of the year. This woodpecker who then screams, often
seasonal decrease could also explain the resulting in the arrival of a secondary
woodpeckers’ neutral response to the jay predator, distracting the primary threat
call. While acorn woodpeckers still defend (Koenig et al. 1991). Based on this, we did
their preferred trees during the spring, not expect woodpeckers to approach the
defense events are much less likely as there played distress call. However, we observed
is little motive for outsiders to come into the the woodpeckers flying into nearby trees
territory. Insects are much more available at and appearing to search for the source of the
the end of winter, and the woodpeckers distress call. We also had one instance where
themselves spend more time flycatching in the grouped woodpeckers responded with
the spring and summer (MacRoberts & their own distress call. This contradicts the
MacRoberts 1976). Acorns contain less distress call study because they did not
nutrients than insects and therefore provide observe a conspecific response to the
a lower reward in the warmer months, distress call. We propose that the habitat
perhaps lowering the interspecies differences affect the behavioral response.
competition for acorns (Koenig et al. 2008). Acorn woodpeckers are disproportionately
Hawks, as raptors, do not perform calls for studied in oak woodland ecosystems and
the same reasons that a songbird might; these studies are not representative of
hawks mainly call for breeding or territorial populations in different ecosystems. Acorn
purposes (Sibley et al. 2001). Despite this, woodpeckers are heavily dependent on
we expected the acorn woodpeckers to acorns but do not require extensive amounts
perform avoidance behaviors, such as hiding of oak stands to subsist themselves, so they
in dense trees and remaining silent for a are able to thrive in mixed woodland
time, to keep their location hidden. The environments (Koenig et al. 2008; Koenig &
woodpeckers did become less active at two Benedict 2002). Mixed coniferous forests are
of the three sites we studied, however, there more dense than oak woodland or savanna,
were no consistent results regarding the and this can affect an animal’s expected
amount of time the woodpeckers were behavior due to changes in cover. In exposed
present. We observed that after the hawk areas, like those that have been burned,
call was played, the woodpeckers were less animals tend to be more vigilant because
active and did not call as frequently as they are more visible to potential predators
before. While this reduced activity is most (Drake et al. 2020). Additionally, hawks call
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prey species will have to be more reliant on
visual predator cues. Dense foliage impacts This work was performed at the University
how far visual signals can be conveyed, of California’s James San Jacinto Mountains
making audio signals even more crucial for Reserve, doi:10.21973/N3KQ0T and
animals to be aware of. This difference in composed at the University of California
coverage may provide some explanation to Steele/Burnand Anza Borrego Desert
the observed differences in behavior Research Center, doi:10.21973/N3Q94F.
between these ecosystems. Special thanks to Dr. Tim Miller and Dr. Sarah
Overall, we found that reactions to Kingston for your guidance in completion of
intruder calls differed between colonies this research project.
more than expected. This may be due to
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