Clay-colored Robin (Turdus grayi). In this sample they use this "mono-duet" as a mobbing call as they harrass a pair of Spectacled Owls that are too close in the river bottom at El Gavilan, near Puerto Viejo de Sarapiqui 6/20/99.
An American Robin (Turdus migratorius) sings an overture of Spring vocalizations in this mid-March, 1998 recording. Just about every voice and call a Robin can make, he makes in this sample. The session lasted over 20 minutes, so I've compressed this to demonstrate all the different sounds he uses.