Anti-predator Behavior of Feeding Songbirds in the Presence of Predatory Sounds

Justin Congdon and Nicole Szostak

April 13th, 2012

Abstract:

Anti-predator behavior of feeding songbirds is a very important learned behavior

that helps to decrease the chances of being caught by a predator. When predator species

are present, avian species must make sure to be vigilant and be ready to fly away if

danger becomes imminent. In our experiment we tested whether a sound of a predator

would cause the birds at a feeder to fly away and be cautious of that area. We tested this

by setting up two bird feeders near a woody area and then we played an experimental and

a control playback to see how these two would differently affect the birds at the feeder.

We found out that when we played the experimental playback, red-tailed hawk, the birds

at the feeder rarely returned to the feeder and if one were to return it would take an ample

amount of time. Then when we played the control playback, northern crow; most all of

the birds returned within a short period. These results show that when birds hear the

predator playback they do flee the area and to not return unless they feel that enough time

has passed to make it safe enough. Overall this indicates that the predator playback does

cause the birds to show avoidance behavior.

Introduction:

Anti-predator behavior is an ongoing avoidance process that aids in the continual

survival of a species against a predator. These behaviors are a set of strategies used to

avoid being killed and consumed by a predator. In avian species, these anti-predator

tendencies are integral to survival against larger and more aggressive birds.

Recent studies have shown that their needs to be a trade-off based on a risk and

reward strategy. Examples of traits that are balanced with this risk/reward tactic are

vigilance, flightiness, and predator avoidance (Gluck, 1987). While predators are

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present, avian species must make sure to be vigilant, and be ready to fly away if the

danger becomes imminent (Slotow & Rotherson, 1995). While constant avoidance

strategy would help to alleviate the risk of predation, it would not allow much time to

feed and achieve homeostasis; vice versa, if no anti-predator strategies are employed, that

bird would likely be predated upon and not pass on the genes associated with weak anti-

predator use.

The trade-off between anti-predator and feeding strategies would also be different

based on the size of the bird species (Sih, 1991). A question that arises is whether or not

the size and mass of the bird (species-wise) affects the amount, and overall time of anti-

predator behavior use. Is it that a smaller species, based on overall lower percentage

chance of survival against a larger avian predator, uses more vigilance, flightiness, and

time away from feeder, as compared to a larger prey species?

In our experiment, we hoped to find if the species of bird affects the overall usage

of anti-predator behavior when the sound of a predator is played. Some variables that

were tested are the group size after return to the feeder, duration of visit (continual stay

and feeding, or get food and leave), vigilance per minute, seeds eaten per minute,

duration of time away from feeder, and if the same species of bird returns to the feeder.

These values were then be compared to the sound of a non-predatory threat (the control

variable).

We hypothesize that birds of different species will display different amounts of

anti-predator behavior (flying away from the feeder, time spent away from the feeder

before returning, duration of visit, feeding and vigilance rates, etc.) after hearing the

sound of a known predator. Our prediction is that when the sound of a predator is played,

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the birds will fly away, and spend a considerable time away from the feeder. Birds

should fly back to the feeder (after a period of time) and have decreased duration of visit.

The size of the bird should cause the rates of change in the bird to be less extreme overall

than that of a bird of a smaller size (rates should still trend in a similar fashion; the

numbers should just be less extreme than a smaller bird).

The types of questions that are examined are description, adaptive significance

and evolutionary effects. The descriptive aspect of the experiment should describe the

normal actions of a bird before the predatory sound is played. The adaptive significance

portion of the experiment describes what the bird does after the sound has been played,

how and why they do what they do to avoid the pseudo-predator. Although natural

selection cannot be looked at directly through one experiment, the significance of natural

selection that predation plays upon the spreading of balanced anti-predator traits cannot

be ignored.

