Abstract: Many bird species are known to differ in their vocal repertoires between populations across a range of geographic scales. This is generally assumed to be caused by acoustic drift and social learning of small differences in songs among individuals in separate populations. To determine the extent to which vocal repertoire is structured in a highly nomadic species with a low degree of isolation among populations, we characterized the vocalizations of the 'Apapane (Himatione sanguinea) and described the variations in its songs on a microgeographic scale. 'Apapane had significant shifts in their songs in both fragmented and non-fragmented forest habitats, with little to no overlap in song meme structure within distances as short as 2 km, despite birds moving freely between areas with distinct songs. Forest fragments had unique song compositions and shared more syllables with closer fragments than with the ones further apart. Furthermore, microgeographic variation was relatively stable at a given recording location even over multiple years. This pattern of song differentiation in a highly mobile species at the microgeographic scale may be a consequence of their ability to learn new vocalizations over their life and of intraspecific mimicry, or "vocal matching" by individuals visiting other populations.