"Mom Always Liked You Best"

Clyde Freeman Herreid
Department of Biological Sciences
University at Buffalo, State University of New York


This case is based on a fascinating article that appeared in 1994 entitled "Parental choice selects for ornamental plumage in American coot chicks" (Nature 37l: 240-243) written by B. Lyon, J. Eadie, and L. Hamilton. Also see the commentary by Mark Pagel, "Parents prefer pretty plumage," in the same issue on page 200. As the case reveals, the authors concluded that coot parents "feed ornamented chicks over non-ornamented chicks, resulting in higher growth rates and greater survival for ornamented chicks."

They sum up "that parental preference is relative, rather than absolute, an important element in the evolution of exaggerated traits. These observations provide the first empirical evidence that parental choice can select for ornamental traits in offspring."

I have used this case in courses whenever I wished to emphasize how scientists go about solving problems. Obviously, the subject matter of the case is appropriate for courses in biology, especially those focusing on evolution and ecology. Yet the case is accessible to students without any background in science at all, so I have also used it in general courses in "Scientific Inquiry."

I am particularly fond of using this case for faculty development workshops where I wish to show instructors how to use case studies with small groups. The way that the case unfolds mimics the real way that scientists go about their work. Scientists do not have all of the facts all at once; they get them piecemeal. Moreover, the "interrupted case method" is far easier for most faculty to use than practically any other.

Case Objectives:

There are several objectives for this case beyond teaching coot biology:

  • To reinforce the value of the cooperative/collaborative learning method in which students work together to solve problems.
  • To help students develop a clear, rigorous, and structured approach to solving problems, i.e., to enhance critical thinking.
  • To give students practice in designing experiments.
  • To give students practice in making predictions and interpreting data.
  • To give students an explicit experience with the hypothetico-deductive method of reasoning, i.e., "the scientific method," where a question is asked, a hypothesis suggested, predictions or deductions made in light of the hypothesis (i.e., if the hypothesis is true, then such and such should occur), tests accomplished, and the data evaluated supporting or rejecting the hypothesis. To reinforce this model, it is essential at every opportunity as new information and questions are added that the instructor always ask the students what they now expect if the hypothesis were true.


This case is an example of something I call the Interrupted Case Method. The basic technique is to give the students a problem to work on in small groups (in this case I gave them a real research problem that was reported in the journal Nature). Then, after the groups discuss this a short time, I add information and let them go back to work. This sequence is repeated several times as the problem gets closer to resolution. At each point just before I give them new information, I ask each group to briefly report out their conclusions so that other groups can hear their progress. This has the effect of keeping the groups alert to other possible interpretations of the data and information.

As noted below, the only parts of the case I hand out to the students are Part I and Figures 1 and 2. I do not hand out Parts II-IV but instead read this information out loud to the entire class.

This case takes me about 75 minutes for analysis, but it can be easily shortened or lengthened. Let me lay out the timing of the case along with my strategy at each stage.

Part I--What's the Problem?

(Here students work in small groups and report out, then they receive new information.)

I use about 15 minutes for this. I first hand out Part I to each person in the class sitting in their groups. I instruct them to read the case and as a group answer the questions at the end of the page. (It is sometimes preferable to give each group only one copy of the case. This forces them to cooperate from the outset. If this is not done, individuals tend to read the case and then start making notes alone. It often takes them a long while to start talking together. To prevent this isolation, you must emphasize that the answers must be a group effort.)

After the small group discussion of Part I is in full swing, about five minutes along, I walk around the room with a color copy of the cover of Nature magazine (September 15, 1994) which has a photograph of the orange baby coots. The students don't know it at the time (some hardly glance at it, others are simply enamored with how cute the babies look), but what the youngsters look like is essential to what the authors did in the study. I then put a color overhead picture of the cover of the magazine on the projection screen for the rest of the class so the coots are omnipresent.

When the 15 minutes have passed, I interrupt the discussion, telling them that I know that they aren't really finished but I'd like to hear their thoughts. I ask for a representative from each group to tell me what they thought the question was that the authors were addressing. I quickly have the groups report out their conclusions. I do not comment at this point. There are always differences of opinion as to the question, usually because some groups have been specific, focusing on coots, while others have focused on the big evolutionary picture.

After this round robin is over, I ask the groups to quickly report on the hypothesis that they think that the authors are pursuing. Again, when the reporting is done, it will be apparent that there are differences among groups. I choose not to comment as this process is occurring except to say that clearly people differ even though they have the same information. I could intervene and tell them my version of the truth or explore the reasons for their differences, but I choose not to do this because I would never get to other critical points ahead.

At this point I say that I know that they have begun to consider the ways that the authors might test their hypothesis. But to help them on their way I thought that they might like to hear a little bit more from the authors.

Part II--The Authors Find a Method to Attack the Problem

(Again students work in small groups and report out, then they receive new information)

I now read Part II out loud to everyone. I finish by saying that now that they now know what the authors are up to, they are to use the method of clipping the feathers and design an experimental program which will test the hypothesis. It is essential at this point to emphasize that, yes, you know there are other ways to attack the problem, but you want them to use the authors' approach. If you don't do this, some groups tend to go off on their own and produce radically different designs, and although this might be productive in some ways, it will surely be diversionary to the present case strategy.