Methods:

This experiment was conducted during the academic semester during the spring of

2012. This study experimentally determined how different playbacks affect different

species and how the species react when hearing their predator playback. The independent

variables are the crow and red-tailed hawk playback and the dependent variables are the

vigilance rate, time for the birds to return after hearing the playback, and the bird’s

habituation to sound. The predator playback that was used is from The Arizona Field

Ornithologists and it was of a red-tailed hawk. The control playback is from North

American Bird Sounds and it is of the northern crow. The sexes of both playbacks are

unknown and the duration of the playbacks were 14 seconds.

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Two small birdfeeders were used and they were placed at two different locations.

The feeders consisted of small lunch trays nailed to a wooden post about five feet tall.

The feeders were set up two weeks prior the experiment so that birds became acquainted

with them. A basic bird food with medium sized seeds was used so that all species of

birds can feed on it. One feeder was placed between an apartment complex and woods

and the other was set up between a house and woods. These feeders were placed over

fifteen miles apart. The purpose of having two different bird feeders set up is to make

sure to eliminate any variable that may only apply to one and this helped to ensure

random sampling. The feeders were placed about five meters from the house or apartment

and five meters from the woods in an open area. The bird food was placed on the feeder

and on the ground so that it attracted lots of birds.

To set up the experiment, we placed food on the birdfeeders and then we placed

the bird feeders in-between the woods and apartment or house. An I-home speaker was

placed outside approximately ten meters from the feeders next to the apartment. The I-

homes was completely hidden with leaves and other organic materials. The speakers were

placed on loud so that all surrounding birds were also able to hear the playback.

The experiments were conducted between 12:00A.M till 5:00P.M. During the

experiment, we sat inside of the apartment and watched the birds through a window. We

first chose one bird and watched it for five minutes and recorded the approximate

duration of visit and vigilance rate. The duration of visit is whether the bird left the

feeder during those five minutes or not. For the vigilance rate, we counted how many

times the bird looked up during its visit and then calculated the average amount of times

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per minute. These factors were recorded for one bird that we saw on the feeder during

these first five minutes.

The I-home was placed on a timer so after five minutes the first playback

automatically went off. The first playback was the red-tailed hawk. Immediately after the

playback was finished playing, we watched the bird feeder for another ten minutes.

During this time we still paid attention to the type of species present, and the duration of

visit. Also, we recorded which species returns directly after hearing the playback and

how long it took for it to return to the feeder.

On the first day of conducting the experiment we did a playback set with the red-

tailed hawk three consecutive times. Everyday when we conducted the experiment we

switch between playing the red-tailed hawk playback and between the Crow playback.

This entire experiment took place on five different days, meaning a total of eight red-

tailed hawk and seven Northwestern Crow replicates were conducted. The entire sample

size for this experiment was fifteen test runs.

To determine how the length of time to return to feeder, duration of visit and

vigilance is affected over the course of six red-tailed hawk playbacks in a day, the first

playbacks of each of the three days for each type of playback was averaged for each set

of data. A t-test was done to determine if there is a significant difference between the two

playbacks and between the different trials for both duration of visit and vigilance. As a

control, these averages were compared using a t-test to the crow trials. Our final sample

size for the entire experiment was 15.

Results:

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Bird species that heard the control sound returned to the feeder within 45 minutes

85.7% of the time. Birds that heard the experimental sound returned to the feeder within

45 minutes only 12.5% of the time (Fig 1) (6/7 returned after the control sound, 1/8

returned after hearing the predator sound).

When a bird came back to a feeder (not necessarily the same bird) it took the

experimental sound group an average of 1561 ± 1248.7 seconds (N=3). The control

sound group returned to the feeder on average of 307.8 seconds ± 217.2 seconds (N=6).

These values can be seen in Figure 2.

Blue Jays had an average feeding rate of 19.8 ± 5.34 seeds per minute and an

average vigilance rate of 10.5 ± 2.34 per minute(N=6). Cardinals had a feeding rate of

13.8 ± 5.34 seeds per minute and a vigilance rate of 13 ± 2 per minute (N=6). The black-

capped chickadee population had an average feeding rate of 8.5 ± 4.9 seeds per minute

and a vigilance rate of 19 ± 4.24(N=2). The red-winged blackbird had a feeding rate of

30 seeds per minute and a vigilance rate of 18 per minute (N=1). The Blue jay had the

largest average feeding rate, and the black-capped chickadee had the largest average

vigilance rate.

