Toward Great Horned Owls (Bubo Virginianus) and Raccoons (Procyon Lotor)

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AGGRESSIVE BEHAVIOR OF TREE SWALLOWS (TACHYCINETA BICOLOR) TOWARD GREAT HORNED OWLS (BUBO VIRGINIANUS) AND RACCOONS (PROCYON LOTOR)

July 26, 2013

ABSTRACT

Tree Swallows (Tachycineta bicolor), greenish-blue birds native to North America, are protected by the Migratory Bird Treaty Act. During their breeding season (May through August), they live in the northern half of North America, and in the winter, they migrate to Mexico and Central America. During breeding season, they exhibit defensive behaviors, such as alarm calling, dive-bombing, and predator mobbing, more than any other time of the year. The purpose of our study was to compare Tree Swallows’ aggression toward two predators, the Great Horned Owl and raccoon. Our null hypothesis was that there will be no difference in the average number of dive-bombs the Tree Swallow takes toward the Great Horned Owl and raccoon. We conducted our experiment at Carey Run Bird Sanctuary at varying times between June 28 and July 1. We placed mounts of a Great Horned Owl (Bubo virginianus) and a raccoon (Procyon lotor) at nest boxes with nestlings and eggs in them. Each predator was presented to a nest box for 5 minutes, and the order of predators presented switched every nest box. Prior to the start of each trial, predator calls were played using an iPod connected to a speaker 12m from the nest, while investigators stood 15m away from nest box. After we analyzed the videos, we found the average number of dive-bombs for each predator. We graphed our results and added confidence intervals, which suggested that the Great Horned Owl was dive-bombed more than the raccoon. This means that the Tree Swallows reacted with a different level of aggression toward the different predators. The t-test confirmed that there was a statistically significant difference in the average number of dive-bombs per predator (t=3.64, p=0.0001 (2-tailed), d.f. 18). Since there was a significant difference, we rejected our null hypothesis. We can use these results to improve our understanding of Tree Swallows behavior and how we can effectively protect them under the Migratory Bird Treaty Act.

INTRODUCTION

Tree Swallows (Tachycineta bicolor) are small migratory birds that have blue-green plumage and a white underbelly. Their habitat generally consists of open meadows and fields near waters. While in the air, Tree Swallows eat insects such as grasshoppers, ants, and beetles, known as aerial feeding. They also eat berries, and because they eat both plants and animals, Tree Swallows are omnivores (Cornell Lab, 2013). Tree Swallows are diurnal, they are mostly active during the day, and they are gregarious, usually living in large groups and traveling in flocks. During their breeding season (May-August), Tree Swallows live in the north half of the United States and most of Canada. When the weather turns colder, Tree Swallows migrate south to the lower half of the United States, Mexico and Central America, so they inhabit a similar temperature year round (Cornell Lab, 2013). Tree Swallows return to the same site every year because they exhibit nest-site fidelity. Because they practice nest-site fidelity, Tree Swallows exhibit monogamous behavior, which means they only mate with one other bird during every breeding season. Therefore, both parents take care of the young by protecting and defending the young from predators and supplying them with food (Roof & Harris, 2013). This behavior is classified as bi-parental care. A previous study stated that Tree Swallows are most territorial before eggs are laid because they want to protect the young from predators (Schneider & Griesser, 2012). As a result, they will become more aggressive toward the predators in order to protect their nest-site (Cornell Lab, 2013). Since the nests are home to the Tree Swallows permanently, people are not allowed to take down their nests, because the Migratory Bird Treaty Act protects the Tree Swallows (Roof and Harris, 2013). This act makes people responsible for knowing how the Tree Swallows are affected, and to do so, it is important to know their behaviors in order to protect them. After Tree Swallows return from their wintering grounds, mating and nest building begins within two weeks. Nest building takes two days to two weeks and after two to eight eggs are laid the incubation period lasts 14 to 15 days (Cornell Lab, 2013). At 15 to 25 days old, when nestlings begin to fledge or leave the nests, they are called fledglings (Roof & Harris, 2013). The fledglings are fed for 3 days and afterwards they become completely independent. Tree Swallow predators change depending on the Tree Swallows’ life stage. Predators of Tree Swallow eggs and nestlings include black rat snakes (Elaphe obsoleta), long-tailed weasels (Mustela frenata), and raccoons (Procyon lotor) (Roof and Harris, 2013). In our study, we used raccoons, which are an omnivorous (eat plants and animals), generalist (broad diet), and crepuscular (active during dawn and dusk) species. Predators of adult Tree Swallows include Merlins (Falco columbarius), Peregrine Falcons (Falco peregrinus), and Great Horned Owls

