Question

due to which similar character birds and fish are in the same phylum

Answer 1

The similar character birds and fishes are of the same phylum is presence of scalesd post and tales. All of the birds and fishes and birds that are known to you, fall under Phylum Chordata. The most unique character that all animals belonging to this phylum have is the existence of notochord."

Answer 2

because both are oviparous