Translation:A goose, a hen and a duck live together.
Why do we sometimes say hen instead of chicken? Because hen is more specific than chicken and we sometimes need to be specific about the type of chicken we are talking about. As this is a language learning site, it seems a safe bet that Duolingo wants students to understand the difference between le poulet, le coq and la poule.
I don't think "hen" is more specific unless you know you're talking about chickens. "Hen" can be the female of many species. Here are the first two definitions from among several in a Collins dictionary:
the female of any bird, esp. the adult female of the domestic fowl
the female of certain other animals, such as the lobster
True. "Une poule" is specifically a hen of the chicken variety. "Les poulets" are chickens in general or those to cook & eat, and "les poules" are egg-layers. I don't think Duo goes into any of the other female bird designations, but your comment piqued my curiosity: a female turkey is "une dinde"; a peahen is "une paonne"; a female duck is "une cane", tho male ducks or ducks in general are "les canards" and a drake is specifically "un canard mâle"; a swan of either sex is "un cygne"; and an ostrich of either sex is "une autruche". Go figure! Have a lingot and thanks for the deep dive into my dictionaries! :-)
anais67mb, I'm very pleased to accept your lingot, but it's obvious you have done more research on hens than I have, and you are much more deserving of a lingot than I am. I gave it back before I realised it could seem unfriendly. Therefore I had to give you two, but please let's not enter an arms race - please don't give me three!
One would think that in the context of this exercise, they would have anticipated someone using "drake" for "canard".
I live in a bilingual community and once saw someone mocked for asking about "oeufs de canard", rather than "oeufs de canne". It would seem that "canard" is very clearly perceived as male, when gender counts, and in English, when gender counts, a male duck is a "drake"