Island Birds Can Adapt to Predators, Study Suggests

Anne Minard, National Geographic News 9 Jun 08;

An island bird in New Zealand is able to change its nesting behavior to outwit predators—offering a ray of hope for island species worldwide facing threats from exotic invaders—researchers have discovered.

Melanie Massaro, a biologist at the University of Canterbury, and her co-authors found that New Zealand bellbird mothers spend longer periods of time on their nests when the risk of predation rises.

Both male and female bellbirds also make fewer trips to and from the nests, which reduces activity that might draw predators' attention.

Although the research focused on a single New Zealand bird, the conclusions suggest that island birds worldwide may be able to adapt to predation, the authors say.

The new study flies in the face of a widely accepted theory that suggests that island birds are especially vulnerable to predators because they've missed the opportunity to evolve alongside them, unlike their mainland counterparts.

"The main findings of our study are that naïve endemic island birds are not necessarily trapped by their evolutionary history as is generally considered to be the case, but they have the ability to change their behaviors in ways that appear adaptive," Massaro said.

"More importantly, our study demonstrates that such a change can occur over an ecologically relevant time scale of years and not centuries."

The work appears in this week's online journal PLoS ONE.

Island of Opportunity

The introduction of predatory mammals such as rats, cats, and weasels to oceanic islands has caused the extinction of many native birds. On New Zealand, casualties have included huias, piopios, and bush wrens.

The study authors point out that exotic predators threaten the survival of a quarter of the world's remaining endangered bird species.

But studying the effects of exotic invasion is difficult, because "very few island bird populations still exist in habitats that have not been affected by human-mediated changes," Massaro said.

"This study took advantage of the unique situation in New Zealand where exotic predators were introduced, but a few offshore islands remained undisturbed."

Her research team looked at three sites: a site where exotic predators have become well established, a site where exotic predators were experimentally removed, and an offshore island lacking exotic predators.

They found that, as the risk of predation increased, female bellbirds spent more time on their nests each round of incubating, a strategy that minimized activity at the nest and decreased the risk of predators locating and destroying the eggs.

"Parental activity during the nestling period, measured as number of feeding visits per hour, also decreased with increasing nest predation risk across sites, which would further reduce the risk of an exotic predator destroying the nest," Massaro said.

Massaro's team also compared the parental behavior of New Zealand bellbirds with that of two related honeyeaters in Tasmania, an environment where native birds co-evolved with a variety of predatory mammals.

"We were able to confirm that parental care patterns in previously naïve populations of bellbirds that have since been exposed to exotic mammalian predators are converging with those of birds that evolved in continental areas with a high risk of predation," she said.

More Work Needed

Tim Blackburn, who heads up the Institute of Zoology in Regent's Park, London, noted that in general "the longer that a species has been isolated on an island, the more likely it is to go extinct when predators are introduced. Isolation causes bird species to lose their responses to predators."

Blackburn said it's possible that bellbirds may be relatively recent arrivals to New Zealand, and thus more likely to redevelop antipredator behaviors than other species that have gone extinct.

He's also unsure about whether the bellbirds' strategy is working.

"The bellbirds in the predator treatment have changed their behavior but still have higher nest predation rates than bellbirds on predator-free islands," he said.

"It is not clear from the paper whether the behavioral changes have made sufficient difference to nesting success to allow the bellbird population to persist in the face of predators, or whether the population with predators persists because of immigration of individuals from other areas."

And he expressed concerns about drawing grand conclusions from a study of limited scope.

"The main shortcoming is that they only have single field sites for each of their experimental treatments," he said. "It would be very interesting to see whether the same results hold for other bellbird populations with and without predators."

More Work Needed

Massaro said that her team largely agrees with Blackburn "that likely many island bird species are doomed because of their evolutionary history," Massaro said.

"For example, we are quite aware that [the] kakapo, the famous flightless parrot of New Zealand, will unlikely ever [to] be able to coexist with mammalian predators.

"In general, what evolved over millions of years can't change within a few decades or hundreds of years."

But Massaro and her co-authors believe other conservation efforts could benefit from the knowledge that at least one island bird can respond to exotic predators—especially when the elimination of such predators is not possible.

More work, particularly to see if other birds are able to adapt as well as bellbirds, is needed, she said.

"While we have shown in this study that bellbirds have adjusted behaviorally to the presence of exotic predators to some degree, it is presently unknown whether other endemic island birds have responded in a similar fashion," she said.

Her team is already looking at responses to new predation by other island bird species.