"A chicken is a bird."
Translation:Курица - это птица.
Since the Russian language dropped the verb 'to be' in most cases, sentences like «X is Y» became easily confusable with appositions «X-Y». So, «Курица — птица» 'A chicken [is] a bird' can be confused with «курица-птица» 'a chicken bird, a bird of a chicken'.
If you think this is not someone would say, you should check Russian folklore: it has lots of things like «Сирин-птица» (a bird Sirin). Well, you don't find many traditional poetic texts about chicken, alas! But still, the construction is ambiguous.
So, to distinguish these two usages, Russian has introduced a pronoun 'э́то' in the construction «X is Y»: «Ку́рица — это пти́ца» (literally "Hen, it [is a] bird"). It might seem redundant from the English point of view, but in Russian э́то serves a purpose. And while you still can say «Ку́рица — пти́ца», it's not what Russian speaker would usually say, it would sound unnatural.
I am a native speaker of Russian, have lived in Russia all my life, but to me «Курица — птица» doesn't sound unnatural at all as a sentence (in fact, I've chosen it as an answer). As long as you say it with a proper intonation, it will not sound as an apposition, so это IS redundant after all. It's true that это is widely used in giving definitions, but it doesn't make it a mandatory thing.
«Курица не птица» doesn’t literally mean what it says. It is part of the old Soviet saying «курица не птица, Болгария не заграница», which means “just like a chicken is not good enough to be considered a real bird (it can’t fly far and high), Bulgaria is not sufficiently different from the Soviet Union to be considered a foreign country”.