Abundance, Zonation and Foraging Ecology of Birds in Mangroves of Darwin Harbour, Northern Territory.

Abstract

The density, seasonality, habitat utilization and foraging ecology of birds in mangals (mangrove forests) at a site on the upper reaches of Darwin Harbour were investigated by transect censusing and colour-banding over three years. Despite having only 10 species of plants, the site supported 17 confirmed and five probable breeding resident species of birds, and was visited by 30 more. From variable-width transect censuses, the mean density of birds on a 4-ha plot was estimated to be 25 ha-1, fairly consistent with densities obtained from territory mapping of colour-banded birds. Nearly 70% of the individuals belonged to just 4 species: two mangal-dependent species, the red-headed honeyeater (Myzomela erythrocephala) and the yellow white-eye (Zosterops lutea) and two more generalised species, the large-billed gerygone (Gerygone magnirostris) and the brown honeyeater (Lichmera indistincta). Only the red-headed honeyeater showed significant seasonal variation in abundance, with highest numbers during the late dry season after breeding. This coincided with the period of greatest food (nectar) availability in the mangal, due to flowering Bruguiera exaristata. Lowest numbers of the red-headed honeyeater (and the brown honeyeater) occurred in the late wet season when nectar was scarce in the mangal but abundant in Melaleuca cajuputi fringing the mangal. Several resident species held permanent territories, while others apparently shifted landward during the wet season, possibly due to the wetter conditions created by freshwater runoff and high spring tides Many species showed strong associations with particular mangal zones. Large-billed gerygones, grey whistlers (Pachycephala simplex), shining flycatchers (Myiagra alecto) and mangrove fantails (Rhipidura phasiana) were associated with the Rhizophora zone at the wetter (more frequently inundated) end of the plot; mangrove robins (Eopsaltria pulverulenta) and mangrove gerygones (Gerygone laevigaster), were encountered most in the Ceriops zone; and green-backed gerygones (Gerygone chloronata) strongly favoured the dry landward edge. Four foraging guilds were evident among 13 of the most abundant species, the largest of which was the insectivorous foliage-foraging guild. Species in this group partitioned resources by differential selection of mangrove species, heights and foraging techniques. The tiny (6.4 g) mangrove gerygone was the most specialized species, spending 80% of its time on Avicennia marina. Contrary to the literature, breeding of mangal-dwelling birds peaked during the dry season. The ecology, evolution and biogeography of mangrove-endemic birds is reviewed in the light of this study and recent information from Western Australia.