Search Paths of Foraging Robins

Developed by Theodore E. Burk, Dept. of Biology, Creighton University, Omaha, NE 68178-0103

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BACKGROUND

One of the areas of research in animal behavior that has been most exciting in recent years is known as "optimal foraging theory." In this area, scientists attempt to analyze the food requirements and foraging opportunities for various species, then develop models predicting how the animals should forage in order to achieve specific nutritional objectives, and finally carry out observations or experiments to see if predictions based on optimal foraging models hold true for real animals. If a model seems to work, then the scientists can provisionally accept that they have made an advance in their understanding of animal foraging behavior. If not, the ways in which the model failed may suggest what additional information needs to be obtained by the scientists or what additional factors they need to consider in order to achieve a better understanding of the foraging behavior of real animals. Among the questions that have been modeled by optimal foraging theorists are: How many different kinds of foods should an animal pursue? When should it change its diet to become more specialized or generalized in its foraging? When foraging in an area, how should it decide whether to continue searching for prey in that area or to leave to find another area? Should it forage alone or in a group? If taking its food back to a central spot (such as a nest full of baby chicks), how many prey should it collect on one foraging trip before returning?

One other question that has been studied by students of foraging behavior is: What pattern of movements should a forager follow while moving through an area in which it is seeking prey? Obviously, a predator should move in such a way as to make the number of encounters with prey as large as possible, and to limit the number of times it forages in areas that have already been searched. In exploring this question, students of animal foraging behavior have discovered several general patterns in the movements of foragers. One particularly common set of patterns is observed when prey are distributed in clumps within the area that a forager is searching. In such a situation, a forager should move in such a way that, if in a clump, it stays there until it has eaten most of the prey in the clump. If not in one of the clumps, the animal should move in such a way as to maximize its chances of encountering one. In 1974, J.N.M. Smith carried out a classic study of such a forager searching in a habitat where prey occurred in clumps. The subject was the European blackbird, Turdus merula, a close relative of the American robing, Turdus migratorius, and a bird that like the robin forages in short grass areas for worms, beetles, and other invertebrate prey. Smith watched blackbirds forage on a small island between the two channels of the Thames River in Oxford (the island being known locally as "Mesopotamia"); he observed them from the pedestrian walkway of the bridge over the river.

You are probably familiar with the general searching behavior of American robins, which the searching behavior of the blackbird closely resembles. The bird makes short "runs" either by alternate-leg running or by hopping along on both legs. At intervals, the bird stops and peers at the ground. If a prey is seen, the bird then strikes down at it with its bill. After a prey capture, or if no prey is seen, the bird turns away from the direction it has just been hopping, and makes another run before stopping to search again.

Smith found that if a blackbird was foraging in an area where there were few prey, (so that bird did not usually stab at a prey when it stopped), it: 1) would take long runs between stops; 2) would make only small turn angles between the direction of one run and the direction of the next run; and 3) would tend to alternate right turns and left turns. All of these actions had the tendency to move the bird quickly straight out of the area where it was having little or no success.

On the other hand, if the blackbird was frequently catching prey, it: 1) would make much shorter runs between stops; 2) would make large turn angles between one run and the next; and 3) tended to string together turns in the same direction (a series of all right turns or all left turns). The effect of these actions was to keep the blackbird moving in small circles in the area where it was experiencing success, as long as it continued to be successful.

The type of search behavior shown by the blackbirds in the low density prey areas has been called RANGING. The alternative, shown by the blackbirds in high prey density areas, is usually called AREA RESTRICTED SEARCHING. These two movement patterns have been shown in a variety of animals (for example, ladybug beetles foraging on aphids), and have even been seen in mate-seeking animals (by Burk in male fireflies, by Turchin and associates in male and female butterflies).

In this lab, you will perform observations to see if American robins demonstrate the same ranging vs. area restricted searching patterns as their blackbird relatives.

METHODS

Select a robin foraging in a short-grass area. Work in pairs, with one observing and the other recording data. Alternatively, if working alone, carry a tape recorder and set it to record continuously. Speak your observations into it for later playback and transcription onto paper. Follow the bird from a distance that allows you to see what you need to see but that will not alarm the bird and change its behavior (recommended following distance is about 30 feet, or 10 meters). Each time the bird stops, record whether or not the robin makes a prey strike. Having recorded that, count the number of moves by the robin on its next run. A "move" is defined as either one stride if the bird is using alternate-leg running, or one hop if the bird is using two-leg hopping. After each stop, also record whether the bird turned to the left or to the right of its previous direction. A typical observation on tape might sound like "Yes, left, three moves." You will not be asked to try to determine the size of the turn angles. However, this could be done if you videotaped your observations and used the freeze-frame feature of your videorecorder.

