Ornamentation in Birds
Students use video specimens from the Macaulay Library to investigate elaborate traits of plumage or behavior that can be found in some bird species due to sexual dimorphism. They develop systems for scoring the extent of variation, and frame hypotheses to investigate using Macaulay Library and the Birds of North America Online. more ...
Students investigate the connection between competition for mates and the evolution of elaborate ornaments in birds. This relationship is the premise of sexual selection, an evolutionary force. Inspired by plumage variation they see in video specimens of displaying birds, students pose hypotheses and then test them using an authoritative source of life history information (the Birds of North America Online).
The instructor first engages student interest in the phenomenon of sexual dimorphism by showing examples of species where the male is much more elaborate than the female, in contrast to examples where both the male and female look the same. Students then explore this phenomenon by developing a coding scheme to rate the extent of sexual dimorphism in video specimens from a number of taxa. This leads to a class discussion of life history or evolutionary factors that might explain the observed patterns of sexual dimorphism, and to posing of a small set of hypotheses. After elaborating on what they might need to know to test these hypotheses, students groups then research focal species in the online resource Birds of North America Online (BNA). The class will reconvene to present the results of their research, evaluate the extent of support for their hypotheses, and discuss potential future investigations related to this topic.
- Why are some bird species more elaborate than others?
- What causes the varying degrees of sexual dimorphism we see across bird families?
- What happens when natural selection is in conflict with sexual selection?
After completing this investigation, the students will be able to:
- Describe and compare plumage traits in a number of bird species
- Pose and test hypotheses using authoritative online sources of multimedia and scientific literature
- Discuss theories and evidence for the evolution of sexually dimorphic plumage traits
The following materials are needed to complete this investigation:
- 1 laptop per group of 2 - 4 students and one for the instructor
- Internet access
- Flash or QuickTime player installed for viewing videos from the Macaulay Library (http://macaulaylibrary.org)
- Subscription to the Birds of North America Online (most college libraries have access, or temporary and educational licenses are available at http://bna.birds.cornell.edu)
- Computer projector to show videos and demonstrate site navigation
- Student resources, including “Tutorial-MacaulayLibrary” (recommended) and “Mating Systems Definitions”, “Mating Systems Activity”, and “Hypothesis Testing Flowchart” (all optional)
- Instructor resources, including “Presentation-MatingSystems” for defining key terms and concepts, “Presentation-MacaulayLibrary-RavenViewer” for introducing Macaulay Library navigation, and “Presentation-BNA” for introducing Birds of North America Online navigation (all optional)
Appropriate for mid- or upper-level courses in evolution, animal behavior, or ornithology. Students should have some familiarity with the scientific method (observation, hypothesis, test, predictions, results, conclusions) and evolution by natural selection. They should also be introduced or already be familiar with key terms and concepts associated with animal mating systems (see optional support materials). For example, one way to incorporate this lesson in an Animal Behavior course would be to cover parental care, mating systems and then sexual selection in that order. This exercise on sexual dimorphism could then be part of the sexual selection lesson. This investigation can be adapted for use as a self-directed assignment in distance education or large lectures.
Locate and bookmark the initial Macaulay Library videos you will show in the Engagement portion of the lesson. Make sure to install or update your Flash player or QuickTime if necessary to play these videos. Verify that your institution has access to Birds of North America Online site. Alternatively, information on the appearance of the species may be gathered through bird field guides, but more specific texts will likely be needed to investigate life history and mating systems.
This investigation could be begun in a 50-minute class period with independent student investigations in Birds of North America Online conducted as homework, and the presentation of findings done at the next session. Alternatively, a 90-plus minute period in which students have internet and computer access might allow most of the investigations to be completed and presented.
- Assessment will take place before the investigation via the class discussions.
- During the investigation you can assess student understanding by circulating around the room and asking questions.
- After the investigation the students will complete homework questions that assess their understanding of the material and evolutionary theory in general. (see below for details)
Engagement - 10 minutes
- Hook up your computer to a projector. Begin by showing video clips of two bird species with very different degrees of sexual dimorphism using a free online library of natural sounds and videos, Macaulay Library (http://macaulaylibrary.org). (See Instructor Guide for suggested video specimens).
