New Predators Lead to Changes in Nesting Behavior

Invasive species – plants, animals, or others – pose major problems for native species. Some of the worst cases of invasion come from Pacific islands where many birds that evolved without predators have become extinct or endangered due to introduced mammals. A recent study of bellbirds in New Zealand provides a counter example of birds changing their behavior to cope with an introduced predator.

The study focused on the New Zealand bellbird (Anthornis melanura), an endemic species that was exposed to introduced mammalian predators within the last 200 years. It compared nesting behavior at four types of nests:

1. Mainland bellbird nests with introduced predators present
2. Mainland bellbird nests with introduced predators removed
3. Island bellbird nests with no introduced predators
4. Tasmanian honeyeater nests with native predators
   

Bellbirds adjusted their behavior to the presence of exotic predators by reducing the number of parental visits to the nests and increasing the amounts of time spent on the nest and away from the nest. Parents at both mainland sites fed their chicks far less often than parents at the island site, but more often the Tasmanian birds. Fewer visits means that a nest location is less obvious to nest predators, which will have fewer chances to track a parent to the nest. (I wonder, though, if the lower fledge rate of mainland chicks – 29%/39% mainland vs. 65% island – is partly a result of fewer feeding visits.)

The number of parental visits to the nest per hour to feed nestlings for bellbirds on Aorangi Island, where exotic predators were never introduced, for bellbirds in Waiman Bush, where exotic predators were removed, for bellbirds in Kowhai Bush, where all exotic predators are present, and for honeyeaters in Tasmania, which evolved with a range of native mammalian predators. (Click to enlarge)

This study gives some hope that other native island birds can adapt relatively quickly to the presence of new predators. These particular adaptations are specifically suitable for woodland birds, whose nests can be easily hidden. Seabird colonies and flightless birds face greater challenges in protecting their nests from predators. The best policy is still to prevent the spread of harmful invasive species or remove them where possible.