One small step at a time. Let's actually look at, you know, the evidence,
First of all, it wouldn't be a 'frog into a bird'
It would be 'an amphibian to a dinosaur to to bird over the course of 400 million years'.
First, let's look at the development of amphibian to land dwelling animals. The major development that allowed amphibians to become full land dwelling animals is the development of the egg shell. This page details a list of the transitional fossils we have from amphibians to the early egg laying land animals
https://en.wikipedia.org/wiki/List_of_transitional_fossils#Amphibians_to_amniotes
Over time, such features as shells around the embryo, and changes to the kidneys allows early egg laying land animals to appear.
Aminotes moved into alasours , to dinosaurs, and then some of those, developed into birds, over a 200 million year period.