- Ask the students what type of mating system they think each set of birds has, and why. Ask for initial suggestions on how to test these hypotheses. (see Instructor Guide and Student Sheets for details)
- Define the term “sexual dimorphism.” Ask the students to brainstorm other examples of sexual dimorphism that they have noticed in birds.
Exploration - 5-15 minutes
- Point out to the students that there was quite a difference in sexual dimorphism between the first and second pair of birds. Ask the students, “are there even more extreme cases of sexual dimorphism? How common do you think sexual dimorphism is in birds?”
- You can then explore variation in sexual dimorphism across the bird world in one of two ways. If time is limited, you could show the students a series of slides prepared by the instructor showing species with a range of sexual dimorphism. Consider selecting at least some birds that the students could see locally. Alternatively, involve the class in helping to design an investigation to briefly assess the extent and variation of sexual dimorphism in birds. This investigation should be doable in pairs or small groups using field guides or the Macaulay Library’s online archive (http://macaulaylibrary.org). Decide as a class on what kind of scale you will use to score variation in sexual dimorphism. For example, you might want to use a scale of 1 to 5, with 1 showing least dimorphism/most similarity between sexes, and 5 showing most dimorphism. Make sure the class is measuring things on the same scale by agreeing on how the two initial video examples would each be scored under the class system.
- Assign pairs of students a certain number of species to research and score, so you get a broad sample and little to no overlap when you pool the class data. You can use the species from Table 1 (see Instructor Guide) or give the students more freedom in exploring the birds available to them in the Macaulay Library or field guides (which may or may not have detailed information in the Birds of North America relevant to the hypotheses that the class tests). If you are using the Macaulay Library instead of field guides, discuss how they will know the sex of the animals in the videos, and how they will differentiate sexual dimorphism from dimorphism related to age or season.
- If using the Macaulay Library, it may be a good idea to demonstrate how to use the “Browse by Taxonomy” function to find video examples of specific species (http://macaulaylibrary.org/browse/taxa/aves; see the Macaulay Library tutorial in the Student Sheets for more information). If you decide to only look at one species within a family, make sure the students know which Latin ending denotes a family grouping: -idae.
- Pool the class data. This could be done by creating a table or histogram showing the number of species with each dimorphism score (see examples below). At this point the students should be able to see that there is quite a bit of variation among bird species in how sexually dimorphic they are. (see Instructor Guide for example tables and graphs).
Explanation and Elaboration - 15 minutes
- Remind the students that at this point they have investigated what variation in sexual dimorphism exists across different bird families, but they haven’t yet collected evidence as to why this variation exists, or what explains the patterns of dimorphism they are seeing. At this point, you may want to post some form of the question, “What causes the varying degrees of sexual dimorphism we see across bird families?” in front of the class.
- Ask the students to brainstorm factors that might account for the patterns of sexual dimorphism they see (you may or may not want to formalize these into hypotheses at this point). If students have trouble getting started, ask them what they already know or might need to know about life history or social behavior of these bird families to answer this larger question. Also, what selective pressures might be at work?
- After you have brainstormed factors that might come into play, define concepts and terms that may have arisen if these are concepts you have not already covered in class. (See Instructor Guide and Student Sheets for details.) Some terms you may need to define explicitly include mating systems, monogamy, polygyny, intra- and intersexual selection, and extra-pair paternity. This is best done as a Socratic style discussion: e.g., Ask “Are all monogamous birds truly monogamous in that they mate only with their social partner?” as a lead-in to defining extra-pair paternity.
- Use these concepts and terms to guide the formulation of testable hypotheses and predictions. (See Instructor Guide for details on possible hypotheses and predictions.) Minimally, the students could test whether mating system or parental care type are better predictors of the degree of sexual dimorphism.
- Demonstrate how to access BNA Online (http://bna.birds.cornell.edu), and what general information it contains in the species accounts. Highlight sections that will be particularly relevant based on the hypotheses chosen. (e.g., Distinguishing Characteristics, Appearance, Behavior and Breeding). See Instructor Guide for slides that can be used to introduce students to using BNA.
