How does woody succession affect population densities of passerine birds in NewﾠZealand drylands?

The density of shrubs is increasing in many dry grassland ecosystems worldwide. In the dry interior of the South Island, New Zealand, secondary succession generates novel woody communities, as shrubs colonise anthropogenic grasslands where fire frequency has decreased. The avifauna of this dry region is also novel, with many indigenous birds extinct, extirpated or uncommon, and exotic species predominant. We studied how succession from grassland to shrubland habitats affects the abundance of dryland birds, by estimating population densities of nine common exotic and indigenous passerine bird species in dryland habitats in grassland, mixed grassland–shrubland and shrubland, at three sites in Central Otago, South Island. We used distance sampling along line transects to estimate densities. Generally, succession from grassland to forest leads to decreased numbers of granivorous birds and feeding generalists, and increased numbers of insectivores. We therefore predicted that density of birds in five guilds would change as follows during succession to woodier habitats: (1) granivores: decrease, (2) a primarily ground-nesting, ground-feeding granivore–insectivore: decrease; (3) a tree- and shrub-nesting granivore–insectivore: increase; (4) ground-feeding insectivores: increase; (5) indigenous insectivores: increase. Our predictions were supported for only one guild. Yellowhammer (Emberiza citronella) (exotic primarily ground-nesting, ground-feeding granivore–insectivore that prefers open country) declined in density as woody-species frequency increased. Exotic European goldfinch (Carduelis carduelis) and common redpoll (C. flammea) (granivores) were most abundant at intermediate woodiness. Densities of other guilds did not vary consistently with woodiness. We conclude that continued succession to shrubland in Central Otago will have little effect on densities of many common exotic passerine birds, but may lead to local declines in yellowhammer, and ultimately goldfinch and redpoll. Our density estimates for silvereye (Zosterops lateralis) and grey warbler (Gerygone igata) were lower than densities reported in forests; therefore later stages of succession may benefit these widespread indigenous insectivores.