Size matters: the effects of forest fragmentation and resource availability on the endemic Sumba Hornbill Aceros everetti

Abstract

We examined the influence of forest fragmentation and resource availability on the abundance and distribution of Sumba Hornbill Aceros everetti, a large, canopy-dwelling bird endemic to Sumba Island, Indonesia. Hornbill numbers were estimated monthly from August 1998 to September 1999. Estimates were made in three large (≥1,000 ha) and three small (<1,000 ha) forest fragments, using a standard line transect method. Phenological patterns of canopy trees were assessed in 10 × 50 m plots. Our data indicated that forest patch size may be a better predictor of Sumba Hornbill abundance and distribution than overall resource availability. Hornbills occurred at higher densities in large forests (6/km²) than small forests (2.4/km²). Small forests produced more fruit/ha per month but lacked a number of important hornbill food species. Monthly fruit availability in large and small forest fragments had no significant effect on fluctuations in hornbill density. However, hornbill densities were significantly higher in forests with high densities of strangling figs, after controlling for patch size, and in larger forests hornbill densities correlated with the abundance of ripe figs. We hypothesize that small patches still have conservation value if they are within hornbill ranging distance, and speculate that Sumba's forests are in a dynamic phase before the full impact of fragmentation has been expressed.