Ecological Release and Behavioral and Ecological Flexibility in Marsh Harriers on Islands

Abstract

Marsh Harriers Circus aeruginosus in New Zealand have undergone both niche expansion and niche shifts. Niche expansion has occurred in the range of habitats hunted over and nested in, because Marsh Harriers in New Zealand ex- ploit all of the range of habitats used by Australian Marsh Harriers as well as hunting over short grass and frequently nesting in tall vegetation well away from wetlands. Harriers in New Zealand have shifted from exploiting the traditional Marsh Harrier food of waterfowl to consuming more of the abundant Sheep Ovis aries carrion and road-killed animals. These niche changes and increases in the density of the island population have been facilitated by ecological release from diffuse interspecific competition and differences in resource availability. The interspecific ecological changes observed in New Zealand are vrobablv the result of behavioural and ecological flexibility within the species rather than recent genetic change.