How did Louis arrange Serena's freedom in The Trumpet of the Swan?

Serena the swan, the love of Louis' life, crash lands in a zoo. After this unfortunate incident, she's kept in captivity with numerous other animals. As the zoo authorities are concerned that the swans will fly away at the first opportunity, they clip the swans' wings to prevent them from escaping. Serena is now scheduled to have the same treatment. Louis is treated differently, however, because unlike all the other swans, he can read and write. He can also play the trumpet, which brings in the crowds.
Nevertheless, Louis' love for Serena leads to his arranging her freedom. He does this by agreeing to donate a couple of his own children to the zoo every few years in exchange for Serena's being released from captivity. He does this even though he knows that his children will have their wings clipped and will therefore never be able to fly to Montana.