Life-history and behavioural responses to nest predation in Australian and New Zealand birds: can naïve birds adapt to exotic predators?

Over the last 500 years, the introduction of exotic species into new areas has been one of the primary drivers of native species’ declines and extinctions. The impacts of introduced species, particularly predators, are especially profound on islands such as New Zealand, where the native fauna evolved in the absence of terrestrial mammals. Whereas the life histories of most continental species reflect their shared evolutionary history with diverse predators, island species typically lack traits which allow them to recognise and avoid novel predators. However, there is increasing evidence that evolutionarily naïve species have acquired anti-predator responses over historical time periods. In this study, I investigated whether New Zealand songbirds have adjusted parental activity and nest site selection in the 800 years since the arrival of the first terrestrial mammals, and whether behavioural responses to nest predation are lost under relaxed predation pressure.
As parental activity at the nest can attract predators, it has been hypothesised that birds should reduce activity levels under high nest predation risk, but only if the predators are diurnal (i.e. active at the same time as the parent birds) and hunt by sight. To ascertain whether parental activity is likely to influence nest predation risk in Tasmanian birds, I used remote infra-red cameras to identify nest predators in Tasmanian woodlands. Having established that the main predators of Tasmanian songbird nests are diurnal birds, I compared parental activity between Petroica robins in Tasmania (where they evolved with diverse nest predators), the South Island of New Zealand (where the birds evolved with no terrestrial mammals, but are now threatened by introduced mammalian nest predators), and the Chatham Islands (where the robins have never been exposed to mammalian nest predators). The Tasmanian species showed reduced activity at the nest compared to the Chatham Island species. Despite evolving with no mammalian predators, South Island robins from a mainland population exposed to high levels of nest predation by introduced mammals, showed activity levels more similar to the Tasmanian species than to the Chatham Island species. However, South Island robins secondarily isolated from mammalian predators on offshore islands showed activity levels more similar to the Chatham Island species than their conspecifics at the mainland site.
The height of South Island robin and New Zealand bellbird nests was compared between mainland sites with uncontrolled invasive predator communities, mainland sites with controlled invasive predators, and islands with no mammalian predators. In both species, nests were built higher in areas with terrestrial mammals than areas without terrestrial mammals, suggesting low nests are avoided in the presence of mammalian predators. However, nest height did not significantly predict nest survival in any of these populations, or in Tasmanian species exposed to abundant and diverse nest predators.
Previous studies have shown that predation by invasive European starlings are the main cause of nest failure in endangered Chatham Island black robins, but that some nests are more likely to be targeted than others. I show that 250 years after the arrival of starlings in the Chatham Islands, black robins can respond to personal experience of nest predation by this novel predator, by recognising and selecting safer nest sites for re-nesting attempts.
Overall, this study demonstrates that evolutionarily naïve species are capable of developing and expressing predictable behavioural responses to novel predators, but that these behaviours can reverse under relaxed predation pressure. I discuss the potential management implications of these results, and the place of studies such as this in the broader context of life history evolution.