Some frame data seems off, for example, is his V Skill 6f startup? Trying to punish -7 moves with it, and no joy.
Anyway, not sure if the following belongs in the MU thread, or for here, but I’ll post in both anyway. Sorry if anyone already covered this. Played some games against a friend’s Birdie and after examining frame data, some stuff was negative that I let go unpunished (f+hp, bull heads, etc). Here’s what I found
Birdie’s f+HP is -7, so the following punishes it:
st.fierce is a 1f punish (not hard given the buffer). Works as a punish at pretty much all ranges, unless the f+hp hits super far. st.hp xx MK/HK SS seems to be the best meterless punish.
st.mp has a great hitbox and punishes it too, 6f startup so this punish has an extra frame of leniency. Might be better going for this if you aren’t confident in st.fierce as a punish. Worth nothing is that st.mp will punish at ranges where st.hp wont, so this might be the best go to option. You give up 30 or so for more consistency. Fair trade imo.
At max distance, this move seems unpunishable, but 90% of the time, st.mp xx HK SS/MK SS will be the go to punish. You can get fancy with V Trig and one EX by doing stuff like st.mp xx ex boom (2 hits), f v trig, nj.hp, cr.hp, st.mp xx HK SS, pretty standard stuff. As an extra tip, if you’re cornered and they do f+hp, you can use v trigger in your punish to corner them =)
Bull Head punishes:
lp/mp/hp/ex bullhead are -10/-10/-5/-10. st.fierce xx SS is a perfect punish on lp and mp. HP is -5 to st.mp/st.hp punishes wont work, however it leaves Birdie right in range for a cr.jab xx lk ss punish. There’s also an audio cue to tell which version is the HP one, as well as the distance it travels.
EX is annoying in that even though it’s -10, it leaves him out of range of every nash normal. However, LK SS reversal timing is a 1f punish too. Super will punish it too. EX SS and EX TA will work too, for meter. So overall nash has some good punishes meterless for each bull head.
Bull head in V Trig
This is where it gets annoying. lp/mp/hp/ex are now -9/-9/-9/-7 respectively, but very annoying to punish meterless. The best nash can get meterless on lp/mp/hp is cr.mk, sweep, cr.fierce. That’s really all I have found. On block Birdie’s hurtbox reels back out of range for other punishes. Meterl punishes include ex SS and EX TA.
EX bullhead is annoying in that there is no meterless punish. EX SS seems to be the only one. EX TA is 8f startup so no go 
So all in all, nash has meterless punishes for everything birdie can do, apart from EX bullhead in VT, which requires a bar to punish.
V Reversal is also an option if you want to get away from Birdie too