New Zealand robin populations have been found to lose their fear of rats after only one generation in a predator-free environment, raising questions about the reintroduction of birds from island sanctuaries to the mainland.
Researchers Ian Jamieson and Karin Ludwig from Otago University compared robins on Stewart Island, where Norway rats are common, with birds on Ulva Island, where rats were eradicated in 1996.
The birds on Ulva are descendants of birds from the Stewart Island population, and Ulva does have native predators – the flightless weka and a native owl.
A report on the research, published in Animal Behaviour, said the Stewart Island robins were more agitated than the Ulva birds in the presence of a rat model, and were more hesitant about approaching mealworm food, which they tended to collect one at a time.
Many of the robins on Ulva appeared to show little fear or recognition of the model rat and were much less agitated and more likely to consume all five mealworms in a shorter time, despite the nearby model rat.
Vigilance and antipredator behaviour were costly for the birds, as they needed to be traded off against other activities, such as feeding or resting, the report said.
New Zealand robins appeared to possess proper antipredator behaviours, such as mobbing and alarm calling, but needed to learn to recognise a mammalian predator as a specific threat.
If experience were important to develop predator recognition, then isolation for just one generation would have a significant effect on the performance of a population. That seemed to be the case for Stewart Island robins reintroduced to rat-free Ulva, the report said.
“These results raise the question of whether established populations on island sanctuaries are appropriate sources for harvesting for reintroductions back to the mainland.”
While predator-naïve birds might be more likely to forage in the presence of a potential threat, as long as some individuals survived an initial attack that incident might be enough to restore predator recognition capabilities.