Differences in song repertoires and characteristics of island and mainland populations of the same avian species are usually explained by dispersal, cultural evolution and/or habitat differences. The influence of morphology is often overlooked, even though island populations are frequently morphologically distinct from mainland populations, and morphology could affect vocalizations. I compared morphological features, songs, contact calls and alarm calls of six isolated island populations of silvereye Zosterops lateralis with those of two mainland populations to examine whether differences between mainland and island vocalizations were consistent across vocalization types, and whether these differences could be linked to morphological differences. Vocalizations were lower in frequency on islands. Island individuals were larger (both in mass and body structure), and body mass was an important predictor of frequency in contact and alarm calls. I argue that this strong association results from the island rule (islands promote larger body sizes) and cascading effects of morphology on vocalization frequency in this species.