In the great redwood forests in California, hummingbirds were sipping nectar from flowers that were contaminated with chlorofluorocarbons, causing them to grow to gigantic size and forcing them to seek out new sources of CFCs. These monsters made their way to San Francisco, attacking a military jet. They were unable to be seen, because their wings, which had reflective feathers, reflected the light, therefore rendering them invisible. As H.E.A.T. investigated the situation, Zilla Junior came into contact with the creatures, but was unable to retaliate because he couldn't locate them. Finally, H.E.A.T. managed to construct a special pair of goggles that when fitted on Zilla Junior's head would allow him to see the Mutant Hummingbirds. He was then able to knock them out of the sky with tail whips and atomic blasts. According to Hicks, they weren't fatally injured, just stunned. The Pentagon then decided to study their stealth capabilities to create stealth jets.
The Giant Mutant Hummingbirds could also spit a liquid that was as corossive as battery acid on targets had incredible speed to be unseen by humans or Zilla Junior, and the ability to fly high in the sky.