Acid-tongued reality singing show judge Simon Cowell is celebrating his 54th birthday today (Oct. 7). Former 'X Factor' contestants helped him celebrate by toilet papering his Beverly Hills abode.

No, they were not disgruntled artists and performers sent home after receiving a verbal throttling from Simon, thus seeking revenge on him for his hard-nosed, tough-to-take critiques. In fact, this batch of contestants is close to his blackened heart, since he mentored them deep into Season 2.

The culprits? The girls of Fifth Harmony. Woot woot!

It was certainly a way for the girls to get the expectant father's goat!

The band -- comprised of all females with gorgeous voices who signed to Si Co's label despite not winning their season -- tossed TP all over the grounds and left a sign outside the gates of the home.

Only they turned Cowell into an octogenarian, writing "Happy 87th Birthday," no doubt an ode to his crotchety old man personality. See the funny photo here. That's definitely not a typo, but a dig at Cowell.

Happy birthday from your pals at PopCrush, Simon! We won't toilet paper your house…but only because we don't know where it is.

We'll give this one to Fifth Harmony -- game, set and match.