Animalshave long been known to alter their behavior to avoid predation.
Most attentionin the study of predation effects has been given to lethal effects ofpredation, while nonlethal effects have been more or less ignored. Many studieshave reported that the perceived risk of predation has a direct impact onforaging activity (Frid et al. 2002, Lima et al. 1990, Beale 2007, Cresswell2008, Lind and Cresswell 2005). If individuals of a species differ in theirvulnerability to predation, behavioral changes can result usually concentratedon the starvation-predation trade-off. (Cresswell 2008, Lind and Cresswell2005, Cresswell 1994). Foraging involves the balance of energy gains againstthe risk of being consumed.
Under the risk of predation, it may be to the animal’sadvantage to avoid a profitable food source and adjust by choose foragingtimes, feeding areas and other foods that are subprime with regard to reducingstarvation risk alone (Houston et al. 1993). Much research has been conductedon birds to observe their potential to assess their food supply and implement aprofitable feeding technique (McNamara et al. 1994, Pravosudovet al. 2001).
In light of the optimal foraging theory, we would expectto see a bimodal relationship between foraging intensity and time of day bylimiting intensive feeding to times when the threat of starvation exceeds therisk of predation (McNamara et al.1994, Bednekoff and Houston 1994). Some experimentalevidence suggests patterns of daily weight gain (foraging activity) during thefirst three hours after sunrise among blackbirds (Cresswell 1998). However, themajority of empirical data of foraging patterns appears to follow therisk-spreading theorem, which entails continuous foraging throughout the dayuntil a critical threshold of stored energy is reached (McNamara et al. 1994,Wolf and Hainsworth 1977). Studies showing a relatively constant daily weight gainsuggests that predation was either steady during the day or not a significantdriver of foraging behavior. A recent study of direct measurement of dailyfeeding activity has been conducted and provides more support for therisk-spreading theorem regarding bird-feeding times (Bonter et al.
2013). Ithas also been found that birds favor foraging at feeders situated in tallvegetation/more tree cover versus areas with less vegetative growth (Lee etal. 2005).
Tallfoliage can provide better shelter against predation and birds are expected totake it into account and yield the most profitable feeding technique. Although,researchers were able to get significant results in understanding bird behaviorby analyzing feeding times, and weight gain, foraging still involves a complexbalance between survival, growth, and reproduction, making it difficult to examinea single major element of fitness since they are all interconnected. Daily climatic changes can also influencewhen, where, and how birds search for food. Short-term environmentalstochasticity has one of the most significant indirect impacts on dailyforaging patterns of a number of wildlife species worldwide (Lesley 2015, Bost2015, Hughes 2000, Parmesan 2006, Walther 2002, Hunt 2011, Baier and Napp 2003).Daily climatic variation, such as changes in temperature, humidity, rainfalland airflow all play a role in altering the feeding behavior of bird populations.In every population, certain individuals are able, to varying degrees, to copesuccessfully with a range of conditions within their immediate habitat. Adirect correlation between altered foraging behavior and breeding has been maderegarding global climatic variation under El Nino Southern Oscillation (ENSO)conditions among albatrosses and penguins (Lesley 2015, Bost 2015). During thecolder days of the year, birds tend to feed more because they demand moreenergy to maintain a stable body temperature (Bedneckoff and Houston 1994).
Thereare many other factors that influence prey vulnerability, which may constrainour ability to assess the effect of the starvation-predation trade-off on birdfeeding. 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 ofunderstanding of bird behavior (Poulin 1994). Although logical, most birdbehavioral studies were conducted without taking in to account the complexinteractions involved in the starvation-predation tradeoff that birds mustassess when foraging. Scientists have not yet been able to directly test birdforaging behavior while controlling for other factors that may drastically interruptthe analysis of their results. Wequantified temporal changes in the daily foraging behavior of birds in fall bytracking seed mass at feeding stations.
Our objective was to quantify theday-to-day feeding patterns in response to three simulated levels of predation.We tested whether predation and distance of the predators had direct impacts onbird’s food intake in areas of high and low cover. Wepredicted that foraging activity would be directly related to the distance thatseparates the predators from the feeder and that there will be an overallincrease in foraging activity in the area of high cover. The average amount of seed mass depleted during nighttime feedingwas observed to be slightly higher in relation to daytime feeding within thebird community. There was no significant difference of foraging during the dayand night: on average, the average amount of mass depleted from birdfeeders didnot vary throughout the length of the experiment. Theoverall quantity of seed consumed by wild birds was considerably higher in theforest cover relative to the amount of seeds eaten from the birdfeeders with nocover (Figure 1).
