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 regard to reducing
starvation risk 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
limiting intensive feeding to times when the threat of starvation exceeds the
risk of predation (McNamara et al.1994, Bednekoff and Houston 1994). Some experimental
evidence suggests patterns of 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 steady during the day or not a significant
driver of foraging behavior. A recent study of direct measurement of daily
feeding activity has been conducted and provides 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).
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.
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.
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.
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).
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).
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).
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.
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).
experimental setup allowed resource predictability, which is likely to 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.
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.