Science mystery solvers

This exercise is a part of Educator Guide: Elusive Killer in Eagle Die-Offs ID’d / View Guide

Directions for teachers:
Ask your students to read the online Science News article A toxin behind mysterious eagle die-offs may have finally been found,” which explores scientists’ quest to ID a suspect in mass bird deaths, and answer the following questions. A version of the story, “Elusive killer in eagle die-offs ID’d,” appears in the April 24, 2021 issue of Science News.

1. What types of birds have died in massive numbers around lakes in the southeast United States? When did the die-offs start?

Massive die-offs of bald eagles, mallards and other began in the mid-1990s.   

2. What behavioral observations did scientists make about the birds before and after the animals’ deaths? How were the observations connected?

Birds lost coordination, struggled to fly and walk, and had seizures. Examinations of dead birds’ brains found unnatural holes called vacuoles. Those holes contributed to the observed symptoms.

3. What disease did scientist classify the birds as having?

Scientists classified the affected birds’ symptoms as a disease — vacuolar myelinopathy, or VM.

4. What invasive plant did scientists find around lakes where die-offs took place? Based on this observation, what hypothesis did aquatic ecologist Susan Wilde come up with that might explain the die-offs?

Lakes with die-offs all had dense growth of the invasive water plant Hydrilla verticillate, which water birds like to eat. Wilde thought that the plant, and the cyanobacteria that sticks to it, may somehow be causing the die-offs. 

5. How did Wilde and colleagues test this hypothesis in the lab? Describe each step and the evidence that the researchers gathered from it.

The scientists first collected samples of the weed and looked at it under a microscope. They saw cyanobacteria covered in goo on leaves. An analysis of the cyanobacteria revealed it was a new species. Wilde’s team grew the bacteria in the lab and tested it on birds, and observed no effects. 

6. Why did Wilde’s team take their investigation from the laboratory to the field?

The team thought that maybe something was wrong with their lab setup, so they went straight to the source.The scientists collected wild weeds covered in cyanobacteria from places with confirmed VM outbreaks.

7. What did the researchers find during their fieldwork? Explain how the discovery led to an additional procedural step and evidence.


They detected an unknown compound made by the bacterial colonies. Tests of the compound showed it was rich in the element bromine. When the scientists gave bromine to lab-grown bacteria, the colonies turned toxic.
8. How did the invasive plant play a role in the die-offs? Explain.

The plant builds up concentrations of bromine that are up to 1,000 times greater than in lake water, and likely leaks bromine under certain conditions turning the cyanobacteria on it toxic. Birds that eat the plant get a deadly dose of toxin.

9. Why did birds that didn’t eat the plant get vacuolar myelinopathy, or VM?

Predator species like bald eagles and owls ingest the toxin when they feed on affected water birds.

10. What does Wilde’s team plan to do next?

The team is investigating how the toxin might affect mammals.

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