Up to 700 of their young would be killed during construction of the site in the Ivanpah Valley near the Nevada border.
The U.S. Bureau of Land Management's assessment is far worse than BrightSource Energy Co.'s prediction that the project would only disturb a few dozen animals.
BrightSource was ordered this month to halt construction while the plans are re-examined.
The federal government may have many of the animals captured and moved.
The Associated Press contributed to this story.