(Bubo virginianus) (Roof and Harris, 2013). Our study used Great Horned Owls, which are a carnivorous (eats other animals), specialist (narrow diet), and nocturnal (active during the night) species (Ashcraft, 2013). Since the Great Horned Owl is a specialist, avian and adult Tree Swallow predator, the Tree Swallows might believe that the Great Horned Owl is a greater, or direct, threat. If a predator eats the adult Tree Swallow, then it cannot care for its young, and the chain of Tree Swallows is broken and their fitness is negatively impacted. However, if a

predator eats the Tree Swallow’s young, then the adult Tree Swallow can come back next year and reproduce again. So, Tree Swallows may react dissimilarly to different predators based on the predators’ traits. If a Tree Swallow feels it is threatened, it may react in different ways. Researchers have indicated that Tree Swallows fly around the predator, while emitting warning calls of different rates and frequencies. The Tree Swallows may also dive-bomb the predator, which is when the Tree Swallows fly up 5-20 meters into the air, then dive down swiftly within 0.5 to 2 meters (Betts, Haley and Doran, 2005, p. 822). Tree Swallows may also exhibit passive defensives, in which they gather for predator mobbing. This is when the birds circles around, while remaining a safe distance from the predator. Tree Swallows do this passive defensive behavior to strengthen their social bonds with other Tree Swallows, and therefore, will be more likely to function as a team later (Winkler, 1992). However, there are negative impacts to defensive behaviors on the Tree Swallows’ fitness, or the ability to survive and reproduce. These negative impacts include the fact that it takes time and energy to defend themselves or their nests, meaning the birds will have less time to search for food or participate in other fitness enhancing activities. In addition, the Tree Swallows could be hurt or even killed while dive-bombing.

The purpose of our study is to observe Tree Swallows aggression toward predators by the average number of dive-bombs Tree Swallows take towards raccoons and Great Horned Owls. Studying Tree Swallows aggressive behavior is important because they are a protected species under the Migratory Bird Treaty Act, which indicates that Tree Swallows are a form of responsibility for humans to protect. It is important to understand how Tree Swallows protect themselves because we can apply that knowledge to protecting them. Tree Swallows are also important in the ecosystem by controlling the pest population since they eat insects humans consider as pests (Roof & Harris, 2013). Studying Tree Swallows behavior is important because we can see how animals evolve certain abilities and behaviors that can enhance or harm their fitness, which means the ability to survive and reproduce. These abilities may help Tree Swallows when reacting to different situations such as reproduction and predation by ensuring survival and reproductive success. Therefore, Tree Swallows have to find a balanced behavior between being aggressive and passive to protect themselves, their young, and nest sites. We measured the average number of dive-bombs because Winkler (1992), indicated that dive-bombs are an effective way to measure aggression in Tree Swallows. Dive-bombs are also one of the most common defensive behavior Tree Swallows use, and they are an easy way to measure Tree Swallow aggression. It was less subjective to test because we were only recording one variable, which helped us to focus better. Recording the average number of dive-bombs allowed us to compare the difference of Tree Swallows aggression towards predators that have different characteristics (direct and indirect threats). We chose to use mounts because a previous study by Wheelwright and Dorsey (1991) demonstrated that mounts are just as effective as live animals in elucidating an aggressive response from Tree Swallows. Therefore, we chose to not use live animals as the predators. We were able to conduct this experiment without affecting the Tree Swallows reactions because

Winkler found that Tree Swallows tolerate humans. Therefore, human interference will not change the aggressive behaviors of Tree Swallows toward predators (Winkler, 1992). We chose to use Great Horned Owls and raccoons as the predators because they are common predators in our study site. Because Tree Swallows exhibit nest-site fidelity (return to the same nest-site every year), they may be able to better recognize the predators as a threat. We played calls to make the predators seem more realistic. The Carey Run Bird Sanctuary was our study site location because the sanctuary naturally contains Tree Swallows and nest boxes that house Tree Swallows. We chose to videotape as a source of documentation, because the videotapes allowed us to count the number of dive-bombs more precisely. We chose this time of year because it was during the breeding season when the Tree Swallows are most aggressive. Given these choices, our alternative hypothesis was that the Tree Swallows will be more aggressive toward the raccoon if the average number of times they dive-bomb the raccoons is greater than the average number of times the Tree Swallows dive-bomb the Great Horned Owl. Our null hypothesis was that there will be no difference in the average number of dive-bombs the Tree Swallow takes toward the Great Horned Owl and raccoon.

