Predator outwitted – nest cages take eggs off the menu

As a last-ditch effort to prevent the extinction of Sydney’s endangered White-fronted Chats, we installed cages at nests in an attempt to exclude predators. We were pleased to discover that parents would not desert their nests when subjected to this intervention and that the cages drastically reduced predation by superabundant Australian Ravens.

The White-fronted Chat (Epthianura albifrons) is a small, insectivorous bird that is threatened with extinction in the north-eastern part of its range, largely due to loss and degradation of its saltmarsh habitat. It was once common in wetlands across Sydney but only one small population remains and it is listed as endangered. Our research has shown that this population consists of fewer than 20 birds – and falling.

White-fronted Chats build their nests close to the ground where they are vulnerable to a range of predators, and for the endangered population, the main offenders seem to be Australian Ravens (Corvus coronoides). Ravens thrive in the urban environment that surrounds the Chats’ saltmarsh home, and flocks of Ravens make forays into the saltmarsh. Reducing the impact of ravens seemed to us like the best chance of reversing population decline.

We constructed 160 artificial nests and enclosed half of them with cages made of 50-mm wire mesh. Cages lowered predation rate from 96 % to 14%, and automated cameras revealed that almost all predation was by Australian Ravens (watch the video above). We also caged some real Chat nests (in a secure population) and monitored parental response from a hide. The parents readily flew through the mesh to incubate their eggs and feed their young – no nests were deserted.

Finally, we caged three nests from the endangered population and all three successfully raised young, at least one of which survived until the next breeding season. This increased the breeding population from three to four breeding pairs, demonstrating that nest-caging might prove useful in last-ditch recovery of endangered species.

Now for the bad news . . . By the time just three or four breeding pairs remain in a population, it is very time-consuming to find and protect nests. We do not have the resources to continue this work and it seems that Sydney will soon be diminished by the loss of one more species. Fortunately, White-fronted Chats are still secure in southern Australia, and hopefully we will learn from our mistakes and give their saltmarsh strongholds greater protection. But then again, Coastal Saltmarsh is an endangered ecological community that is threatened by rising seas levels associated with global climate change . . .

Dr Richard Major
Principal Research Scientist

More information:
Major R. E., Ashcroft M. B. & Davis A. (2015). Potential benefits of nest caging for a threatened shrub-nesting bird. Wildlife Research 41, 598–605.

Major, R. E., Johnson, R. N., King, A. G., Cooke, G. M. & Sladek, J. L. T. (2014). Genetic isolation of endangered bird populations inhabiting saltmarsh remnants surrounded by intensive urbanisation. Animal Conservation, 17, 419-29.

Ashcroft, M.B. & Major, R.E. (2013). The importance of matrix permeability and quantity of core habitat for persistence of a threatened saltmarsh bird. Austral Ecology 38, 326-337.

Acknowledgements:
We are grateful to Aphra Byrne and Lauren Ashcroft for their assistance in the field, and to Mike Major for video editing.