The original sentence is: "Woodpecker nests are valuable real estate that are used by other birds over many years."
This sentence is mostly correct and clear. However, to make it slightly smoother, you could say: "Woodpecker nests are valuable real estate that is used by other birds for many years."
Explanation: The word "real estate" is singular, so we use "is" instead of "are" after "that." Also, "for many years" sounds a bit more natural than "over many years."
So, the corrected sentence is: "Woodpecker nests are valuable real estate that is used by other birds for many years."