After hearing the sound of the control and returning to the feeder, the cardinal had

an average change in vigilance of 2 ± 1.41 more per minute and feeding rate went down

2 ± 1.41 seeds per minute (N=2). The Blue Jay, in the same situation went up 1 vigilance

per minute and went down 1 seed eaten per minute, in comparison to the cardinal (N=1).

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.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

0.857000000000001 0.125Control Experimental

N=7 N=8

Percentage of birds returning to feeder with-ing 45 minutes

Fig 1: The data shows the perecentage of a bird returning to a feeder after the control or experimental sound has been

played

Control Experimental0

5

10

15

20

25

30

ControlExperimental

Time in Min-utes to Return to bird feeder

after sound has been

played

Time for any bird to return to Feeder

Fig 2: The time in seconds it takes for a bird to return to a feeder if a bird actually returns.

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.0

5

10

15

20

25

30

35

Vigilance (Red) and Feeding

(Blue) Rates Per Minute Per

Species

Blue Jays Northern Cardinal

Black capped Chickadee

Red Winged Blackbird

Fig 3: Shows average vigilance and feeding rates compared between songbird species

.

-2.5

-2

-1.5

-1

-0.5

0

0.5

1

1.5

2

2.5

Blue JayBlue JayCardinalCardinal

Change in Vigi-lance (Blue) and

Feeding (Red) Rates after return

from control sound

Blue Jay

Blue Jay

Cardinal

Cardinal

Fig 4: Shows the Change in feeding rates and valiance rates after the bird has returned from the control variable sound.

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Discussion:

The results from our experiment showed that when playing a red-tailed hawk

playback, none of the original birds returned to the feeder except for one red-winged

black bird. However, a few blue jays that was not originally at the feeder came to the

feeder after the playback but it took quite a bit of time. On the other hand, all of the birds

returned to the feeder after the northern crow playback was played except for one blue

jay. A few other birds that were not originally on the feeder returned to the feeder as well

such as some black-capped chickadees and a mourning dove. These results showed that

the red-tailed hawk playbacks did cause the local birds to be extremely cautious of their

surroundings and caused most birds to not want to come back to the bird feeders after

hearing their predator playback.

The vigilance rate and the seeds eaten per minute estimation seemed to have no

correlation with the cautiousness of the birds and whether or not they would return to the

feeder. For the most part, the vigilance rate was pretty low and the seeds that were eaten

per minute were pretty high. Perhaps if we had focused on two species then we would

have seen a positive correlation between vigilance rate and cautiousness to return to the

feeder amongst the birds.

Our results do support our hypothesis, which was that the sound of a predator

would cause the birds to show anti-predator behaviors. We predicted that after the sound

of a predator was played, the birds would fly away and spend a considerable amount of

time away from the feeder. Our predictions were also supported by our results. This

shows that when a predator playback is conducted amongst avian species, the species will

most likely show anti-predator behavior by leaving the feeder and seeking shelter and not

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returning to the feeder until they feel that it is safe enough to return. Templeton (2005)

received the same results when conducting a similar experiment. This experiment

revealed that when a predator playback if performed to chickadees, the chickadee would

flee the area and not return until quite a bit of time has passed.

Other data that we found showed that in general, once the control sound was

played, and the same bird (or at least, same species) returned to the feeder, the amounts

of vigilance increased and seeds eaten per minute decreased. This was found in both

cardinals and in blue jays. On average, the cardinal seemed to be more unpredictable

compared to the blue jay, as the changes to feeding and vigilance rates were more

extreme in the cardinal.

