Differential Range Expansion and Population Growth of Bulbuls in Hawaii

Population growth and distributional patterns of Red-whiskered (Pycnonotus jocosus) and Red-vented (P. cafer bengalensis) bulbuls were examined from their introductions to Oahu in the mid-1960s to the present. Population growth rates for the two species did not differ significantly (P > 0.05). The red-whiskered was observed only in the central Honolulu area, primarily in wet, exotic, residential habitats above 150 m elevation; whereas, the red-vented was observed in most habitats below 200 m island-wide. These observed differences in habitat preference were quantified and found to differ significantly (P < 0.05) using 12 monthly transect censuses in Manoa and Nuuanu valleys. Differences in the distributions of the two bulbuls can be explained as a result of differences in habitat preference. The wet, exotic, residential habitat preferred by the red-whiskered is of restricted size and occurs in disjunct patches, thus limiting the spread of the Red-whiskered Bulbul across Oahu. Lower elevation, drier habitats form a continuous belt around the perimeter of Oahu and through the center of the island and thus, have apparently served as a corridor for dispersal and range expansion of the Red-vented Bulbul on Oahu.