Animals more or less ignored. Many studies have reported

Animals
have long been known to alter their behavior to avoid predation. Most attention
in the study of predation effects has been given to lethal effects of
predation, while nonlethal effects have been more or less ignored. Many studies
have reported that the perceived risk of predation has a direct impact on
foraging activity (Frid et al. 2002, Lima et al. 1990, Beale 2007, Cresswell
2008, Lind and Cresswell 2005). If individuals of a species differ in their
vulnerability to predation, behavioral changes can result usually concentrated
on the starvation-predation trade-off. (Cresswell 2008, Lind and Cresswell
2005, Cresswell 1994). Foraging involves the balance of energy gains against
the risk of being consumed. Under the risk of predation, it may be to the animal’s
advantage to avoid a profitable food source and adjust by choose foraging
times, feeding areas and other foods that are subprime with respect to lessening
the risk of starvation alone (Houston et al. 1993). Much research has been
conducted on birds to observe their potential to assess their food supply and
implement a profitable feeding technique (McNamara et al. 1994, Pravosudov
et al. 2001). In light of the optimal foraging theory, we would expect
to see a bimodal relationship between foraging intensity and time of day by reducing
rigorous feeding to times when the threat of starvation surpasses the threat of
predation (McNamara et al.1994, Bednekoff and Houston 1994). There is some experimental
evidence that reveals patterns of an increase in daily weight gain (foraging
activity) during the first three hours after sunrise among blackbirds
(Cresswell 1998). However, the majority of empirical data of foraging patterns appears
to follow the risk-spreading theorem, which entails continuous foraging
throughout the day until a critical threshold of stored energy is reached (McNamara
et al. 1994, Wolf and Hainsworth 1977). Studies showing a relatively constant
daily weight gain suggests that predation was either stable during the day or
not a significant driver of foraging behavior. A recent study of direct
quantification of day-to-day feeding behavior has been conducted and offers
more support for the risk-spreading theorem regarding bird-feeding times
(Bonter et al. 2013). It has also been found that birds favor foraging at
feeders situated in tall vegetation/more tree cover versus areas with less vegetative
growth (Lee et al. 2005).
Tall
foliage can provide better shelter against predation and birds are expected to
take it into account and yield the most profitable feeding technique. Although,
researchers were able to get significant results in understanding bird behavior
by analyzing feeding times, and weight gain, foraging still involves a complex
balance between survival, growth, and reproduction, making it difficult to examine
a single major element of fitness since they are all interconnected. 

 Daily climatic changes can also influence
when, where, and how birds search for food. Short-term environmental
stochasticity has one of the most significant indirect impacts on daily
foraging patterns of a number of wildlife species worldwide (Lesley 2015, Bost
2015, Hughes 2000, Parmesan 2006, Walther 2002, Hunt 2011, Baier and Napp 2003).
Daily climatic variation, such as changes in temperature, humidity, rainfall
and airflow all play a role in altering the feeding behavior of bird populations.
In every population, certain individuals are able, to varying degrees, to cope
successfully with a range of conditions within their immediate habitat. A
direct correlation between altered foraging behavior and breeding has been made
regarding global climatic variation under El Nino Southern Oscillation (ENSO)
conditions among albatrosses and penguins (Lesley 2015, Bost 2015). During the
colder days of the year, birds tend to feed more because they demand more
energy to maintain a stable body temperature (Bedneckoff and Houston 1994).

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There
are many other factors that influence prey vulnerability, which may constrain
our ability to assess the effect of the starvation-predation trade-off on bird
feeding. Close proximity to other individuals can create competition over food,
which is not taken into account and may impose variability in our results.
Varying diets of relatively different species also increases the complexity of
understanding of bird behavior (Poulin 1994). Although logical, most bird
behavioral studies were conducted without taking in to account the complex
interactions involved in the starvation-predation tradeoff that birds must
assess when foraging. Scientists have not yet been able to directly test bird
foraging behavior while controlling for other factors that may drastically interrupt
the analysis of their results.

We
quantified temporal changes in the daily foraging behavior of birds in fall by
tracking seed mass at feeding stations. Our objective was to quantify the
day-to-day feeding patterns in response to three simulated levels of predation.
We tested whether predation and distance of the predators had direct impacts on
bird’s food intake in areas of high and low cover. We
predicted that foraging activity would be directly related to the distance that
separates the predators from the feeder and that there will be an overall
increase in foraging activity in the area of high cover.

 

METHODS

Study
Site

We
examined the feeding activity of birds in two isolated experimental sites on
the outskirts of The College of New Jersey, NJ, USA (40.272994, -74776983) over
a 10-day period during the month of November 2017. Bird species in the area
were not identified. All 18 birdfeeders were originally filled with 200 grams
of sunflower seeds. Each morning (9AM) and afternoon (4PM), we measured the levels of seed mass (in grams), calculated
the amount of food eaten, and refilled all birdfeeders back to 200 grams. To
simulate high and low cover, we set up two identical experimental sites, one in
the forest and the other just outside of the forest. Plastic owls suspended on
poles represented our predators. To simulate high, moderate, and low levels of
predation, owls were placed 5,10, and 20 meters away from the center of each
site. AM samples represent nighttime feeding and PM samples represent daytime
feeding.

