NotWhatISignedUpFor said: Strands doesn't take into account that ducks have a life span of ten years or less.
Well, she's only calculating three generations ahead. And duck hens can lay eggs as yearlings, so she's probably assuming new clutches once each year. Although she's either assuming very small clutches or that half of the hens won't be laying, as cluches are usually in the range of 7-12 eggs, and can go as high as 20. Really, it's not the english that's strange, it's the math or the nuns' duckkeeping that seems a little odd.