i personally would use s.mp, c.mp > s.hp for whiff punishing against birdie, just because it’s hard to punish itself, and for whatever reason seems to work better against s.mp and s.mk which are the two buttons birdie’s prob using the most.
EX CS sets up a nasty mix-up against Birdie, since Cammy’s at point-blank, +1, and Birdie has no 3f normals. It’s also Cammy’s only “cheap” way of getting in against Birdie - I base the match-up around this tool.
I play the Birdie matchup primarily by counter-poking - as a couple of other people have mentioned, eating a s.mp xx bull head for a whiffed s.mk seems like way too much. I fish for an in by whiff punishing, and once birdie is knocked down, it’s really lights-out. He has more trouble dealing w/ Cammy’s pressure than pretty much any other character in the game.
By the time I get pushed out again, I’ve usually built up a bar for EX CS, which gives Cammy an edge in neutral. It’s going to make Birdie more scared to press buttons, since any time he does I might hop over it and get inside. Depending on how the Birdie plays, I can either a) start bullying him more if he clams up, and find opportunities to dash in, harass w/ s.mk, etc. or b) if he doesn’t adjust and keeps poking, I find the right spot for EX CS, and throw him in the blender.
I don’t think it’s an easy matchup, but I do think that if you have the right strategy and stick to it, it’s going to be just as (if not more) frustrating for the Birdie as it is for you.