The average mass depleted in response to types of cover andtime showed a significant difference with an increase of seed intake underforest cover relative to no cover (F1.379=3.91, p=0.
049). Thequantity of birdseed eaten under the high-simulated predation level wassignificantly 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 inwhich foraging performance decreases as levels of predation increase. Inrelation to time of feeding and levels of predation, we observed that bird-feedingactivity was the highest in birdfeeders furthest away from predators (Figure2).
The amount of seed consumed was the lowest during the day in the open areaexperimental site. (F2.379=7.329, p=0.
0071).By exploring seed mass depletion, wedemonstrate the nonlethal effects of cover and simulated predators on the foragingbehavior of free-living wild birds. Energy reserves in birds displayed abundantdisparity, being depleted overnight and refilled during the day, thus birds arelikely to feed abundantly during the early hours of the day. Though we couldnot find any evidence in support of bimodal foraging since we only sampledbirdfeeders twice a day, we observed no difference in response to daily feedingpatterns. In support of this idea, previous studies reveal that bird feeding didnot differ in respect to the time of day (McNamara et al. 1994, Wolf andHainsworth 1977, Bonter et al. 2013). Inlight of previous experimental exploration, birds tend to localize and foragein habitats of increased cover (Lima 1998).
This allows them to better detectand avoid predators, thus increasing their survival and reproduction. Our data correlatedwith this observation, throughout the day, birds consumed more in forest coverand significantly less in no cover. During the night, foraging decreased inforest cover. It’s likely that they met their energy reserve before the eveningsampling.
Interestingly, birds spend less time feeding the higher their foodavailability (Olsson 2000). An elevated level of seed mass consumption duringthe night in the open field experimental site also agrees with our predictions.At nightfall, the darkness can pose as shelter while they feed.
Thestrongest effect on bird behavior was predation, in particular a high levelrisk of predation. The results from the present study suggest that reducedactivity in birdfeeders in closer proximity to simulated predators (high) isdirectly associated to predation. The risk of predation almost certainly outweighedstarvation and may have caused birds to completely avoid those selectbirdfeeders.
Although bird activity was not monitored otherwise to assess theirstrategies when subject to predation risk, this general pattern concurs withother reports done on the analysis of bird foraging in response to predation (Lima1986, Houston et al. 1993, Pravosudov and Lucas 2001). Our results conflictwith the idea that predation risk is relatively constant or is not a strongenough driver to change foraging behavior in small passerines. They came tothis 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). Byevaluating weight gain of birds; they failed to make a direct correlationbetween predation risk and feeding activity by analyzing only fat load.Predation risk is a function of both fat load and feeding activity and in orderto effectively test for it, scientists must control for the variability of activityof individual birds. Only until recently has the daily temporal changes inbehavior of individual, free-living birds been quantified (Bonter et al.
2015).Theexperimental setup allowed resource predictability, which is likely to increasepredatory risk (McNamara and Houston 1990). Taking this into account, it isdifficult to determine whether the birds avoided the birdfeeders altogether andobtained a different food source or simply flew to another feeding post thatexhibited a lower level of simulated predation. Additionally, increasing foodavailability in a controlled manner does not reflect the organization ofresources exhibited by nature. Wedid not implement a control group in this study for two purposes.
First, ourexperimental setup limited the number of extra birdfeeders available. Second, thereis enough evidence to support the hypothesis that bird foraging increases whenno predators are nearby (Lima 1988, Pravosudov 2001, Frid and Dill 2002). One shortcomingof our methodology was that we were restricted to assessing the consumption ofsupplementary food. We know that birds undoubtedly fed on other naturalresources but we were unable to monitor such this activity due to a lack of monitoringmachinery (Bonter and Bridge 2011). For future experiments, it may be beneficialthe forest and have cameras set up to observe what species is consumingbirdfeed 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 practicalto set up separate feeding sites each testing a single level of predation.Additional research of free-living birds is necessary to further investigatethe comparative significance of predation in shaping foraging behavior.