I give the groups about 10-15 minutes to come up with an experimental design using the feather clipping method of the authors. As the discussion occurs, I wander about from group to group checking on their progress and making sure that they are using the authors' method. I try not to answer many coot questions and do mention that, yes, I know that they don't know much about coots but they should just make some reasonable assumptions as they design their experiment. I do reassure them that they don't need to worry about budget at this point. Their goal is simple: design an experiment to test the hypothesis.

When the time is up, I once again interrupt. I ask the groups to take turns in reporting their findings. Incidentally, I always take the groups in a different order each time they report. Most groups will have very similar experimental designs now. All will have decided to have a nest with orange chicks and to have another with all black chicks. Some will stop there, not realizing that they have to have a third group, a nest condition with 50 percent black and 50 percent orange. It is only in this situation that the parents will have a choice between the two conditions. Also, there will be variations in what data to collect and how to go about collecting it. Once again, I really don't comment except occasionally to ask questions for clarification.

Part III--What Should Be Measured?

(Small groups work, then report out, and new information is added)

Then I read out Part III, telling the students what the authors really did. This passage is instructive because it reveals details about how the authors handled the coots that some groups will have considered. At the end of the passage, I specifically ask them to decide what data the authors should collect and how might they do it. The groups will have talked about this earlier, but now they have a chance to revise their ideas in light of other groups' comments and the further information. Also, it is essential to emphasize to them that they should begin to seriously think about what the data might show.

After about five minutes, I have the groups briefly report out their proposals. There will be a few surprises here as some groups will have some novel and often very expensive ways to monitor the birds.

Part IV--At Last, Here Are Some Data!

(Small group work and the students plot some data)

Then I read Part IV to them and hand out Figure 1 to everyone. Sometimes I only hand out a single copy of the data to save paper and to force each group to work together.

The figure shows part of the data where the coots have been left orange. The data show what happens in a nest where every chick has the normal plumage. Now I want them first to predict (guess) what would happen to the chicks that are in nests where all are black. Of course, there are several possibilities, but I tell them to plot the data that they would hope to see occur if the hypothesis were correct. Recall they have the data for nests where all the chicks are orange for comparison.

This point is troublesome. I have not figured out how to say this so everyone clearly understands the issue without telling them the answer. Most students get the point when I say: "Let's consider the ideal experiment, the one with the least complications in interpretation. Now, if the hypothesis were correct that parents feed some chicks better than others based on their plumage when there is a choice, what do you want to happen when all of the chicks are black?" (Obviously, you are hoping that they will realize that the ideal would be that the chicks in the all-black nests will have the same survival rates as the chicks in the all-orange nest. If this didn't happen, and say, the black chicks fare more poorly even without orange ones in the nest, then this complicates the story a lot. It would mean that any survival differences that occurred when both orange and black were in the same nest could not be simply attributed to parental feeding. It could be due to problems of temperature regulation facing the black chicks, etc.)

Then I tell the students that after they have filled in the graphs for the all-black nestlings, they should fill in the data for the experimental nests where there is a 50:50 mixture. Again you emphasize the point, what would they predict if the hypothesis were correct.

Part V--The Rest of the Story

(You show them the actual results and ask them what they make of it)

The plotting of the data will take about 10 minutes and after some hemming and hawing, most groups end up with the appropriate predictions. You will see this as you meander around the class. If a group is having difficulties, invariably it will be that they haven't gotten the black ones correctly positioned. They typically will have given them a lower survival rate than the all orange normals. You'll have to spend time with these folks to work through what the ideal would be.

I don't have the groups report out here at this point. But when they have filled in their graphs I hand them a competed version of what happened (Figure 2). After they have had a few moments to compare their predictions with the real data, I ask the entire class to respond to the questions listed at the end of the passage. Do black chicks survive more poorly because they are black or because they are "inferior" (less fit) than the orange chicks? Do the data support the hypothesis? Do the data prove the hypothesis? By this point students will recognize the distinction between support and prove.

At this point I stop. The case is over but the work is not. The authors are still working on the problem. To intrigue them further I might mention what the authors say at the end of their abstract:

They write:

…the survival benefits from increased parental care did not accrue to all orange chicks in experimental broods, but depended strongly on a chick's position in the hatching order. Late-hatched orange chicks had dramatically higher survival than late-hatched black chicks, but there was little difference in survival between early hatched black and orange chicks.

So we could continue the case by giving the students the graph depicting hatching order and ask them what they make of it.

As a last point, the authors of the Nature article suggest three possible answers to the question "Why do parents prefer orange feathered chicks?"

  1. Ornamental plumage may be a signal of a chick's high genetic or phenotypic quality, leading the parents to invest more care-giving in them.
  2. Orange plumage may be a signal of age, allowing parents to selectively feed the chicks in an optimum way.
  3. The color preferences may not be directly related to feather color but simply due to a parent color preference for other reasons. Perhaps it is the chick's head color that signals the offspring's need.

What experiments might resolve the issue?


Lyon, B., J. Eadie, and L. Hamilton. 1994. "Parental choice selects for ornamental plumage in American coot chicks," Nature 37l: 240-243.

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