"Pleydell-Smith said, ‘The guanay is the most valuable bird in the world. Each pair produces about two dollars’ worth of guano in a year without any expense to the owner. Each female lays an average of three eggs and raises two young. Two broods a year. Say they’re worth fifteen dollars a pair, and say there are one hundred thousand birds on Crab Key, which is a reasonable guess on the old figures we have. That makes his birds worth a million and a half dollars."
Ian Fleming - Dr No: James Bond 007
I kid you not, Fleming dedicated a whole chapter to the economics of bird poop.