If your bird flies away or in some other way stops foraging, find another bird and resume your observations, being careful to record in your data sheets and notebook that you have switched to a different bird. Continue observations until your instructor tells you to stop.

DATA ANALYSIS

Your null hypotheses are that there are no differences in the length of runs (number of moves) following stops when the robin did not make a prey strike and those following stops in which the robin DID make a prey strike; also, that the number of right and left turns followed each kind of stop in a random manner, regardless of whether or not the bird made a prey strike. Your experimental or alternative hypothesis is that the birds will make longer runs after stops that resulted in no prey strike, and that the birds will tend to turn in the same direction as the previous turn after making a stop that results in a prey strike (but the opposite direction as the previous turn after stops that do not result in a prey strike).

To analyze your data statistically, use some simple statistical tests described in the book Nonparametric Statistics for the Behavioral Sciences, by S. Siegel and N.J. Castellan. To see if the average length of runs (operationalized here by the number of moves) was greater when no prey strike, follow the procedures described for the "Mann-Whitney U Test." To test your hypotheses about turn directions, there are two alternatives. In one, use the "runs test" as described in Siegel and Castellan, to see if turns tend to go right-right-right... or left-left-left...following either stops with or without a prey strike. Alternatively, record for each of your turns whether it was in the same direction as the last turn, or opposite that direction. Then use the Chi Square Goodness-of-Fit test to see if the proportion of same-direction turns versus different direction turns varies between turns following stops with prey strikes and stops without. Be sure to analyze each bird separately.

An excellent way to present your results is by graphing them, using the graphing function in MS Excel, or a program like Cricketgraph.

Background Reading

On Animal Search Paths:

Smith, J.N.M. (1974). The food searching behavior of two European thrushes: I.

Description and analysis of search paths. Behaviour, 48, 276-302.

Bell, W.J. (1991). Searching Behavior: The Behavioral Ecology of Finding Resources.

NY, NY: Chapman & Hall. See especially Chapter 7: "Restricting search to a path."

On Statistics:

Siegel, S. & Castellan, N.J. (1988). Non-Parametric Statistics for the Behavioral

Sciences, 2nd Ed. NY, NY: McGraw-Hill.

NOTES TO TEACHERS

Since this is a field exercise, you do not have to worry about obtaining and caring for the animals used. For a location to carry out the study, the best sites are open lawn areas, where trees are not far away. A public garden or park, unoccupied ball field or schoolyard, or an open area in your local zoo are good possibilities. Robins can be seen foraging in such locations in the northern part of the United States from April to October. If you are watching birds from mid-summer on, you can distinguish the adults from the young birds of the year (the young have spotted breasts). You could have the students see if adults and youngsters differ in their foraging behavior.

The data to be collected are straightforward, and the major problem that students may have is just getting used to the continuous fast pace of the birds' movements. Tell them to be patient, and they will soon fall into a rhythm; it might be a good idea to have them practice taking the data for five minutes to get used to the pace before actually collecting data to keep and analyze. Students should be careful to follow the birds at an appropriate distance--close enough to see the behavior, but far enough away so as not to upset the birds and alter their behavior. They may need to adjust their distance based on the birds' reactions in the first few minutes. If the birds are alarm calling, spending long periods of time looking at the observers, or flying away after only very few brief foraging bouts, the students are probably too close.

The exercise can be carried out in any time block. If the class period lasts an hour or so, a site very close to the classroom would be preferable. The data could be collected in one period, with data analysis (including, if necessary, a little background on the purpose and procedures of statistical analysis) carried out in a second period, and graphing (including an introduction to the needed software) in a third period. If a longer period of time is available, such as the 3-hour block typical of college labs, one could try to do all 3 parts in one period, or spend one period for data collection and a second for data analysis and graphing of results.

As alternatives or supplements to the exercise described above, one could videotape the behavior in the field and record the results during slow-speed freeze-frame analysis of the tape playback in the classroom; also, one could put food (mealworms, fly maggots, small earthworms, etc.--available from bait and/or pet shops) out for the birds in a particular area for a few days and get them used to foraging there. Then, one could experimentally alter the food density in various patches by placing food in some locations and not in others, to see if the birds' behavior changes in the predicted manner as they move from areas of low food density to areas of high food density.

