Kaitlin Olson and Rob McElhenney attend the premiere party for "It's Always Sunny In Philadelphia" and "The League" on September 13, 2011 in Hollywood. Credit: Jason LaVeris/FilmMagic

The boys are taking over!

It's Always Sunny in Philadelphia costars Kaitlin Olson, 36, and Rob McElhenney, 34, welcomed son Leo Grey April 5 at their home in L.A. and four days later, the couple posted the very first pic of their bundle of joy.

"Leo Grey McElhenney…born April 5th after 23 very very long hours," Olson tweeted late Monday. "We're in love."

Leo Grey weighed in at 8 lbs., 5 oz. and measured 20 inches long. He joins older brother Axel Lee, 19 months, who was also born at home.

Fellow It's Always Sunny in Philadelphia star Charlie Day (with wife Mary Elizabeth Ellis) welcomed a baby boy in December, and Glenn Howerton (with wife Jill Latiano) became a first-time parent in September.

Olson and McElhenney married in Malibu in September 2008.