Whitman loved the bel canto style of opera.  Bel canto consists of long passages of simple melody alternating with outbursts of elaborate vocal scrollwork, which turns the voice into a complex wind instrument.  The desired effect was to heighten the dramatic meaning and significance of the words through attention to pitch, dynamics, melody, and rhythm.  This highly emotional and intense use of the human voice was in Whitman’s view the highest form of art.
His favorite singer: Marietta Alboni (in NY 1852-1853). Her work influenced the aria of the mockingbird in “Out of the Cradle endlessly Rocking” and the carol of the hermit thrush in “When Lilacs Last in the Dooryard Bloom’d.” These 2 poems employ a recitative-aria structured modeled on Italian operatic style. (WW Enchclopedia 485).