The bird could easily be a chicken or duck or turkey. If he eats it all by himself perhaps it is a Cornish hen or a quail. I had this exercise as one of the hearing ones where you have to write down in German what they are saying. At first I thought she said "Der Großvater IST den Vogel." (the grandfather IS the bird). I thought it was another crazy duolingo sentence. But then realized it would be "der Vogel" in that case. Remembering my grammar helps.

Why doesn't "den" translate to "this" here? How would you say "The grandfather eats this bird"?