Adaptive Anti-predator Behaviour of South Island Robins (Petroica australis australis) in Mammalian Predator-free and Predator-dense Regions

Abstract

Anti-predator behaviours in birds often exhibit adaptive plasticity with the presence and abundance of predators in their local habitats. I described and compared adaptive anti-predator behaviours of South Island (SI) Robins (Petroica australis australis) residing within a mammalian predator-free region (Orokonui Ecosanctuary) with those inhabiting predator-dense regions (Silver Peaks and Silverstream). I investigated:

a) Differences in the intensity of anti-predator behavioural displays towards introduced mammalian predators between the regions. I discovered that SI robins in Orokonui had lost their predator recognition skills towards rat models and showed lower intensities of anti-predator displays than the SI robins at Silver Peaks and Silverstream. Orokonui’s robins may have retained behaviours towards stoat models due to a recent stoat incursion in the ecosanctuary. These results suggest that SI robins lose their anti-predator responses with the absence of specific predators in their habitat, and regain those behaviours upon encountering those predators again;

b) Seasonal variation in the intensity of anti-predator behavioural displays between the regions towards introduced mammalian predators. I discovered that SI robins showed less intense behaviours towards rat models during the non-breeding season than in the breeding season, though the decrease was significant with varying predation pressure. The absence of breeding mates, chicks, and altricial fledglings in the non-breeding season possibly increased the costs of displaying anti-predator behaviour to avoid predation of self, and lowered the intensity of anti-predator display. No significant seasonal variation in responses towards the stoat model was observed, perhaps implying that the predation threat of stoats may be consistently high irrespective of the season;

c) Vocal responses towards native avian and introduced mammalian predators. SI robins in this study gave alarm calls to avian predators but not to mammalian predators, and the alarm calls showed some acoustic differences that distinguished calls in response to moreporks from calls in response to New Zealand falcons at the species level. However, the functional reference of these alarm calls could not be conclusively determined;

d) Differences in nest-site selection between the regions. Comparisons of protective habitat features between predator-free and predator-dense areas did not reveal statistically significant variations. Protective habitat features within regions reflected some adaptive selection of nest sites in response to both mammalian and avian predation, with SI robins preferring higher nests, greater nest concealment, and more ground cover in some predator-dense regions. A broader understanding of the complex interactions between habitat structure, predation pressure, and behavioural plasticity is required to confirm adaptive nest-site selection in SI robins.

The study provides evidence of behavioural loss in the absence of mammalian predators, and some verification of adaptive anti-predator behaviours in multipredator environments based on the predator’s hunting technique (mammalian versus avian tactics). Its ease of detection and susceptibility to predation makes the SI robin an ideal bird to assess the effectiveness of predator-control operations, and knowledge of its adaptive anti-predator behaviour could further help indicate the predation pressure in predator-managed areas.