Conservation ecology of the White-fronted Chat - a declining bird in an endangered ecological community

A project undertaken at the Australian Museum and supervised by Richard Major and Rebecca Johnson

Introduction

Habitat fragmentation and modification associated with urban development are key threats to native fauna and their intensity compounds with global climate change. Coastal saltmarsh is an endangered ecological community that is particularly vulnerable to such pressures with rising sea-levels inundating the community from below, urbanisation encroaching on it from above, and intensification of coastal development isolating remaining patches.


The White-fronted Chat is an iconic, saltmarsh-dwelling bird that is in decline Australia-wide, yet is still sufficiently common to serve as a model species for understanding processes of population decline and recovery. It is increasingly being reduced to what appear to be isolated declining populations, each heading toward extinction. The aim of this project was to measure demographic parameters and gene flow amongst key populations which will assist both in the recovery of this species and our general understanding of the extinction pathway.


Background


White-fronted Chats occupy moist habitats including inland swampy areas and coastal saltmarsh. There has been a significant decline in populations in inland swamps associated with changed hydrological conditions, and in coastal wetlands due to urban development. The prognosis for populations in both habitats is poor under projections for climate change, with reduced rainfall extinguishing many swamps, and sea-level rise extinguishing saltmarshes that are bordered by urban development.


The species has undergone a 36 % decline nationally between 1980 and 2000 and, as a result, the species is listed as Vulnerable in NSW. Within the greater Sydney region more than 80 % of saltmarsh vegetation has been destroyed by coastal development, leaving two isolated White-fronted Chat populations from 55 former localities. These populations are now listed as Endangered.


Objectives


Using the White-fronted Chat as a model species, the main objective of this project was to investigate the process of regional extinction as the endpoint in a series of local extinctions of small fragmented populations. The specific objectives were:

  • to accurately determine the population size of key populations using mark-recapture.
  • to evaluate evidence of inbreeding within isolated populations using genetic markers.
  • to assess the level of genetic diversity in each of ten populations distributed over a range of bioclimatic regions and spatial scales.
  • to determine the level of gene flow (if any) between apparently isolated populations.
  • to measure breeding success and population recruitment in two isolated populations.
  • to test the effectiveness of predator-proof caging (on a secure population) as a last-ditch tool for reducing local extinction rates.

Findings


By catching, colour-banding and conducting regular surveys we monitored the endangered populations at Homebush Bay and Towra Point for three years. During this period the Homebush Bay population declined from 9 down to 2 individuals, both of which are male. This population is now functionally extinct. The population at Towra Point ranged between 18 and 24 birds, with a more even sex ratio, but with so few birds remaining, the Towra Point population is at serious risk of following the same trajectory as the population at Homebush Bay.

We captured and extracted feathers from 310 birds across Australia to obtain DNA samples. Eighteen microsatellite markers were developed to compare the genetic structure of the potentially isolated populations with less-fragmented control populations. Overall, the genetic structure of 17 populations of White-fronted Chats was sampled across Australia, including three island populations (Rottnest Island, King Island and Tasmania). Genetic diversity expressed in terms of both allelic diversity and heterozygosity was lower on the islands than on the Australian mainland, except for the two endangered populations in Sydney (Towra Point and Homebush Bay) which showed the lowest levels of genetic diversity of all populations. Evidence of inbreeding was apparent in these two populations.

Extensive population structuring was evident amongst the seventeen populations studied. Populations from Rottnest Island, King Island, and Tasmania were strongly differentiated both from each other and from mainland populations. This genetic evidence contradicts previous assumptions of dispersal across these oceanic barriers. There was no discernible difference in genetic structure amongst mainland populations ranging from inland New South Wales across Victoria to Western Australia. However, the two endangered populations, isolated by urban development, were strongly differentiated from the nearest populations beyond the urbanised zone. Two individuals in the Towra Point population were genetically distinct from the remainder of the population indicating that they were recent migrants. These birds have the potential to effect genetic rescue of the population, but to date they have not reproduced successfully.

Breeding was monitored for three years at Towra Point, with intensive surveys conducted throughout one full breeding season. This involved following the nesting attempts of 10 pairs of birds many of which nested twice during the season. Despite a high abundance of potential nest predators (up to 80 ravens foraging in a 40 ha site), reproductive success was relatively high, with more than 50% of nests successfully producing young in 2010. Monitoring was less intensive in 2011 and there was a high rate of nest failure (only one of 6 nests successful), with flooding by spring tides the major cause of mortality. Despite a reasonable level of nest productivity, only two birds (one from each of the 2010 and 2011 cohorts) were recruited into the adult population.


