Translation:Are they birds?
Because that's improper English. You almost never say "are that ..." in a question. You would use "is." Is refers to the group as a whole, in this case the birds. You're not speaking about the individual members of the group of birds. If you were, you'd use "are." For example "The birds are eating worms." It's all the more so because your pronoun "that" refers to a single item. If you wanted to use "are," then you'd need to use the pronoun "those."
Does that make sense?