It turns out that Birdo has been in jail on Mimin Island for using a women's restroom. She asks Captain Rainbow to find proof that she's a female so she can get out of jail, and he does -- buzzing, under her pillow. When he picks it up, it is censored.
Maybe Nintendo will rewrite this bit so it's about sandwiches. Then it could be released outside Japan.
The weirdest thing about Birdo's gender is that the thing seems to be gender-free except for the bow. So, then, does it mentally identify as a man identifying as a woman? And let's ignore the egg-laying issue entirely.