The sentence literally translates as "ducks - they are birds." Also "these ducks" would be "эти утки"
I believe you, but I thought "ducks are birds would be like "утки -- птицы." Do you need это when you're saying that one noun is equivalent to another noun?
Yes. Since Russian dropped copula (the verb 'is, are, am') in most cases, it needed a way to distinguish appositions and «X is Y»-type sentences. To do so, Russian speakers started adding «э́то»: i.e. we say «Ducks, they [are] birds» instead of «Ducks [are] birds».
A similar process occured in Chinese: in 鸭子是鸟, 是 was originally a pronoun in Classical Chinese. In fact, Mandarin Chinese went even further than Russian: it made 是 a verb. In Russian, «э́то» is not yet a verb, but it's used in «X is Y»-type sentences and without them, they generally sound much less natural.