Carneau Pigeons Display Their Ability to Distinguish Colors

Introduction

Classical conditioning (CC) has long been studied in animals, and it has proven to be an effective means of learning through association.  Through CC, an organism learns to respond in a particular way to a stimulus that usually does not bring about that type of response.  Ivan Pavlov stumbled upon this information by accident through digestion experiments with dogs.  By pairing a dinner bell with food, Pavlov discovered the dogs would begin to salivate to the sound of the bell, even if no food was present.  Pavlov termed the food an unconditioned stimulus (US) and the salivation an unconditioned response (UR).  The term unconditioned implies that no prior experience is necessary for a response to be triggered.  The food in the dogs’ mouth always caused salivation, a behavior that required no learning.  For CC to be successful, another stimulus must be consistently paired with the US.  Pavlov called this additional stimulus a conditioned stimulus (CS).  A CS is only plausible if it does not cause the desired response prior to conditioning.  The bell in Pavlov’s experiment was not related to when the dogs were going to be fed.  However, through several pairings of the bell with presentation of food, the dogs soon began to salivate to the sound of the bell.  Pavlov explained that the response to the bell was a conditioned response (CR).  Through increasing pairings of the bell and the food, Pavlov also showed that the CR would strengthen in correlation with increase of pairings.  Conversely, when the pairings decreased, the CR also weakened.  If the CS is presented alone consistently, the response begins to decrease until it is extinct.  (Wortman, 1999)

            Nearly half a century later, CC techniques have shown promising means of teaching pigeons to respond to a stimulus for food.  This procedure has been replicated several times, and it has since adopted the name of autoshaping.  Through autoshaping procedures, pigeons learn to peck (CR) at a light key (CS) that has previously been paired with access to food.  Autoshaping serves as an effective model for CC in pigeons because the association between the light and the food can be measured by the amount of key pecks during the presentation of the light source. 

            Farmers raise Carneau pigeons to sell as poultry for human consumption.  Commonly referred to as squab, these pigeons are “processed” at 4 weeks of age and are available at http://www.gamefarm.com.au/squab.htm for purchase.  Squabs have never flown, and have a strict diet of only pigeon milk.  Squabs are ready for flight around 26-30 days, and usually average about 500 grams.  Farmers sell these birds on the market to many exquisite restaurants and private buyers.  For our experiment, 2 pigeons were bought from one such meat company.  We trained them to reveal their ability to distinguish varying wavelengths for, what they believed to be, a food reward.  After a few weeks of training, the pigeons began to peck the light key during illumination even if there was no food presented afterward.  A manuscript by Dennis Passe laid the foundation for the procedures involved in this training.  Passe reliably demonstrated a fast, reliable way to train pigeons to respond to a light source for food reward. The challenge of this experiment was to begin a discrimination-training interval that would model the behavior of the pigeons so that they only pecked at a green light, 525nanometers. 

(http://www.ricecrc.org/reader/poultry/squab-raising.htm#Introduction)

 

 

Method

Subjects

            During the days prior to the training, two white carneaux pigeons were reduced to 80% of their free-feeding bodyweight to ensure hunger during testing.  The animals received a food reward during the procedure, and only received additional food if performance and meal portion were not significant during testing.

Apparatus

            The viewing chamber was made from a modified Skinner box.  The interior of the box was painted flat black, and all screws were later painted to match.  A small light was housed above the color key to provide a small amount of illumination within the chamber.  A fiber optic cable connected to a monochromator was placed at pigeon eye level.  The computer that recorded the number of pecks the pigeon made also controlled the light.  A hopper filled with bird seed was placed below the key, and was operated by the computer.  A small light illuminated the food hopper, and a door closed when the food was not being presented. 

Procedure

            After the pigeons’ body weight was reduced to 80%, training trials were run for a period of 2 weeks.  During this time, the pigeons became accustomed to pecking at the light, and receiving a food reward.  Once this was accomplished, the autoshaping procedure began.  The sequence of training was as follows: the pigeon was placed into the box and allowed to light adapt for several minutes.  When the pigeon began to explore the cage, testing was initiated.  The shutter blocking the fiber optic light was removed for six seconds.  The hopper was then raised, and the subjects were allowed to feed for an additional six seconds. For 48 seconds following, there was a time out interval.  This sequence was repeated thirty times during each session.  During each one-minute interval, the computer and a pulse counter logged the number of times the pigeon pecked at the light.  Once the pigeons consistently pecked at the light, a partial schedule was introduced.  During this period, half the trials resulted in food being presented, while the other half did not.  When the pigeon showed a more consistent pattern of pecking during light illumination, new light wavelengths were introduced to the testing.  For the period of testing, only 525nanometer wavelengths were followed by a food reward.  Beginning at a minimal distance of 25nanometers, the wavelengths were eventually moved closer and closer until 5nanometers difference was reached. 

