9 - 12
Students will understand how photoperiodism impacts plants and animals in the environment and learn how egg farms use this science to manage the production of eggs by their hens.
- Scenario image to display for class
- Blank sheet of paper or interactive science notebook, 1 per student
- Photoperiodism Notebook Cutouts, 1 copy per student
- Scissors and tape or glue
Essential Files (maps, charts, pictures, or documents)
Photoperiodism Notebook Cutouts
Egg Phenomena picture
Animals Who Lay Eggs picture
American Egg Board
protein: an essential nutrient responsible for building tissue, cells and muscle
clutch: the name of a group of eggs produced by birds, amphibians or reptiles in a series of days
photoperiodism: the physiological reaction of organisms to the length of day or night
Did you know? (Ag Facts)
- In 2017 the average American ate 274 eggs per year.1
- Eggs provide the least expensive source of high-quality protein for our diets. Eggs are followed by milk and chicken.2
- Hens can lay up to 1 egg per day. It takes approximately 25-26 hours for a hen to produce an egg.3
Background - Agricultural Connections
Protein is an essential nutrient in our body responsible for building tissue, cells and muscle as well as making hormones and anti-bodies. Sources of protein include milk, yogurt, meat, beans/pulses, and eggs. One egg has around 6 grams of protein. Eggs have been consumed by humans for thousands of years. Eggs are laid by many species of birds, reptiles, and fish. However, due to the ease of raising chickens (compared to other egg-laying species) and the amount of eggs a hen can produce, Eggs from chickens are mostly commonly consumed.
All species of chicken lay eggs. The size and color of the egg varies by the breed of the hen. Egg shell colors typically range from white to deep brown. Hens with white feathers and ear lobes lay white-shelled eggs. Hens with red feathers and ear lobes lay brown eggs. Breeds of chicken that lay white eggs are typically smaller and eat less. This makes white eggs more cost efficient to produce and typically cheaper than brown shelled eggs. There is no nutritional difference between a white and brown shelled egg, but some consumers still prefer one shell color over the other. For a lesson plan discussing labels and consumer choices, see Weighing in on Egg Labels, Supply, and Demand.
Hens begin laying eggs between four and six months of age depending on the breed and size of the chicken. Smaller breed hens start laying sooner than larger breed hens. Chickens do not need to be in the presence of a rooster to lay an egg. Farms raising eggs for consumption do not house roosters because there is no need to produce a fertile egg. In the wild, a hen would lay an egg per day until she had a clutch of eggs (10-15) at which time she would "set" on the eggs to begin the incubation process and hatch her eggs 21 days later. When hens are raised on farms, the eggs are collected each day. This stimulates the chicken to continue laying an egg per day.
Many factors impact a hen's production of eggs including her age and body condition. Another important factor is photoperiodism which is the physiological reaction of organisms to the length of day or night. This phenomena occurs in both plants and animals when the length of day (light) and night (darkness) triggers a specific response in the plant or animal. For example, a poinsettia plant is triggered to flower when the days are short and the nights are long. Sheep and goats begin their estrus cycle in the fall when the days get short. In contrast, horses begin their estrus cycle in the spring when the days are long. Hens are also impacted by photoperiodism. Egg production decreases and may even stop in the winter when the nights (darkness) are long and days (light) are short. Egg producers can manipulate this natural response by adding lights to their hen houses to simulate 14 hours of daylight in their environment.
Egg laying farms vary widely by size and production style (conventional, cage-free, free-range, etc.), but all large-scale farms use the science of photoperiodism to manage their hen's egg production.
Interest Approach – Engagement
- Define the word phenomenon as, a fact or situation that is observed to exist or happen.
- Introduce students to the following phenomena by displaying the following image:
- Discuss scenario one. Ask, "Could there be a scientific explanation for why the hens slowed their production of eggs?"
- Discuss scenario two. Ask, "Is it possible that there is a scientific explanation for this economic phenomenon?"
- Leave the questions open-ended and instruct students that they will be seeking to find the answer in the lesson.
Activity 1: The Science of Egg Laying
- Display the Animals Who Lay Eggs image for your class to see. Ask students, "What do all of these animals have in common?" Allow students to think and offer answers until they identify that the female of each of these species lays eggs. Clarify that with some exceptions most birds, reptiles, and amphibians lay eggs. Each egg varies in size, shape, and consistency (soft or hard shells).
- Ask students to raise their hand if they had an egg for breakfast or any food with eggs as an ingredient in the last 24 hours. All or most students should likely raise their hand. Ask, "What species of animal lays the eggs we typically eat?" (chicken)
- Ask students why we don't typically eat eggs from ducks, turkeys, or other species like snakes or lizards. As students offer answers, provide guiding questions to lead them to think about raising each animal on a farm. Point out that our food is produced on farms, not hunted and gathered. Therefore, our food comes from plants and animals that produce desirable products efficiently. In the case of eggs, efficiency can be measured by considering the size and quantity of eggs the animal can produce compared to the cost of breeding, feeding, and housing the animal. It may be helpful to provide the following statistics for illustration:
- Turkey hens can lay around 100 eggs per year.
