"It is another bird."
Translation:Se on eri lintu.
So correct me if I'm wrong but this is how I've always understood the difference between "eri" and "toinen". If I saw a pigeon first and then a crow, I would say about the second bird "se on eri lintu" (it's a different bird), but if I saw an oyster catcher and then an oyster catcher again I would say "se on toinen lintu" (it's another bird). In the first scenario I could easily use either sentence but in the second I could only use the second sentence.
That's how I understand the difference between eri and toinen. I would also point out that if you hover over the English words it comes up with "toinen" as a suggested translation for "another" but if you use it in your answer it's not accepted.
That's one way to put it but you can also use "eri" about two birds (or just anything) of the same species. Let's say you're having a conversation with someone and you are watching birds, and there's a pigeon and it goes away and then a pigeon appears again, one says "It came back!" (Se tuli takaisin!) but if you know for sure that is not the same bird, you can say "Se on eri lintu!", or "Se ei ole sama lintu." (It's not the same bird.)
And you can also use "toinen" in the place of this as well, but it's more often used when you see more than one. If you still see the first bird and another one appears, you can say "Tänne tuli toinen(kin) lintu." ((Yet) another bird came here.)
I feel like "eri" more of corrects a mistake you or someone else made, and "toinen" is to state that there's more than one thing out there. I think this might also have more difference in the written Finnish and then in the spoken Finnish we probably use these more interchangeably, since it's yet another case where I totally stop being able to figure this out in my head the more I think about it :D Hearing from other Finns what they do with these words might also help, since I might also imagine these in different contexts and therefore give examples that might sound weird for someone else, like with the word "pidgeon", do I mean an individual bird or the whole species? That also kinda affects if I'm going to choose "eri" or "toinen".