Science time: a bird is a bird, right?

So how come they have evolved sooooo differently from each other since they evolved from the dinos?

Penguins (my faves) stick together for life. Lots of other birds kiss and tell, or kiss and fly away.

Some birds could care less about their mate/young; others mate for life and even raise their chicks for as long as the human equivalent.

What makes them so different, and don’t tell me geography, climate and other stuff like that, please.