METHODS

Our study took place at Carey Run Bird Sanctuary located in Garrett County, Western Maryland (Fig. 1). The sanctuary is the valley of two streams and made of open fields and forests. In the sanctuary, 150 species of birds are known to be living in the sanctuary including the Wild Turkey (Meleagris gallopavo), Vesper Sparrow (Pooecetes gramineus), and Black-capped Chickadee (Poecile atricapillus). Some predators of the Tree Swallow living at Carey Run are weasels, raccoons, and Great Horned Owls (The Maryland Ornithological Society, 2010). We collected videos from June 28 to July 1, which were sunny days, with temperatures between 70o-80oF from 10am to 3pm.

Fig. 1-This is an image of the Carey Run Bird Sanctuary. The red circle is the specific place we collected our data.

We conducted our study by first confirming the presence of nestlings and eggs because if the nest boxes had contained nothing, Tree Swallows would not be around to shield their eggs or nestlings from the predators. The nest boxes were tested at different parts of the field and days to ensure that it was not the same Tree Swallows dive-bombing the raccoon and Great Horned Owl. Ten nest boxes were used out of the 30 nest boxes available; the nest boxes were separated 15 meters apart. The predator mounts were placed in natural attack positions to make the predators seem more realistic. The raccoon was positioned holding on to the bottom of the nest box pole to make it seem as if the raccoon was climbing up to the nest box to obtain the nestlings or eggs. The Great Horned Owl was placed on top of the nest box using a broom stick and wire to secure its place. After setting and securing the positions of the predators, the numbers of dive-bombs were captured at approximately 15 meters away from the nest box using a video camera that was held up by a tripod. Prior to the start of each trial, the calls of the raccoon and Great Horned Owl were played using an iPod that was connected to speakers, which was 12 meters from the nest box. At each nest box, two trials were conducted. We placed the raccoon first and video recorded for 5 minutes. We removed the raccoon and waited 10 minutes. After the 10 minutes we placed the Great Horned Owl and recorded for another 5 minutes. After each nest box, the predators were counter- balanced or switched for each subsequent nest box. Therefore, the Great Horned Owl was placed first in half of the nest boxes, and the raccoon was placed first in the other half of the nest boxes.

To analyze the data, 3 people all watched two videos together and recorded the number of dive-bombs (Tree Swallow flies up, and dives down, missing the target). Multiple people watched the videos to assure precision. Then, the number of dive-bombs collected and recorded on a spreadsheet. Next, the total number of dive-bombs for each predator was averaged.

Descriptive statistics were used to analyze the data by observing the Tree Swallows aggression towards raccoons and Great Horned Owls through the average number of dive-bombs. We displayed the average number of dive-bombs per predator on a bar graph and used 95% confidence intervals to indicate that we were 95% confident that if this experiment was performed again, the average number of dive-bombs would fall within the confident interval bars. We used to use a two-tailed paired t-test to compare the means of two independent paired variables.

RESULTS

The confidence intervals in Figure1 suggest that the average number of dive-bombs the Tree Swallows took toward the Great Horned Owl was greater than the raccoon. The confidence intervals also show that the number of times the Tree Swallows dive-bombed the Great Horned Owl varied more than the raccoons, because the bars have more of a range on the Great Horned Owl. The results of our t-test indicated the Tree Swallows were more significantly more aggressive towards the Great Horned Owl (mean = 58) than the raccoon (mean = 8, t = 3.64, p = 0.0001, d.f. 18).

Average Number of Dive-Bombs For Each Predator

100

bombs 80 - 70 60 50 40 30 20

10 Average Average NumberofDive 0 Raccoon Great Horned Owl Predator

Fig. 2-The average number of dive-bombs the Tree Swallows took against the Great Horned Owl and raccoon.

CONCLUSIONS AND DISCUSSION

The averages and confidence intervals support that the Tree Swallows took more dive-bombs, on average, toward the Great Horned Owl than the raccoon. We therefore reject the alternative hypothesis that states that the Tree Swallow will take more dive-bombs toward the raccoon than the Great Horned Owl. Since the t-test supports that there was a significant difference in the number dive-bombs, we also reject our null hypothesis that states that there will be no difference in the average number of dive-bombs taken toward the raccoon and Great Horned Owl. This means the Tree Swallows were more aggressive toward the Great Horned Owl than the raccoon.