- Female geese can lay around 40 eggs per year.
- Female ducks can lay between 60 and 150 eggs per year depending on the breed.
- Chicken hens can lay up to 300 eggs per year.
- Note that each of these numbers represent birds raised on farms where the eggs are collected each day. In the wild the bird would only lay enough eggs to complete a nest, often called a clutch. After the clutch was complete the bird would stop laying eggs and set on them until they hatch. On a farm, collecting the eggs each day causes the bird to continue to lay eggs longer than if it was in the wild.
- Open the Egg Farms Prezi and review it as a class. At the end of the Prezi, list the following observed phenomena students have been introduced to so far.
- Chickens lay more eggs in the spring and summer than they do in the fall and winter.
- Chickens do not lay eggs late in the day or during the night. Rather, they will skip a day of egg production and lay the next morning.
- The price of free-range chicken eggs can increase in the winter. The cost of conventionally produced eggs usually does not.
Activity 2: Explaining Photoperiodism
- Give each student 1 sheet of blank paper or have them open to a new page in their interactive science notebook.
- Instruct students to title the page with the word "Photoperiodism" and record the definition (the physiological reaction of organisms to the length of day or night). Clarify that photoperiodism takes place in both plants and animals.
- Have students draw a diagonal line from the top right corner of their page to the bottom left corner. Draw and color a sun on one side of the line and a moon on the opposite side to signify day and night. (display image for illustration)
- Ask students, "How can sunlight impact plants and animals?" Follow up with the question, "Can darkness also impact plants and animals?" Allow students to draw from their own background knowledge and observations to offer answers.
- Next, ask students to think about the length of day (amount of light vs darkness) in the spring and summer compared to the length of day in the fall and winter. Is there a difference? (Yes, daylight is long in the spring and summer and short in the fall and winter due to the rotation of the earth around the sun) Have students record in their notebooks that spring and summer have long days and short nights while fall and winter have short days and long nights
- Give each student 1 copy of the Photoperiodism Notebook Cutouts. Instruct students to cut out each diagram and place it on their note paper or in their science notebook.
- Assign as homework or allow students time to research examples of photoperiodism in nature. Students should find five examples of plants or animals reacting to long days and five examples of plants or animals reacting to short days. Write the name of the plant or animal on the outside of each tab and describe the photoperiodic reaction on the inside of the tab. Many examples exist in nature. A few examples your students may find include:
- Female sheep and goats only have an estrus (breeding) cycle when the days are short.
- Female horses only have an estrus (breeding) cycle when the days are long.
- Poinsettia plants flower and their leaves turn red when the days are short.
- Chrysanthemum plants flower in the fall when the days are short.
- Pea, lettuce, and wheat plants will only flower when the days are long.
- After the notes page is complete, draw your students' attention to the list of phenomena you listed on the board at the end of Activity 1. Could egg production in hens be influenced by sunlight? (Yes. Hens will naturally slow their egg production as the days get shorter in the fall and may even stop producing eggs in the shortest days of winter.) Explain to students that egg farmers have various types of housing for hens that are specially engineered with automatic lighting. When the days begin getting shorter, the lights turn on at dusk to mimic a long summer day. This results in hens sustaining their egg production through the winter providing a year-round supply of eggs. See the Enriching Activity below for an explanation of why free-range eggs may be more expensive in the winter while conventional eggs are typically not.
Concept Elaboration and Evaluation
After completing these activities, review and summarize the following key concepts:
- Photoperiodism is the physiological reaction of organisms to the length of day or night.
- Many plants and animals in our environment, including domestic hens that lay eggs, are impacted by either the presence or absence of sunlight.
- Understanding science, such as the principle of photoperiodism, allows farmers to increase the productivity of their farms to make them more efficient.
We welcome your feedback! Please take a minute to tell us how to make this lesson better or to give us a few gold stars!
To help students better understand various styles of hen housing, give students a video tour of Burnbrae Farms, a large egg farm in Canada which uses several styles of hen-housing. Have students watch each video and compare and contrast the benefits and disadvantages for each housing system in a T-chart. After completing the activity, students should recognize that when accounting for all factors there is no single "best" or "worst" system. Each producer and consumer chooses their preference of egg production systems. As a class, discuss how free-range housing may prove more difficult to provide supplemental lighting in the winter than other styles of housing. While supplemental lighting can be provided in nest houses of free-range egg farms, the hens have more access to the outdoors leading to more likelihood of the hens decreasing their winter egg production. Following simple economic rules of supply and demand free range egg prices may rise in the winter while conventional and cage-free eggs do not.
Suggested Companion Resources
Virtual Egg Farm Field Trips (Multimedia)
Eggs 101: A Video Project (Multimedia)
National Center for Agricultural Literacy and American Egg Board