For the benefit of anybody who might have come in late on evolution and birds...
You could start with the original post on this thread.
A flying bird requires a baker's dozen complex systems which you don't find in other animals: flight feathers with their complex structure, wings, super efficient hearts and lungs, beaks (since they don't have hands to feed themselves with), ultra-light bones, special kinds of tails for controlling flight, specialized balance parameters etc. etc.
For a non-flying creature to simple hatch an egg with all of that at one stroke would be an ultimate probabilistic miracle, totally impossible.
Unless of course, structures can have multiple functions, and oops what a coincidence... THEY DO.
Feathers can be used to fly, but they can also be used to keep you warm, and to produce colorful displays, and to protect you from rain.
Fins can help you swim, but they can also help you crawl along the ocean floor, or on rocks, or onto land (mudskippers).
Irreducible Complexity makes the same error that Gunga makes above when it assumes that all morphology has a linear developmental track and no flexibility to function. In short, it's an invalid assumption resulting in an incorrect conclusion.