Do you think they're happy when they swim across the water?
In the sense that I think you mean.... yes. Assuming there is not any suffering going on - same as it is with us - it's OK to project the idea that they're happy. Projection nonetheless.
I hope that typed ok - even with my glasses I can't see for shit tonight
Animals experience emotions. You can tell when an animal is afraid, angry, nervous, excited, sad, content. So yes, I would say that ducks swimming on a pond are content and at peace. You could call that happiness.
