In my case "A bird flies" was shown as the correct answer ( I wrote Bird flies). I guess sometimes they are using "The" some times "A" and other times not using any article at all. Confusing !
That's a construction you could use if you specifically wanted to emphasize the habitual nature of the activity. To make a general statement, the example given is sufficient.
Or is it that, if it is singular -- use "The", if plural don't use an article!