Translation:Birds fly in the sky.
Yes, it should be. Without the addition of a counter, a quantifier, or a demonstrative adjective [e.g., 一羽(one+bird counter), たくさん(many), あの(that~)], there is often no way of determining whether an object is intended to be singular or plural. Without context,「とりはそらをとびます」is entirely open to interpretation. Yes, it could be a general statement, "Birds fly in the sky." But it could also be "A bird flies in the sky" (as opposed to "A fish swims in the sea"). I would say that I prefer the plural interpretation in this case. However, had the verb been conjugated to the present progressive (とんでいます), I would definitely prefer the singular interpretation (although the plural is equally plausible). English is the biggest obstacle to learning Japanese.
Just to complement my own answer: In this case, the particle は is making the subject "bird" in general. It's like saying about birds, they fly through the sky. If you want to be specific, you can add この/その/あの to convey the idea one bird in specific can fly (but some others don't). If you want to express the idea about numbers, you can use the sentences shown in this lesson.