So, we know that birds are dinosaurs, right?
As in, all modern birds are dinosaurs
But... what extinct animals are birds? At what cladistic group do we draw the line and say "Everything in this is birds, and everything outside of this is not birds" ?
Well, the MOST INCLUSIVE definition - meaning everything that could POSSIBLY be a bird is in this group - is Avemetatarsalia (The group that includes Pterosaurs and all Dinosaurs)
If you call Avemetatarsalians birds, then you are a Bird Liberal
the MOST EXCLUSIVE definition - meaning ONLY MODERN BIRDS AND THEIR COMMON ANCESTOR and NOTHING ELSE AT ALL - is Neornithes (so the group of dinosaurs that is all modern birds and the most recent common ancestor of all modern birds)
There are reasonings and justifications for each level of bird politics, but ultimately each one is kind of arbitrary because birds aren't really defined by one solitary characteristic in the modern day
They're warm blooded; they have feathers; they fly (except not all of them); they have lightweight bones; they're bipedal; they have a complex respiratory system...
Avemetatarsalia all have feathers. Do you define it like that?
Avemetatarsalia is also probably all endothermic. That's another good reason.
But pterosaurs as birds? What???
Dinosaurs started out bipedal, and have a posture similar to birds... do you use Dinosauria?
Saurischians all had that bird respiratory system... do you use that?
Theropods look a LOT like birds, more than all the others...
Coelurosaurs all pretty much retained feathers - in other dinosaur groups you can find organisms that secondarily lost them (pterosaurs no, ironically enough)... do you use that?
Pennaraptora (potentially Maniraptoriformes) had complex feathers like the kind you use for quill pens... is that where you define it?
Paraves had the small genome required for flight... is that where?
Avialae has Archaeopteryx... this is literally the most arbitrary definition. It's based on what we think is the first bird even though only margianally more bird like than, say, Troodon.
Ornithothoraces had powered flight, but, many birds today don't... that's secondarily lost a lot...
Neornithes is al birds alive today and their common ancestor... definitely no ambiguity there at least, but you're leaving out so much...
WHAT ARE BIRDS?
WE JUST DON'T KNOW
So the more inclusive your personal definition is, the more of a bird liberal you are. Simple as that.
So basically this.