After Trilly and Frank had their babies, they started mating again when the babies were about 6 weeks old. Trilly even laid three eggs, but fortunately completely ignored them.

Every time they started mating I would tell them NO SEX! waving my hands, and I have to say it worked because they don't do it anymore, at least when I am around.

Last night they were all in their night cages. I was in the other room and heard unmistakable noises of mating going on, so very quietly went to their room and opened the door... They were ALL SEVEN perching angelically and looking at me, like "hi there, what's up?".
Impossible to say which couple was the culprit, but I am wondering: do they understand that I don't want them to mate? Is that possible?

You may have a prankster in your flock! I have one in my flock named Henry. Every time I hear mating sounds I go into the bird room to see who is doing what to whom, and sometimes it turns out to be Henry just standing around and imitating the hen mating cry. Little brat.

Well they aren't dummies. I'm sure they know. Beaker and Jaid always go straight for Q-Tip's food bowl and I always tell them, "No cat food!" Then they evilly laugh at me.

And by the way, as a teenager today, I don't just go hopping around like others do. I'm a good girl.

Human teenagers are well-behaved sometimes. But birdie teenagers have no morals at all! Henry once boinked his sister during a bird club meeting while everybody laughed their heads off. He wants to boink his mother too but she's not interested. He is a VERY naughty boy, and he's not even a teenager any more. He's four years old.

Oh great. I just remembered something. Jaid is started his hormonal brat stage too. Shoot, here goes a teenager about to raise a teenager!

I can just picture your club just sipping some drinks and then, "Oh, uh, umm, well then..."

Stephanie you must forgive me, sometimes I forget that you in fact are a teenager
So when I speak of bratty teenagers you are exempt from my remarks lol

Henry sounds like my type of bird, i love pranksters So you are telling me that Darcy or Keith might be doing that? Ah well, I put nothing beneath them, they are delinquents.

The official flock call has now become their own version of pop goes the weasel, which is better than the horrid sound they used to make. Only Frank still needs to be converted to it.

Aww, how is Keith a delinquent? He looks so innocent. I wish I had a flock song. All I have is laughing. Laughing, belly laughing, evil laughing, girly laughing. Just laughing. Welcome to the insane asylum.

Keith looks innocent because he is all white an yellow. His heart is black

no no no it's not true! but as the newcomer he was all nice and calm, and his most rebellious act was to bang on my head.
Now he is becoming exactly like Bubu, hates the girls, kick other people off the perches, steals things, plus he flies very close to my face. I don't even flinch so he might rethink this one, but he definitely has a very strong character.
Then he sits under my chin for scritches and I melt of course.

To be honest I would love to have evil laughing cockatiels.

http://youtu.be/tHfpCUDIwHE

Play the 22 second mark over and over again. You don't know what you are asking for.

I would put Darcy and Keith on the list of suspects until you have proof that someone is actually mating (or masturbating - quite often when I walk in to see who's getting frisky it's Mims entertaining herself). I don't know about your guys, but when my birds are mating they don't stop just because they heard me approaching. If I want them to stop I have to walk over and hold out my finger to get the male to step up off the hen's back. And yes, I have actually done this a few times. What amazes me is that he's willing to do the step up.