The Swomee Swans are first seen when the Once-ler comes to the Truffula Forest. After briefly singing, they tried to attack the Once-ler, but they calmed down when they tasted his marshmallows. They then watched in horror as he cut down the first Truffula tree and mourn the loss of the tree with the Lorax. Later that night after they helped the Lorax put the Once-Ler's bed in the water they (along with the Lorax and other forest creatures) save the Once-ler from going over the waterfalls.
Later they watch in horror and sadness as the Once-ler (under the influence of his greedy and lazy relatives) cuts down all the trees. After the last Truffula Tree is chopped down the Lorax send them and the other forest creatures off to find another place to live. At the end of the movie when the trees start to regrow, a Swomee Swan is seen flying overhead of the Once-ler, showing that they and the other forest creatures are returning.