DERBY LINE, Vt. –

Every morning, these chickens do their duty in the backyard.

“This is it right here. some of the girls are out there already,” Cathy Bussell said.

The girls are a flock of backyard birds owned by the Bussell family of Derby Line. There's Elvira, Morticia and yes, big Butt.

Reporter Anson Tebbetts: So they are pretty good layers?

Cathy Bussell: They are good layers. We are now getting on the average 10 or 12 eggs a day. and that one huge egg that made up for a half a dozen, yes.

One of the birds kicked out a monster. the hen was able to squeeze out an egg three times of the size of a jumbo-sized egg.

“We joked about it,” Bussell said. “Maybe we had a goose in the coop and it was much too big to sit in a jumbo egg carton. We set it in the fridge for two or three days until I decided I wanted eggs and toast one morning.”

Just part of the morning routine, until she cracked the big one open.

“I just stared at it for a minute. what is that? and do I want to touch it? I did pick it up out of the pan and found it was actually a hard-shelled egg,” Bussell said.

So what causes a chicken to lay an egg inside an egg? We asked egg inspector and egg expert Henry Marckres, who's even gone to egg school. But he's never seen the likes of this. Marckres says when a chicken produces an egg it drops a yoke into what is called the oviduct in the chicken. it then takes 24 hours to lay that egg.

“What happens in cases like this, the egg is almost ready to be laid but some trauma happens to the bird. it takes a fall or gets bumped or something. the body has contractions and brings the egg back up through the system, so the body thinks there is another yoke in there and produces another egg around it,” Marckres explained.

“Everyone who first saw it… uh, ah, this isn't happening and yeah, it is,” Bussell said.

A rare day in the backyard thanks to one special bird.

“It was one of those neat things that come along. Probably we won't see it again and we just wanted to share,” Bussell said. “It's fun.”

The family is curious to know what is in the smaller egg. They wanted me to crack the egg but I was too nervous, so that mystery stands.