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Eagles, Fish, and the Food Chain

Largemouth Bass
Eagles eat fish — A LOT of fish. They are also are "opportunistic" in their feeding habits. Their diet varies with the season and with what is available, but fish is their number one food choice. Other foods include certain birds (waterfowl) and an occasional turtle. Some reports tell of bald eagles feeding on deer, whales, or other large animals, but in those cases the eagles found the dead animals and did not kill them.

An eagle's average daily food consumption is from 250-550 grams per day, or between 5-10% of an eagle's body weight.

Sunlight Fuels the Food Chain
Bald Eagles, like other animals, get all their energy from the sun. Of course, they could sit out in the sunshine their whole lives and die of starvation! So, how can the sun be the source of their energy?

Let's start at the beginning. Sunlight is the source of energy for plants. Using the sun's energy, green plants produce food through photosynthesis. Tiny plants called algae grow in lakes. Without the algae, there would be no food for any of the animals, and without the sun, there'd be no algae. Read on to see how the chain unfolds.

Green Algae

Water Fleas

Nymph

Algae are the main source of food for little animals called daphnia or water fleas.

  • It takes about ten grams of algae to grow about one gram of tiny water fleas.

Some aquatic insects eat these tiny water fleas. An example is the dragonfly nymph. Dragonfly eggs hatch into nymphs that live in the water for months or even years. When they leave the water they shed their final nymph skin, and open up their wings to become adults. When these adults breed, they will lay their eggs in the water to start a whole new cycle.

Dragonfly nymphs eat many little creatures in the water, including water fleas:

  • It takes about 10 grams of water fleas to produce about one gram of insect.

Small fish eat many aquatic insects, including dragonfly nymphs:

  • It takes about 10 grams of insects to produce one gram of small fish.

Some medium-sized fish eat smaller fish. An example is the largemouth bass:

  • It takes about 10 grams of smaller fish to produce one gram of these medium-sized fish.

In turn, the medium-sized fish are food for large fish:

  • It takes about 10 grams of medium-sized to produce one gram of these large fish.

Finally, the large fish are food for other living things like eagles!

  • It takes about 100 grams of large fish to produce one gram of eagle.

Why does the eagle need so much more food for its weight than fish, insects, or water fleas need? (Hint: Consider whether a species is warm-blooded or cold-blooded, and what this would mean for food requirements.)

Try This!

  • A food chain is a simple drawing that illustrates how each animal eats some things and is eaten by others. Have individuals or small groups of students ilustrate the eagle's food chain starting with the sun and going all the way up to the eagle. Include labels for each "trophic" (food) level and a title for the completed page.
  • Students can create illustrations for each part of the eagle's food chain, or print the illustrations from this page.
  • Students may want to add facts or math questions to their food chains to challenge other students.
  • Food chains are one way of showing the food levels. Another way is a food pyramid. For example, a food pyramid for eagles would have a large base: the algae. Ten thousand pounds of algae would produce a thousand pounds of daphnia, so this level of the pyramid would be smaller. A thousand pounds of water fleas would produce a hundred pounds of tiny fish. Use this information to draw an eagle's food pyramid.

National Science Education Standards

  • All animals depend on plants. Some animals eat plants for food. Others eat animals that eat plants.
  • For ecosystems, the major source of energy is sunlight. Energy entering ecosystems as sunlight is transferred by producers into chemical energy through photosynthesis. That energy then passes from organism to organism in food webs.

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