Results

            At the end of testing, Marshmallow had some difficulty distinguishing lower wavelengths.  Conversely, Snowflake showed difficulty distinguishing higher wavelengths.  Figure 1 shows Snowflake’s curves generated from the data of all testing days.  There is 8nanometer wavelength discrimination on lower lengths, and 9nanometer discrimination on the higher wavelengths.  Figure 2 shows Marshmallow’s curves.  This data shows 10nanometer discrimination on lower lengths, and 9nanometers on the higher lengths. 

Figure 1.  Snowflake’s Wavelength Graph

Figure 2.  Marhmallow’s Wavelength Graph

Discussion

            One of the major complications that occurred during this study was an attentiveness problem with one of the pigeons.  During several of the testing procedures, Snowflake was not paying attention to the light or the food.  This failure to notice the light inevitably led to many trials that did not score any pecks.  The question why this bird was not attentive may lie in a number of explanations.  There was no examination to see if the birds had sufficient sight because there is no known way to “ask’ the pigeon.  If the bird had trouble seeing clearly, it would certainly inhibit the number of pecks during testing.  However, the data in the results shows that Snowflake was successful at distinguishing wavelengths, and reliably pecked the key when it was illuminated.  Snowflake may have been stressed before testing.  The move from its cage to the Skinner box could have easily caused a physical arousal within the bird that contributed to its inattentiveness.  Although there were several tests that did not register any pecks, in all cases Snowflake eventually began to respond to the illumination of light.  This evidence suggests that the move certainly could have contributed to the bird’s behavior.  However, Snowflake eventually began to peck the key and eat from the hopper.  One way to ensure this error does not occur again would be to give the birds more time to dark-adapt, and time to get over the shock of being picked up and handled. 

            Both pigeons scored relatively the same in respect to higher and lower wavelengths.  Their success at being able to distinguish this closely is noteworthy considering many of the wavelengths close to 525nanometers do not look strikingly different.  As an observer, I was fooled by nanometers within twenty nanometers of 525nanometers!  The clear differentiation in blue, yellow and red significantly improved as the testing progressed.  Hardly any pecks occurred for these colors providing a possible model for color vision in the Carneau pigeon.  Further studies should consider this finding in devising an experiment to study color vision in Carneaux. 

Even more remarkable in this experiment is that these birds actually behaved in response to the light.  The food hopper was always presented following a green light, regardless of whether or not the pigeon pecked the key.  So, if the pigeon did not need to peck the light to get food, why did it do it?  The most popular theory among our lab was that the pigeon “thought” that pecking the key was the cause of the hopper being presented.  However, even this theory is challenged.  When Snowflake was not pecking at the key, he was taking advantage of the free food that was being presented.  Although he eventually started pecking the key, several trials passed in which he received food for just sitting in the box.  The pairing of sitting and food presentation was not far from causing a new CS and CR.  The issue is further complicated in that Marshmallow always pecked the green light before he ate from the hopper.  He did not receive any “free” hopper presentations, yet he still pecked the light consistently.  Clearly, there is more to be learned about the behavior of the Carneu pigeon. 

Conclusion

            Autoshaping served a quick, reliable form of classical conditioning in the pigeon.  While we were successful in getting the birds to display their color vision abilities, we also uncovered many mysteries that should be explored.  Creative experimental designs will be needed to devise a way to ask Carneau Pigeons what color vision is like to them.  In addition, further investigation is necessary to understand why the pigeon pecks a lighted key instead of performing some other repetitive behavior.

References

Chaves, Lin M and Hodus William.  1998.  Color reversal-learning deficits after            tectofugal pathway lesions in the pigeon telencephalon.  Behavioural Brain           Research.  Maryland USA.

Passe, Dennis H.  1981.  Autoshaping as a Psychophysical Paradigm.  Journal of the     Experimental Analysis of Behavior.  Florida State University.

Wortman, Camille and Loftus, Elizabeth and Weaver, Charles.  1999.  Psychology 5^th ed.         Mcgraw-Hill College. 

(An error occurred on the webpage I referenced on the Squab.  I can not get the information, but included the address in text.)