"La frutta" means fruit as uncountable, while "il frutto" as countable: i.e. "L'uccello mangia (la) frutta" ("the bird eats fruit", or the bird eats fruits as part of its diet) and "L'uccello mangia il frutto" ("the bird eats the fruit", or the bird eats that one fruit I'm talking about).
It is write that "gli" only comes before plural masculine nouns, but that noum doesn't necessarily start with a vowel. "Gli" is the plural form of "l'" and of "lo", so it also comes before plural forms of nouns that would take the "lo" in the singular, like "gli squali", "gli zuccheri", "gli gnomi".
I put "birds eat fruit" because as it's not "la frutta", it's fruit in general and the "gli" would be unnecessary in English. Though in English, "the birds eat fruit" would refer to an identifiable group of birds (the birds in my garden for example.). Whereas "birds eat fruit" would be birds in general, or all birds, or some birds always, so I should have been wrong. (That was a confusing sentence.). Italian is an old and evolving language so everything will have a purpose. So, my question is simple. If my answer actually was correct, what does the gli do?
You are taking Italian for English speakers, but you can also take English for Italian speakers, Japanese for German speakers etc. Lots of different languages to learn with lots of different languages to translate to. Just go to your add a language sidebar, scroll down and TADA!