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See a flock of birds fall from the sky? There’s an #EarthWord for that, and it isn’t “apocalypse”

EarthWords is an on-going series in which we shed some light on the complicated, often difficult-to-pronounce language of science. Think of us as your terminology tour-guides, and meet us back here every week for a new word!

Image: Dead Fish Washed Ashore during Golden Alga Toxic Bloom
Dead Fish Washed Ashore during Golden Alga Toxic Bloom. Credit: Mike Hooper, USGS. Public domain

The EarthWord: Mortality Event


  • One of the world’s least enjoyable events, a mortality event happens when a lot of animals die off suddenly.

  • Many times, these events are unexpected or their cause is difficult to determine.


  • Mortality comes from the Latin word mortalitas, meaning “subject to death.”

Use/Significance in the Earth Science Community:

  • Mortality events play a significant role in wildlife epidemiology. Scientists attempt to determine the cause of mortality events to see if a disease is the cause, human activity, or some other reason.

  • For endangered species, mortality events can significantly threaten the population, as there may not be enough surviving individuals to form a viable population.


  • The USGS National Wildlife Health Center studies and responds to mortality events, and we work with wildlife managers to prevent events from occurring.

  • We report mortality events on the Wildlife Health Information Sharing Partnership or WHISPers map, including date, location, and species affected.

  • If you see sick or dead wildlife, please do not handle; we recommend that you contact your state natural resources agency.

Next EarthWord: This week's EarthWord, to quote Indiana Jones, belongs in a museum...

Hungry for some science, but you don’t have time for a full-course research plate? Then check out USGS Science Snippets, our snack-sized science series that focuses on the fun, weird, and fascinating stories of USGS science.

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