This is gonna get close to conspiracy-type thinking, but hear me out. I’m not convinced that Ernest is a male (despite his/her name).
First, and most obvious, it looks pretty clear from that picture that she’s a hen. Not a rooster. I know it hasn’t even been confirmed that she’s a chicken, so, grain of salt, but still.
Second, the name. Ernest is traditionally a male’s name, sure, but it’s possible they just wanted to have a ‘nest’ pun for the chicken character. (what follows is reaching. Like, really, really reaching: is it possible that Ernest is supposed to be similar to "Her"nest (the H is silent in Ernest). Okay, this looks pretty ridiculous as I type it, but I warned you, so I’m keeping it in there.) I’ll admit that Ernestine would have been a more logical choice if this character was/is a female. (Maybe that’s her name and this is all just misdirection).
Third. Going off of the presumed chicken grenadier motif, wouldn’t it make sense for “Sgt. Demobird” to be launching something akin to eggs? Not actual eggs of course. They’re grenades. But the idea of an “egg-launching chicken” seems pretty obvious. If that is the intent/case, then why would Ernest be a male, and not a female?
So, some hope for those wishing for a female. You may use “Keen on Ernestine” for any future rallies if you’d like.