"These are the birds whose food is little."
Translation:Αυτά είναι τα πουλιά των οποίων το φαγητό είναι λίγο.
Why does it use 'των οποίων' and not 'τα οποία' - since it would match the gender neutral of 'τα πουλιά'?
It has to match not only the gender but also the case -- in this case, you need the genitive case, as you're talking about the food of the birds.
Hence των οποίων, neutral plural genitive. (Though this particular form would work for all genders in the genitive plural.)
So, the same reason why we have "whose food" in English and not "which food" or "that food".
I agree (I actually went ahead and reported it as unnatural). In modern English at least, it gives the mental image of the birds eating food that is literally "little" (of small size). As you wrote, the more natural way to convey the idea that is intended is with "these are the birds who have little food" (even that is rather formal-sounding and without context can still convey that the food is small- even more common would be "these are the birds who don't have much food").