The 1956 Summer Olympics, held in Melbourne, Australia, piqued curiosity in all things from the area, including kangaroos and other marsupials.
The story of The Pied Piper of Hamelin dates back to the 13th century. It has been written about and made into films, and cartoons, as seen here.
The background art and secondary characters here were rendered in a more modern, abstract way as the streamlined design of the mid century became the norm. There's still plenty of color and detailed objects, however. The broom and garbage pail on either side of Junior are two examples.
Ben Washam, who normally animated in Chuck Jones's unit, did uncredited work in this short. He animates the sequence from 1:35 to 1:59 when Sylvester and Sylvester Jr. are talking about catching mice.