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Search Paths of Foraging Robins Developed by Theodore E. Burk, Dept. of Biology, Creighton University, Omaha, NE 68178-0103
\|Animal Behavior Teacher's Page Home\| \|Labs & Class Exercises\| \|Course Syllabi\| \|More Psongs My Psychologist Taught Me\| \|Software\|

*BACKGROUND*	One of the areas of research in animal behavior that has been most exciting in recent years is known as "optimal foraging theory." In this area, scientists attempt to analyze the food requirements and foraging opportunities for various species, then develop models predicting how the animals should forage in order to achieve specific nutritional objectives, and finally carry out observations or experiments to see if predictions based on optimal foraging models hold true for real animals. If a model seems to work, then the scientists can provisionally accept that they have made an advance in their understanding of animal foraging behavior. If not, the ways in which the model failed may suggest what additional information needs to be obtained by the scientists or what additional factors they need to consider in order to achieve a better understanding of the foraging behavior of real animals. Among the questions that have been modeled by optimal foraging theorists are: How many different kinds of foods should an animal pursue? When should it change its diet to become more specialized or generalized in its foraging? When foraging in an area, how should it decide whether to continue searching for prey in that area or to leave to find another area? Should it forage alone or in a group? If taking its food back to a central spot (such as a nest full of baby chicks), how many prey should it collect on one foraging trip before returning?
One other question that has been studied by students of foraging behavior is: What pattern of movements should a forager follow while moving through an area in which it is seeking prey? Obviously, a predator should move in such a way as to make the number of encounters with prey as large as possible, and to limit the number of times it forages in areas that have already been searched. In exploring this question, students of animal foraging behavior have discovered several general patterns in the movements of foragers. One particularly common set of patterns is observed when prey are distributed in clumps within the area that a forager is searching. In such a situation, a forager should move in such a way that, if in a clump, it stays there until it has eaten most of the prey in the clump. If not in one of the clumps, the animal should move in such a way as to maximize its chances of encountering one. In 1974, J.N.M. Smith carried out a classic study of such a forager searching in a habitat where prey occurred in clumps. The subject was the European blackbird, Turdus merula, a close relative of the American robing, Turdus migratorius, and a bird that like the robin forages in short grass areas for worms, beetles, and other invertebrate prey. Smith watched blackbirds forage on a small island between the two channels of the Thames River in Oxford (the island being known locally as "Mesopotamia"); he observed them from the pedestrian walkway of the bridge over the river. You are probably familiar with the general searching behavior of American robins, which the searching behavior of the blackbird closely resembles. The bird makes short "runs" either by alternate-leg running or by hopping along on both legs. At intervals, the bird stops and peers at the ground. If a prey is seen, the bird then strikes down at it with its bill. After a prey capture, or if no prey is seen, the bird turns away from the direction it has just been hopping, and makes another run before stopping to search again. Smith found that if a blackbird was foraging in an area where there were few prey, (so that bird did not usually stab at a prey when it stopped), it: 1) would take long runs between stops; 2) would make only small turn angles between the direction of one run and the direction of the next run; and 3) would tend to alternate right turns and left turns. All of these actions had the tendency to move the bird quickly straight out of the area where it was having little or no success. On the other hand, if the blackbird was frequently catching prey, it: 1) would make much shorter runs between stops; 2) would make large turn angles between one run and the next; and 3) tended to string together turns in the same direction (a series of all right turns or all left turns). The effect of these actions was to keep the blackbird moving in small circles in the area where it was experiencing success, as long as it continued to be successful. The type of search behavior shown by the blackbirds in the low density prey areas has been called RANGING. The alternative, shown by the blackbirds in high prey density areas, is usually called AREA RESTRICTED SEARCHING. These two movement patterns have been shown in a variety of animals (for example, ladybug beetles foraging on aphids), and have even been seen in mate-seeking animals (by Burk in male fireflies, by Turchin and associates in male and female butterflies). In this lab, you will perform observations to see if American robins demonstrate the same ranging vs. area restricted searching patterns as their blackbird relatives.

METHODS	Select a robin foraging in a short-grass area. Work in pairs, with one observing and the other recording data. Alternatively, if working alone, carry a tape recorder and set it to record continuously. Speak your observations into it for later playback and transcription onto paper. Follow the bird from a distance that allows you to see what you need to see but that will not alarm the bird and change its behavior (recommended following distance is about 30 feet, or 10 meters). Each time the bird stops, record whether or not the robin makes a prey strike. Having recorded that, count the number of moves by the robin on its next run. A "move" is defined as either one stride if the bird is using alternate-leg running, or one hop if the bird is using two-leg hopping. After each stop, also record whether the bird turned to the left or to the right of its previous direction. A typical observation on tape might sound like "Yes, left, three moves." You will not be asked to try to determine the size of the turn angles. However, this could be done if you videotaped your observations and used the freeze-frame feature of your videorecorder. If your bird flies away or in some other way stops foraging, find another bird and resume your observations, being careful to record in your data sheets and notebook that you have switched to a different bird. Continue observations until your instructor tells you to stop.

