Biologists often ignore the traditional "breed with fertile offspring" definition when it doesn't fit the facts on the ground. For example, the Smew[1] can hybr

Biologists often ignore the traditional "breed with fertile offspring" definition when it doesn't fit the facts on the ground. For example, the Smew[1] can hybridize with the Common Goldeneye. Not only are they different species, but they are different genus! [1] https://en.wikipedia.org/wiki/Smew https://en.wikipedia.org/wiki/Smew