In terms of English "you see birds" and "you watch bird " are about as common in google searches. "Watch" and "see" have a little difference in meaning though. http://www.learnenglish.de/mistakes/watchvssee.html Searching for both "Tú ves" "you see" on google gives 147,000 results and "Tú ves" "you watch" gives 7,740 results. People are probably not translating "ves" to "watch" that often. My guess we have to have more context(another sentence before or after) to know if "watch" is an option.
I translated this as "You watch birds" and it told me I had to have the article "the." "You watch birds" is a legitimate English sentence, but it implies that watching birds is something you regularly do as a hobby instead of watching specific birds.
When I was learning Irish, they have a present tense, and they have a habitual present tense. If I was going to say, "You watch birds" in Irish and I meant it's something you usually or always do, then I would need the habitual tense verb.
In English, "You watch the birds" would be the present tense way of saying you're looking at specific birds right now. "You watch birds" would mean you're a birdwatcher.
So is there an actual real reason in the Spanish language that Duolingo wouldn't accept "You watch birds" as a correct translation for this specific sentence, or did they just goof?