When do you use peștii and when peștele?
'pește' (articulated - 'peștele') is singular
'pești' (articulated - 'peștii') is plural
So literally it should be: "the birds eat the fishes"
No, it should not, unless there a different kinds of fish. The plural of fish is fish.
Both, singular and plural translation are accepted here, because there is no way to know if the birds eat one fish or more.