Any time the heroine says something reflecting her barbaric Blood Knight culture... and Kazuaki replies as though that's a perfectly normal thing for her to say.
At one point in either Sakuya or Oko San's route, the two of them will be having an argument, and you are given 3 choices. Choosing to side with either Sakuya or Oko will result in our hot-blooded heroine throwing the other out the window. Choosing the third option means you side with neither and chastise them for their behavior...by hurling thembothout the window. Double Dove Defenestration!
The extra scene you get as a reward for completing the Bad Boy's Love route. Also doubles as much-needed therapy after everything said route puts you through.
Oddly enough, there's even one little gem in BBL itself that serves as this, where Sakuya somehow connects Yuuya's computer hacking skills to watching copious amounts of porn...which leads Ryouta to think that maybe he should watch more of it!
During Holiday Star the city where the games are set is finally named as part of a things-everybirdie-knows bit, and Hiyoko states that this is the first time she's heard the city's name mentioned! She guesses it never comes up in everyday conversation.
While considering the case of the Christmas Thieves, Hiyoko randomly goes OBJECTION! and HOLD IT! complete with familiar graphic, much to Ryouta's dismay.
Hiyoko: "I need to make sure my finger-pointing is sufficiently dramatic for when I need to use it!"
Ryouta: "Please don't go around pointing and shouting at stuff, you'll give me a heart attack."
An employee at a high society department store tries to shoo out Hiyoko, since they don't cater to low-class people like humans. She is incensed.
OBJECTION! This is blatant speciesism! I hope you're ready to swallow your words or defend them with steel, cretin! Draw, man, draw before I stick you like a rotisserie chicken!
She even questions just what Okosan has that makes him more upper-class than her.
One of Ryouta's odd jobs in the winter involves dressing as Santa. With a miniskirt. Hiyoko wants to do it too!
There's a segment where the birds answer questions fans sent in. One was directed to their human forms - "What underwear do you all wear?" and the responses...
Yuuya: "I'm wearing whatever you want me to be, baby."
Anghel: "On this day of the reckoning, my loins are wrapped in the vestments dyed in the blood of the crimson lotus blossom!"
Ryouta tries reading the first out of two letters directed to Okosan. The letter itself has to be seen to be believed, but the closing remarks are the cherry on top.
Ryouta: "Aaah... I felt like I was gonna suffocate reading that!"
The ending of HolidayStar's second story is so gloriously absurd it needs to be seen to be believed. To quote one Youtube commenter:
This is the only game where the following sentence makes any kind of sense: a magazine editor who's secretly a mad scientist threatens to blow up a school for pigeons with a giant laser powered by otaku birds to get revenge on the school doctor, but is stopped by the heroic birds and cavewoman who turn into magical girls and save the day with the power of kindness. ...I think my brain has died.
And then there's the fact that the laser starts malfunctioning on its own thanks to the fangirls attached to it getting into a ship war.
The April Fools 2014 web episode also centers around Pretty Coore. It's just as absurd, if not more, than the above ending.
In Holiday Star, Doctor Shuu takes in Miru and Kaku, who are obsessed with Christmas. The next time you go to the infirmary, the place is full of Christmas decorations and Shuu is wearing a Santa Hat. The player's dialogue only adds to the hilarity:
"Doctor Shuu? Did you finally get that lobotomy?"
The fact that the in-game character images are of actual birds... though they do show human representations of them - except for San, whose picture is that of a dove in a suit.
This also applies in Holiday Star's Shrine Visits, where you get the option to date with each bird appearing as their human representations... except Okosan, whose portraits still show him as a bird. Even his ICPSS option sounds unsure.
From when you meet Shuu at the New Year's festival in Holiday Star. Shuu says he was led to believe there was a corpse to be examined, but had been led astray. Hiyoko wants to spend time anyways. Shuu says yes, but since he never got to examine a corpse...
Any time someone (or somebirdie) makes a Let's Play of it. From the shock of the whole dating pigeons thingy to the shock of actually caring about the birds.
A special mention is Angie Gallant, who was unfortunate enough to do Shuu's route first. She was so freaked out by it that she became paranoid over the other routes. Nageki's acting suspicious? He's going to kill her. Sakuya just HO HO HO'd like a certain fat bird? He's actually the fat bird in disguise.
Angie: 11/28/11: the day I dated a considerate serial killer cannibal fat bird.
Hiyoko's reaction after being locked into the prep room by Doctor Shuu.