When he had eaten his Devil's Fruit, he had eaten it on a bold whim, and nobody even knew what kind of fruit it was. Was he going to be a Paramecia, a Logia, or a Zoan? Only time and testing would tell.
He didn't feel any different, and kept telling everyone this when they asked him. They told him to hold heavy things and to punch things, and throw all sorts of fancy moves, to see if any that would bring about a change... but nothing was happening.
Just to get everyone off his back, he told them he was going to his room: but he really locked himself in one of the bathrooms. In the mirror he would scrunch up his face, flex his muscles, do some very strange jigs, and if he had done any of this in front of anyone else he'd have felt like a complete idiot — so fortunately for him, who was the mirror going to tell?
He had locked himself away in this bathroom for probably half an hour when, scrunching up his face once more and closing his eyes in concentration, he started to feel strange. He was warm, but at the same time cool; he could feel his body was just, changing, and taking on a different shape — his limbs were growing and folding into positions they shouldn't be, and something was growing in a place it shouldn't be. Through his closed eyes light was flooding in, and when he opened them the room was engulfed in a blue light. It had taken a few moments for Marco to get use to it when the bathroom door banged open, having attracted someone because of the mysterious light, and Thatch screamed, "AAAARGH, THERE'S A GIANT PARROT IN THE BATHROOM!"
The only reason Marco was able to live down being called a giant parrot was because he and Thatch had mutually agreed never to speak to anyone about it, as Thatch had screamed like a little girl and ran away after seeing Marco.