South Australia's infamous 'Big Bird Bandits' have today walked free from court.

The trio were placed on good behaviour bonds after pleading guilty to stealing a large, $137,000 Big Bird costume from a Seasame Street Circus at Bonython Park in April last year.

Big Bird was taken in what Magistrate Briony Kennewell deemed an "impulsive" act, before being returned days later with an apology note placed carefully inside the costume's beak.

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The note said the the three were sorry for being "such a big 'birden'".

"We had no idea what we were doing, or what our actions would cause," it read.

"We were just having a rough time and were trying to cheer ourselves up.

"We had a great time with Mr Bird, he's a great guy, and no harm came to our friend."

Prosecutors have maintained this was a serious example of theft that was treated as a joke.

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During today's sentencing, Magistrate Briony Kennewell said it was at the lower end of the scale describing it as a "foolish and immature prank".

She ordered the group to each enter a $500 good behaviour bond for two years, with the two men required to undertake community service.

The trio told the court they had no idea how much the costume was worth, and feared convictions could affect their future careers.

"You are all young people with promising futures," Kennewell told them.

"The seriousness has been brought home to you all, you now realise it's not a joke".

Trio Cody Milne, 26, Tasman Binder, 23, and Amelia Hurt, 21, were warned if they broke their good behaviour bonds, they would likely face convictions.