DATA ANALYSIS	Your null hypotheses are that there are no differences in the length of runs (number of moves) following stops when the robin did not make a prey strike and those following stops in which the robin DID make a prey strike; also, that the number of right and left turns followed each kind of stop in a random manner, regardless of whether or not the bird made a prey strike. Your experimental or alternative hypothesis is that the birds will make longer runs after stops that resulted in no prey strike, and that the birds will tend to turn in the same direction as the previous turn after making a stop that results in a prey strike (but the opposite direction as the previous turn after stops that do not result in a prey strike).

To analyze your data statistically, use some simple statistical tests described in the book Nonparametric Statistics for the Behavioral Sciences, by S. Siegel and N.J. Castellan. To see if the average length of runs (operationalized here by the number of moves) was greater when no prey strike, follow the procedures described for the "Mann-Whitney U Test." To test your hypotheses about turn directions, there are two alternatives. In one, use the "runs test" as described in Siegel and Castellan, to see if turns tend to go right-right-right... or left-left-left...following either stops with or without a prey strike. Alternatively, record for each of your turns whether it was in the same direction as the last turn, or opposite that direction. Then use the Chi Square Goodness-of-Fit test to see if the proportion of same-direction turns versus different direction turns varies between turns following stops with prey strikes and stops without. Be sure to analyze each bird separately. An excellent way to present your results is by graphing them, using the graphing function in MS Excel, or a program like Cricketgraph.

*Background Reading*	*On Animal Search Paths:* Smith, J.N.M. (1974). The food searching behavior of two European thrushes: I. Description and analysis of search paths. Behaviour, 48, 276-302. Bell, W.J. (1991). Searching Behavior: The Behavioral Ecology of Finding Resources. NY, NY: Chapman & Hall. See especially Chapter 7: "Restricting search to a path."

*On Statistics:* Siegel, S. & Castellan, N.J. (1988). Non-Parametric Statistics for the Behavioral Sciences, 2nd Ed. NY, NY: McGraw-Hill.

*NOTES TO TEACHERS*	Since this is a field exercise, you do not have to worry about obtaining and caring for the animals used. For a location to carry out the study, the best sites are open lawn areas, where trees are not far away. A public garden or park, unoccupied ball field or schoolyard, or an open area in your local zoo are good possibilities. Robins can be seen foraging in such locations in the northern part of the United States from April to October. If you are watching birds from mid-summer on, you can distinguish the adults from the young birds of the year (the young have spotted breasts). You could have the students see if adults and youngsters differ in their foraging behavior.

The data to be collected are straightforward, and the major problem that students may have is just getting used to the continuous fast pace of the birds' movements. Tell them to be patient, and they will soon fall into a rhythm; it might be a good idea to have them practice taking the data for five minutes to get used to the pace before actually collecting data to keep and analyze. Students should be careful to follow the birds at an appropriate distance--close enough to see the behavior, but far enough away so as not to upset the birds and alter their behavior. They may need to adjust their distance based on the birds' reactions in the first few minutes. If the birds are alarm calling, spending long periods of time looking at the observers, or flying away after only very few brief foraging bouts, the students are probably too close. The exercise can be carried out in any time block. If the class period lasts an hour or so, a site very close to the classroom would be preferable. The data could be collected in one period, with data analysis (including, if necessary, a little background on the purpose and procedures of statistical analysis) carried out in a second period, and graphing (including an introduction to the needed software) in a third period. If a longer period of time is available, such as the 3-hour block typical of college labs, one could try to do all 3 parts in one period, or spend one period for data collection and a second for data analysis and graphing of results. As alternatives or supplements to the exercise described above, one could videotape the behavior in the field and record the results during slow-speed freeze-frame analysis of the tape playback in the classroom; also, one could put food (mealworms, fly maggots, small earthworms, etc.--available from bait and/or pet shops) out for the birds in a particular area for a few days and get them used to foraging there. Then, one could experimentally alter the food density in various patches by placing food in some locations and not in others, to see if the birds' behavior changes in the predicted manner as they move from areas of low food density to areas of high food density.

\|Wheaton Home\| \|Morgan Home\| \|Psychobiology Home\|