We installed cages around 80 dummy nests and 80 control nests to determine the effectiveness of nest cages in excluding nest predators. Almost all uncaged nests had their eggs depredated, mostly by Australian Ravens, but there was negligible predation on caged nests. These trials indicate that nest caging may have potential to boost breeding productivity in extreme situations. Cages were successfully installed around four active White-fronted Chat nests without inducing parental desertion, demonstrating that nest-caging is feasible provided expertise is available to locate active nests.

Conclusion

This study demonstrated that the urban landscape can represent a significant barrier to movement of even highly mobile species such as birds. With habitat divided into small fragments and with a low probability of dispersal between fragments, small populations are extremely prone to local extinction. Habitat modelling revealed that coastal populations of White-fronted Chats are strongly associated with saltmarsh, but most saltmarsh patches are now too small to have a large impact on the probability of occurrence. As climate change is expected to further reduce the amount of available saltmarsh through continued mangrove incursion, coastal populations of White-fronted Chats are expected to come under increasing threat. The history of this species is a clear demonstration that even conservation reserves with a relatively high degree of protection from human disturbance cannot protect regional biodiversity in the absence of active management if they are isolated.

We are extremely grateful to the Hermon Slade Foundation for making this research possible.

Publications resulting from the project


Jenner, B., French, K., Oxenham, K. and Major, R.E. (2011). Population decline of the White-fronted Chat (Epthianura albifrons) in New South Wales, Australia. Emu 111: 84-91

Major RE, Sladek JLT (2012) Is an island reserve enough? The decline and fall of the White-fronted Chat (Aves: Meliphagidae) in southern Sydney. Proc. Linn. Soc. NSW. 134: B205-214

Ashcroft, M.B., and Major, R.E. (In press) The importance of matrix permeability and quantity of core habitat for persistence of a threatened saltmarsh bird. Austral Ecology.

King, A.G., Cooke, G.M., Johnson, R.N., and Major, R.E. (In press) Development and multiplex genotyping of eighteen novel microsatellite markers for a threatened saltmarsh bird Epthianura albifrons (Meliphagidae). Conservation Genetics Resources.

Major RE (In Press) Latitudinal and insular variation in morphology of a small Australian passerine - consequences for dispersal distance and conservation. Australian Journal of Zoology.

Cooke, G.M., King, A.G., Boles, W., Johnson, R.N., and Major, R.E. (In Press) Rapid characterization of mitochondrial genome rearrangements in four Australian robins and one Australian honeyeater using next-generation sequencing technology. Journal of Heredity.

Major, R.E., Johnson, R.N., King, A.G., Cooke, G.M., and Sladek, J.L.T. (Draft Manuscript) Urbanisation results in genetic isolation of an endangered bird population inhabiting saltmarsh remnants.

Lindsay, K.J., Allen, A.P., and Major, R.E. (Draft Manuscript) Can spatial and temporal food variability explain the winter foraging movements of a saltmarsh insectivore?

Major, R.E., Ashcroft, M.B., and Davis, A. (In prep) Potential benefits of nest caging for a threatened shrub-nesting bird. Wildlife Research.

Major, R.E., Cooke, G.M., Johnson, C.N., and King, A.G. (In prep) Population differentiation of a small passerine bird at a continental scale: the importance of land, ocean and urban barriers.

 

 

 

Figure 1.White-fronted Chat (male)

Figure 2. White-fronted Chat (female)

Figure 3. Nest of White-fronted Chat

Figure 4. Saltmarsh habitat of White-fronted Chats at Homebush Bay

Figure 5. Mistnets set out across a saltmarsh to capture White-fronted Chats

Figure 6. Urban development encroaching on White-fronted Chat habitat at Lake Illawarra

Figure 7. Preparing feathers for DNA extraction.

Figure 8. Jaynia Sladek with one of the Towra Point chats.

Figure 9. A pair of chats feeding in saltmarsh on Rottnest Island.

Figure 10. Andrew King extracting DNA from feathers.

Figure 11. A full brood of chats on the day of fledging.

Figure 12. Australian raven attempting (unsuccessfully) to prey upon a caged nest.