Why kiwi and kōkako are more vulnerable than fantails

Life history traits explain the vulnerability of endemic forest birds and predict recovery after predator suppression. New modeling, published in the New Zealand Journal of Ecology, has disentangled the limiting effects of predation, forest area, and food availability to predict the outcomes we can expect for different native bird species in a predator-free New Zealand.

New Zealand's "deep endemic" forest bird species, which have evolved here longest (such as kiwi, kokako, rifleman), are more vulnerable to decline than bird species whose ancestors arrived more recently (such as grey warbler, fantail, silvereye). To discover why, Susan Walker, Adrian Monks and John Innes from Manaaki Whenua—Landcare have researched data on species' distributions from the Ornithological Society of New Zealand's two bird atlases (1970s and early 2000s) relative to geographical gradients and species' traits.

They found that three life-history traits (nesting in tree cavities or burrows, large body size and limited ability to travel widely across land) account for the greater vulnerability of deep-endemic species, and make them more vulnerable to both deforestation and predation. In addition, says Walker, "the results give us a new way to predict the future of different forest bird species in different predator-free landscapes."

Walker says, "Our results support understanding that predation is the primary cause of forest bird declines and limitation today, but they also suggest large tracts of native forest will be essential to restore some species, even without mammal predators."

The research leads to three predictions about a future with fewer predators.

First, forest scarcity will limit the population recovery of some, but not all, forest birds. It may be difficult to restore larger, less-mobile endemic species such as kokako, and breeding populations of mobile cavity-nesting species such as kaka, to deforested landscapes.

Second, restoration is likely to be more successful in large tracts of forest, but all large forest tracts are not equal. Cooler forests (the main refuges of endemic forest bird species today) are not the best environments for larger-bodied species that require more food to maintain sizeable populations and are less capable of traveling to obtain it.

Third, the study predicts changes in the bird community, rather than increases in all species. Walker says, "This is the outcome we are starting to see in practice: That deeper endemic species in sanctuaries are replacing silvereye, grey warbler and fantail, which are from more recently-arrived lineages and became common in warm and deforested landscapes since human settlement."