Climate change displacing birds
The breeding range of the Bridled Tern has gradually moved south and east about 1400km over the last two decades.
Image: blundershot, Flickr CC-licensed

Climate change has already contributed to the southward shift in the breeding distribution of some tropical bird species in WA.

A research compiled by the Conservation Council WA (CCWA) and Halfmoon Biosciences, demonstrates that the poleward movement of two dark tern species is in line with general predictions of global warming.

Research involved observing changes in geographical distribution and timing of breeding the Bridled Tern (Onychoprion anaethetus) and the Brown Noddy (Anous stolidus).

The study, since1984 for the Bridled Tern and 1994 for the Brown Noddy, also demonstrates the observed shifts in distribution may affect the long-term population size.

According to the research, the differences in foraging ecology between the two dark tern species explain the contrasting responses to oceanographic changes in shelf and oceanic waters off south-western Australia.

“Tropical species move closer to the poles and up mountains, but cold water species get squeezed into what is left,” CCWA Environmental Science and Policy coordinator Dr Nic Dunlop says.

Bridled Terns were first observed breeding at the Houtman Abrolhos in 1843.

The species gradually moved south and east about 1400km over the last two decades, with the breeding range expanding rapidly along the south coast of WA.

Brown Noddies, however, had not been recorded south of the Houtman Abrolhos before 1991, when only five pairs were discovered that year nesting on Lancelin Island.

Dr Dunlop says the limited range is due to many contributing factors, namely the inability for the Brown Noddy to adapt to changing ocean temperatures and different habitat environments.

Unlike the Bridled Tern, the Brown Noddy prefers only few prey species and shows little capacity for prey switching within or between breeding seasons.

This has caused the bird to suffer low breeding performances.

“There are very few options with suitable foraging areas and suitable conditions for the Brown Noddy,” Dr Dunlop said.

“The Brown Noddies won’t become extinct, but the population size will continue to decrease.”

Bridled Terns however, responded positively to the changes in ocean climate, with the breeding range re-distribution resulting in a population increase.

The Bridled Tern’s adaptable foraging ecology is thought to be a contributing factor to its ability to thrive.

Dr Dunlop says there was not much that could be done to enable the Brown Noddy to adapt to particular climates, except “manage these islands preferentially, such as modifying the habitats to suit the species”.

“The key interest in this study demonstrates that climate change will produce winners and losers and the Bridled Tern is the climate change winner.”

Editor's Note: Original news release can be found here.