This translation is very awkward in english. Nearly any editor would drop the article to: "Many seagulls are shot in the spring." The only exception that I can think of that would be correct is if the specific seagulls had previously been referred to, so that they are a definite quantity. Even so, an editor would have this reworded, perhaps to "These many ..." or "This large number of..."
There's a problem in general with this course. Translation, grammar and pronunciation are at times pretty abysmal. It suffers from the 'direct translation' syndrome which makes some sentences inaccurate and useless.
Why is it "De mange måger" and not "Mange mågerne"? Previous lessons teach that definite form is shown by "den/det/de" only when we have an adjective before the noun. Is this some kind of exception?