There are several possible explanations for our results. The Tree Swallows may react this way because of the threat level of the two predators. The Great Horned Owl is a direct and immediate threat to the Tree Swallow, because the Great Horned Owl will eat the adult Tree Swallows. This is important because if the adult Tree 6

Swallows are eaten or are hurt, they will not be able to provide for their young or return next year to reproduce again. Another reason the Great Horned Owl is a greater threat is that Great Horned Owls are cavity-nesters. This means the Tree Swallows would have to compete against the Great Horned Owl for nests, since the Tree Swallows also live in tree cavities. The Tree Swallows’ nest-site fidelity may make them even more aggressive in order to protect their life-long nests. The Tree Swallows may also have been trying to lead the Great Horned Owl away from the nest to protect their young. More aggression may be shown by responding quicker to the predator’s presence and more Tree Swallows join the predator mobbing or dive-bombing. The raccoon is an indirect threat to the adult Tree Swallows because it does not eat the adult Tree Swallows, but rather, threatens the young. This means the adult Tree Swallows may not feel as threatened, because they could always return next year. Tree Swallows may have just stayed in their nest boxes and hidden, instead of using the energy and time to scare or draw away the raccoon. This means that the Tree Swallows would show less aggression, since raccoons are not as much of a threat and Tree Swallows do not want to risk their fitness. Therefore, the Great Horned Owl may be a greater threat, making the Tree Swallows respond more aggressively to them.

There were some limitations to our study. First, the tall grass may have affected our results because it made the raccoon less visible to the Tree Swallows, meaning it could have been difficult for the Tree Swallows to detect them. This may have made the Tree Swallows’ response to the raccoons slower. To change this, the study could have been conducted in May, when the grass was shorter, or we could have positioned the raccoon further up the pole. Although Dorsey and Wheelwright (1991) previously determined that the Tree Swallows react the same to live animals as mounts, we feel that it would be good to use live animals instead of mounts to in case it affected results. Another limitation is that it was sometimes hard to determine if a dive-bomb had occurred when watching the videos because there was a lack of multiple camera angles. If we had set up multiple video cameras, then we would have had multiple camera angles to assess. The time of day may also have affected our results because neither predator was presented when they were most active; the Great Horned Owl was presented during the day, even though it is nocturnal, and the raccoon was presented during the day, even though it is crepuscular. A suggestion for future studies would be to experiment when the predators are most active (dusk and dawn), or take a sample from all times of day. Future research could examine samples at various times throughout the year because Tree Swallows may react differently. An overall suggestion for future studies would be to increase the sample size in order to have more accurate data.

There are a few research questions that could be examined following this study. One research question is to see which predator Tree Swallows would be more aggressive to after breeding season or earlier in the breeding season. This is an important question because aggressive behaviors might vary toward different predators at different times of the year. For instance, our results indicated that the Great Horned Owl was a bigger threat than the raccoon in the breeding season because of how the Owl is a direct threat and a cavity dweller. However, Tree Swallows could be more aggressive towards other predators in other seasons. Another suggestion for a future 7

study would be to compare the difference of aggressive behavior in two types of avian predators when one is a cavity nester and one is not a cavity nester. This study could confirm our reasoning that Tree Swallows were more aggressive towards Great Horned Owls because they are cavity nesters. It is important for future researchers to continue studying Tree Swallows behaviors, so that we can better protect and care for the Tree Swallows than what the Migratory Bird Treaty Act already does.

REFERENCES CITED

Ashcraft, K. (2013, June). Class Lecture. Frostburg State University. Frostburg, MD.

Betts, M., Hadley, A., Doran, P. (2005). Avian Mobbing Response is Restricted by Territory Boundaries: Experimental Evidence from Two Species of Forest Warblers. Ethology, 111, 821-835.

Cornell Lab of Ornithology. (2013). Tree Swallows. http://www.birds.cornell.edu/nestinginfo/bios/sp_accts/tres.

The Maryland Ornithological Society. (2010, June). Carey Run Garrett County. http://www.mdbirds.org/sanctuary/careyrun.html .

Roof, J., Harris, M. (2013). Tree Swallows (Tachycineta bicolor). http://www.biokids.umich.edu/critters/Tachycineta_bicolor/.

Schneider, N.A., & Griesser, M. (2012). Incubating Females Use Dynamic Risk Assessment to Evaluate

the Risk Posed by Different Predators. Behavioral Ecology, 10, 1093.

Wheelwright, N., & Dorsey, F. (1991). Short-term and Long-term Consequences of Predator Avoidance by Tree Swallows (Tachycineta bicolor). Auk, 108, 719-722.

Winkler, D. (1992). Causes and Consequences of Variation in Parental Defense Behavior by Tree Swallows. The Condor, 94, 502-520.