Foraging Analysis      

To explore the general impact on total amount of foraging, one-way
analysis of variance was performed to test amount of foraging in response to
time, levels of predation, and type of cover. The amount of foraging was
quantified by mass depleted from each birdfeeder. Two-way analysis of variance was
performed to discover the relationship between levels of predation and cover,
cover and time, and levels of predation and time on average bird feeding. To
investigate all the effects on bird feeding at once, a factorial analysis of
variance was conducted. The amount of foraging was quantified by mass depleted
from each birdfeeder. Mass depleted was compared relative to three categorical
variables, time of foraging (am, pm), levels of predation (high, medium, low),
and cover (forest, no cover).

 

RESULTS

The average amount of seed mass depleted during nighttime feeding
was observed to be slightly higher in relation to daytime feeding within the
bird community. There was no significant difference of foraging during the day
and night: on average, the average amount of mass depleted from birdfeeders did
not vary throughout the length of the experiment.

The
overall quantity of seed consumed by wild birds was considerably higher in the
forest cover relative to the amount of seeds eaten from the birdfeeders with no
cover (Figure 1). The average mass depleted in response to types of cover and
time showed a significant difference with an increase of seed intake under
forest cover relative to no cover (F1.379=3.91, p=0.049).

The
quantity of birdseed eaten under the high-simulated predation level was
significantly lower than that from medium and low levels (Figure 2, F2.379=21.58,
p=0.00). There exists an inverse relationship between feeding and predation in
which foraging performance decreases as levels of predation increase. In
relation to time of feeding and levels of predation, we observed that bird-feeding
activity was the highest in birdfeeders furthest away from predators (Figure
2). The amount of seed consumed was the lowest during the day in the open area
experimental site. (F2.379=7.329, p=0.0071).

 

DISCUSSION

            By exploring seed
mass depletion, we demonstrate the nonlethal effects of cover and simulated
predators on the foraging behavior of free-living wild birds. Energy reserves
in birds displayed abundant disparity, being depleted overnight and refilled
during the day, thus birds are likely to feed abundantly during the early hours
of the day. Though we could not find any evidence in support of bimodal
foraging since we only sampled birdfeeders twice a day, we observed no
difference in response to daily feeding patterns. In support of this idea, previous
studies reveal that bird feeding did not differ in respect to the time of day (McNamara
et al. 1994, Wolf and Hainsworth 1977, Bonter et al. 2013).

In
light of previous experimental exploration, birds tend to localize and forage
in habitats of increased cover (Lima 1998). This allows them to better detect
and avoid predators, thus increasing their survival and reproduction. Our data correlated
with this observation, throughout the day, birds consumed more in forest cover
and significantly less in no cover. During the night, foraging decreased in
forest cover. It’s likely that they met their energy reserve before the evening
sampling. Interestingly, birds spend less time feeding the higher their food
availability (Olsson 2000). An elevated level of seed mass consumption during
the night in the open field experimental site also agrees with our predictions.
At nightfall, the darkness can pose as shelter while they feed.

The
strongest effect on bird behavior was predation, in particular a high level
risk of predation. The results from the present study suggest that reduced
activity in birdfeeders in closer proximity to simulated predators (high) is
directly associated to predation. The risk of predation almost certainly outweighed
starvation and may have caused birds to completely avoid those select
birdfeeders. Although bird activity was not monitored otherwise to assess their
strategies when subject to predation risk, this general pattern concurs with
other reports done on the analysis of bird foraging in response to predation (Lima
1986, Houston et al. 1993, Pravosudov and Lucas 2001). Our results conflict
with the idea that predation risk is relatively constant or is not a strong
enough driver to change foraging behavior in small passerines. They came to
this conclusion by observing a relatively constant pattern of weight gain
(substitute for foraging activity) in small passerines when exposed  (Lilliendahl 2002, Koivula et al. 2002, Lange and Leimar 2004). By
evaluating weight gain of birds; they failed to make a direct correlation
between predation risk and feeding activity by analyzing only fat load.
Predation risk is a function of both fat load and feeding activity and in order
to effectively test for it, scientists must control for the variability of activity
of individual birds. Only until recently has the daily temporal changes in
behavior of individual, free-living birds been quantified (Bonter et al. 2015).

This
format of the experiment permitted resource predictableness, which can likely
increase predatory risk (McNamara and Houston 1990). Taking this into account,
it is difficult to determine whether the birds avoided the birdfeeders
altogether and obtained a different food source or simply flew to another
feeding post that exhibited a lower level of simulated predation. Additionally,
increasing food availability in a controlled manner does not reflect the organization
of resources exhibited by nature.

We
did not implement a control group in this study for two purposes. First, our
experimental setup limited the number of extra birdfeeders available. Second, there
is enough evidence to support the hypothesis that bird foraging increases when
no predators are nearby (Lima 1988, Pravosudov 2001, Frid and Dill 2002). One shortcoming
of our methodology was that we were restricted to assessing the consumption of
supplementary food. We know that birds undoubtedly fed on other natural
resources but we were unable to monitor such this activity due to a lack of monitoring
machinery (Bonter and Bridge 2011). For future experiments, it may be
beneficial the forest and have cameras set up to observe what species is
consuming birdfeed so we could control for the varying diets of diverse bird
species. Additionally, since all birdfeeders are relatively close to each
other, it may be practical to set up separate feeding sites each testing a
single level of predation. Additional research of free-living birds is
necessary to further investigate the comparative significance of predation in
shaping foraging behavior.