FOOD SHORTAGE IN SMALL FRAGMENTS: EVIDENCE FROM AN AREA-SENSITIVE PASSERINE

Many forest-interior songbirds are considered “area sensitive” because they are absent from smaller forest remnants in fragmented landscapes. Reductions in food abundance with fragment size could explain area sensitivity, but to date, only one empirical study has investigated this possibility. From 1995 to 1997, we tested the food abundance hypothesis in two small (∼55-ha) and two large (>400-ha) forest fragments located in a matrix of agricultural land in northeastern New South Wales, Australia. We measured differences in food abundance by comparing the biomass of surface-dwelling invertebrates in large and small fragments. We also determined whether food supply was associated with foraging efficiency and reproductive performance in an area-sensitive, ground-foraging insectivore, the Eastern Yellow Robin (Eopsaltria australis), breeding at these same sites. Invertebrate biomass in the small fragments was about half of that in the large fragments. Incubating female robins received 40% less food from males, females left their nests more frequently to forage on their own, and nestlings were provisioned with less food in the small fragments. Females in the small fragments also had a shorter breeding season (by three weeks), laid eggs that were 7% lighter, and reared smaller nestlings. Because our measures of invertebrate biomass, foraging efficiency, and reproduction produced corresponding results, we conclude that Eastern Yellow Robins experienced relative food shortage in these small fragments. Therefore, food shortage probably influences area sensitivity in this species. We suggest that the role of food supply be given greater consideration in other fragmentation studies.