Diversity of native and exotic birds across an urban gradient in a New Zealand city

Few urban studies of avian diversity discriminate between native and exotic species, although highly modified urban environments typically support significant numbers of non-native species. High overall diversity may mask poor native diversity. We identify which native species are successful in a variety of urban habitats in a New Zealand city, and examine the roles that garden size and complexity, native fragments, and vegetation composition and cover play in explaining variations in native and exotic avian diversity. Nearly half (44%) of 39 species recorded were exotic: four (three exotic and one native) comprised 45% of birds counted. While there was little variation between habitats in combined native and exotic species richness, the number and abundance of bush-dependent natives differed. Similarity measures identified three groups: (1) commercial, industrial and residential areas with small structurally simple gardens where 3–4 exotic species were numerically dominant and few bush natives made up <10% of the total count; (2) suburbs with medium/large gardens dominated by lawns, shrubs and trees, with many exotic species but also several bush natives (comprising 24–32% of the total count); and (3) bush fragments where both diversity and species richness of bush natives were highest (comprising 50–75% of the total count), including four insectivores absent from all residential areas. Vegetation structure, composition and cover, and proximity to fragments did not explain much of the variation in total or native species richness and diversity, but did explain 69% and 72% of bush-native species richness and abundance respectively.