"Is it a bird? Is it a plane?"
Translation:Czy to ptak? Czy to samolot?
Is it possible to rephrase that using the instrumental (czy jest ptakem, maybe – not sure about the form), or do you have to use the “to” construct?
First of all, a small correction: Czy jest ptakiem? (you missed the i). While the sentence you propose is per se correct, it does have a slightly different meaning. Czy jest ptakiem? ist translated best as Is he a bird?. The to translates to that, so the whole original sentence gives you Is that a bird?