- Decide as a class what information to collect from BNA Online species accounts, and come up with a list or table format (as shown in Instructor Guide) to keep the group’s researching and reporting this information in consistent ways.
- Assign species either to individuals or small groups, with data to be collected outside of class as homework or in class, depending on how you have structured the exercise. The species assignments can be determined by the instructor prior to class and shown on a slide that lists the students’ names and which species they should research (see Table 1 below). If students collect the data outside of class, it is helpful to assign some points to their response, to ensure they get the data to the instructor in a timely manner.
Evaluation - 10 minutes in class, with optional additional instructor time outside of class
Note: While the “5E” constructivist model of instructional design suggests that an important step at this stage is to engage students in evaluating their own knowledge of the concepts, depending on the learning objectives of the course and the level of the students, you may choose to omit or simplify some parts of the analysis.
- Once all of the data have been collected, decide as a class how to test the hypotheses and present the students’ findings. If in-class time is limited and students have varying degrees of familiarly with statistics, the instructor may want to do the complicated analyses outside of class. The students can email the instructor the data they collected for homework for each species, the instructor can analyze the data outside of class, and then the following class period can be used to briefly summarize the analytical methods used and view and discuss the results. (See Instructor Guide for detailed information on possible ways to consolidate the data and statistically test hypotheses).
Closure - 5 minutes
- Ask the students to summarize in pairs what the class found, then ask one pair to describe the findings to the entire class. Involve others in describing why, in evolutionary terms, they see this pattern – i.e., as competition intensifies, males that are more elaborate are more successful at acquiring mates than those that are not elaborate. At the same time, there are costs to being elaborate and so selection will not favor elaborate females or elaborate males if they provide extensive parental care. Also discuss any exceptions to this pattern and what factors might account for these exceptions.
- If this is a 90-minute class, the students can begin the assessment questions given below, working either individually or in their groups. Otherwise they can do them for homework.
- Why is it important to control for evolutionary history in this type of analysis?
- Are there correlations among body size, mating system and ornaments? What role might body size play in competition for mates?
- How could variation in parental care influence sexual dimorphism?
- What would you expect to happen when different populations of the same species have different mating systems?
- What other traits do birds use to attract mates and what predictions would you make about how these relate to mating systems and to sexual dimorphism?
- Pick one of the above questions and use your answer to outline an hypothesis and how you would test it. Make sure to include the data you would need to collect, your predictions, and how you would evaluate the hypothesis with your data.
- Engage the class in research or discussion on which sex is the more ornamented one in general in birds (the males), and how this compares to humans. What could be some evolutionary explanations for this difference?(See Instructor Guide for answers)
- Investigate or provide examples from bird species that have reversed sex roles and parental care (e.g., phalaropes, spotted sandpipers, jacanas). What would you predict about mating system and appearance based on parental care, and how does this relate to what is actually seen?
- Extend this research to the highly sexually dimorphic species often found in the neotropics, using the free community-created website Neotropical Birds (http://neotropical.birds.cornell.edu). What evolutionary pressures might be different in the neotropics as opposed to temperate environments?
- Show Science as Inquiry, a four-minute video by WGBH about Marion Petrie’s studies of sexual selection in peacocks.
- Introduce Ed Scholes’ research on the evolutionary radiation of the birds-of-paradise due to sexual selection and geographic isolation using videos and other resources at the site: http://birdsofparadiseproject.org.
- Show a video segment from BBC’s Life of Birds to further underscore how elaborate these plumage ornaments and behavioral displays can be, and how they may reinforce one another. Possible discussion questions include:
- What were some examples of showy bird traits you saw in the video?
- Did you see examples where plumage and behavior reinforced one another?
- Who was wearing showy plumage? Are there any case where that is the reverse?
- What appeared to be the function of the plumage and displays?
- Are there possible downsides to employing these displays?
- See Instructor Guide for an annotated list of relevant articles from the primary literature that students could be assigned to read and summarize.
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