Habitat complexity and management intensity positively influence fledging success in the endangered hihi (Notiomystis cincta)

Age and structure of local vegetation (habitat complexity) are commonly assumed to be indicators of habitat quality for breeding birds, but for many species these relationships are poorly understood. The hihi (stitchbird Notiomystis cincta),an endangered New Zealand cavity-nesting passerine that only survives on mammalian predator-free islands or within fenced areas, has been the focus of intensive conservation management and research. Between 1992 and 2004 we examined the fledging success of 347 nests from four island populations. Habitat quality was improved at the two scrub/regenerating sites and one of the two mature/climax sites through management using supplementary feeding, nest-box parasite control or both. At two sites (one mature, one regenerating) management was stopped during the study allowing us to measure fledgling success with and without habitat quality improvement through management. At the population level, the number of chicks fledged per nest increased as management intensity increased and habitat quality increased. The positive effect of management was greatest for populations in lower quality habitats. To assess the relationship between fledging success and local habitat variables around the nesting site we used a height-frequency vegetation survey method sensitive to changes in vertical structural complexity at the two mature/climax sites. For 36 natural nests, a cross-validated regression-tree analysis (R² = 0.69) predicted that as habitat complexity increased, so did fledging success, which was generally higher for nests in trees with larger diameters (present in older forests). Because these habitats are free from nest predators, our results suggest that habitat age and complexity are proxies for habitat quality through effects on nestling food availability and/or nest-chamber characteristics. Our results support the current management approach of providing supplementary food to translocated hihi populations and suggest that supplementary food can be used to overcome resource deficiencies for this species in poorer quality habitats.