Our results also show that the return rate for birds when a sound is played does

affect what species return to the feeder and when they return. The chance of a bird

returning to the feeder when a predatory sound was played is much lower than when the

control sound was played. This supports our hypothesis that birds will return less

frequently and stay away from feeders for a longer period of time when a predatory

sound is being played.

Another recurring theme was that smaller birds were more vigilant than larger

birds. The smaller birds had the largest vigilance rates, while the largest birds, the blue

jays, had the smallest vigilance rate. This could be because the black-capped chickadees

are predated upon more than the blue jay by birds of prey. Hurd’s (1995) results support

this assumption because her results showed that chickadees were more likely to leave an

area after hearing a playback versus other birds. Although the red wing black bird fed

more often, that data set is perhaps an anomaly (explained in next paragraph). Excluding

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that data set, the trade-off factor of anti-predator behavior can clearly be seen. The bird

that had less vigilance feeds more often, and the bird with more vigilance fed less often.

Throughout our experiment we did encounter a few methodological problems.

One of the bird feeders that were set up hardly had any birds present because it was

located in an open area, and the other had a lot of birds because it was in the middle of

the woods. This variation may have skewed our data because birds feeding in the open

bird feeder might have been more cautious to return to the feeder after hearing the

predator playback versus birds as the covered feeder. Also, we obtained a small sample

size, which did not give us a significant amount of data to make any true conclusions of

how different species react to the different playbacks.

As mentioned before, there were a few problems with how this experiment was

set up and conducted and if we were to redo it, we would want to change a couple of

things. We would first narrow down our study by focusing on how the playbacks affect

two specific species such as cardinals and blue jays. This would help us by focusing on

how the playbacks affect certain species versus testing how they affect most species

present at the feeder. We would also like to obtain a larger sample size and conduct more

experimental sets overall. Lastly, we would want to make sure the bird feeders were more

consistent with how they were set up. If we were to change these few things then our

results might have been more prominent and reliable.

Conducting playback experiments are important because they show how different

species react to different predator playbacks. Anti-predator behavior is a learned behavior

that benefits the birds by reducing their risk of attack by a predator. When predators are

present, avian species usually show avoidance behavior by flying to a different area and

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not returning to the site with the predator until a large amount of time has passed. This

learned behavior is extremely beneficial for the survival of the birds and it is important

for it to be passed onto future generations through natural selection.

Overall this experiment was extremely successful because it was a great example

of learned predator avoidance behavior. It was fascinating to see how different species of

birds reacted the hearing the different playbacks. This study shows how through natural

selection birds learn how to avoid predator calls such as the ted-tailed hawk in order to

increase their chance to survive.

Acknowledgements:

The authors would like to thank our professor, Dr. Pangle, for the inspiration to

conduct this experiment. Dr. Pangle also helped us to obtain the materials to conduct the

experiments and she helped with every step along the way. We would also like to thank

Joe Rogers for letting us conduct a few playbacks on his property. With out these two

very important people, we would not have been able to do this experiment.

Literature Cited:

Deviche, P. (2001, August 12). Arizona Field Ornithologist. Retrieved February

18th, 2011, from http://www.azfo.org/soundlibrary/sounds_Page2.html

Hurd, Christine 1995. Interspecific Attraction to the Mobbing Calls of Black-

Capped Chickadees (Parus atricapillus). Behavioral Ecology and Sociobiology 38:287-

292.

Gluck, E. 1987. "An experimental study of feeding, vigilance and predator

avoidance in a single bird ." Oecologia. 71: 268-72.

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Sih, A. 1992. "Prey Uncertainty and the Balancing of Antipredator and Feeding

Needs." The American Naturalist. 139: 1052-1069.

Slowtow, R, and Rothstein, S. 1995. "Influence of Social Status, Distance to

Cover, and Group Size on Feeding and Vigilance in White-Crowned Sparrows." The Auk.

112: 1024-1031.

Templeton, Christopher 2005. “Allometry of Alarm Calls: Black-Capped

Chickadees Encode Information About Predator Size.” Science 308:1934-1937.

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