Long-term viability of the flightless Tasmanian native-hen

Project on Maria Island (PhD project)

hen

Context:

The Tasmanian native-hen (Tribonyx mortierii) is a remarkable flightless bird of the rail family (Rallidae). Rails are ground-dwelling birds and are most often found in wetland environments. There once was over 1,000 flightless rail species present on the Pacific islands (Steadman 2006) – this was 1/5 of all bird species living on Earth at the time! After the arrival of the first human settlers about 10,000 years ago, hunting and introduced species wiped out almost all flightless rails, reduced further by European settlers from the 16th century onwards.

Today, only 20 flightless rail species remain, and all but two are threatened by extinction due to habitat loss, invasive species, over-hunting, and climate change.

The Tasmanian native-hen is one of the two remaining flightless rails that is not threatened. It has survived extreme events in Tasmania, such as culling and population collapses (diseases are suspected), but went extinct on the Australian mainland 3,500 years ago, probably due to the predation by dingoes and other factors (see Interesting facts section).

As an iconic Tasmanian species, but also a rare example of a thriving flightless bird, we need to measure its resistance to changing environments, to forecast its extinction risk in future threatening conditions (e.g., climate change, land-use change and exotic predators like the fox) and ensure the best management. To provide accurate estimates on their survival and population dynamics, we need to use a reliable study population. As it happens, Maria Island is the best location for this.

 

On Maria Island:

Thanks to the existence of a long-term study, undertaken in the 90s (Goldizen et al. 1998), we can estimate the evolution of native-hens’ populations as environmental conditions changed by comparing the measurements taken in the 90s to now. Over 300 native-hens were present in the 90s, just around Darlington. We estimated the population to be around 60 individuals in 2017. Numbers are declining on Maria Island, yet they are thought to be stable in Tasmania. We aim to find out why this population is declining, using this as a case study to explore whether there is cause for concern for other native hen populations in Tasmania.

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Predation: Maria Island is the best place to measure the impact of predation on their survival. Indeed, Tasmanian devils were introduced in 2012 to create an insurance population from the deadly devil facial tumour disease (DFTD; see program here). The devils became abundant on the island and started to have a significant negative impact on bird populations. Along with Tasmanian devils, other native-hen predators are present, such as water-rats, forest ravens, currawongs, and tiger snakes. All of these predators are also found on mainland Tasmania, but since the devils were introduced on the small island, the total impact of predation is more severe. This high rate, not found elsewhere in Tasmania, could be used as a proxy for exploring what might happen to native-hen populations if an exotic predator like the fox was introduced into Tasmania. For this part of the project, we will measure egg, chick, and young survival rates (they are the more vulnerable to predation) throughout the breeding season from September to February.

Competition: herbivores are numerous on Maria Island, and competition for resources like grass can be high, especially in dry years. In particular, wombat populations have considerably increased since the 90s, with an estimated 3,200 individuals in 2017 (density of 335 wombats/km2, Ingram 2018). In 2017, very few groups of native-hens laid eggs on Maria island, suggesting that environmental conditions (potentially a lack of grass) was preventing reproduction (see picture of scarce grass below, taken in November 2017, Darlington)

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To test the effect of habitat quality and competition, we installed 12 m2 exclusion plots in July 2018 (see picture below). These exclusion plots will prevent grazing by wombats and large macropods, such as kangaroos, but will allow entry for the native-hens to forage. To increase grass productivity, the plots will be watered during spring and summer. Half of the native-hens’ territories are left without exclosures (and so have no increase in grass) to measure the effect of resource availability on reproduction success.

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Please don’t hesitate to send me your best pictures of the native-hens on Maria Island, I will post the best pictures on this page. If you are interested in volunteering for this project or hearing more about it, please contact me.

Check out what we are doing in our research group: DEEP lab

Interesting facts about the Tasmanian native-hen:

  • They are known by locals as Turbo chooks, as they can run as fast as 50 km/h!
  • Their Tasmanian Aboriginal name is “Piyura” (palawa kani language).
  • They live on territories in family groups of up to 17 individuals, feeding exclusively on grass
  • While birds generally live and reproduce in pairs, groups of Tasmanian native-hen can be either monogamous (1 female with 1 male), polyandrous (1 female for many males), polygynous (1 male for many females) or polygynandrous (many males and females reproducing together).
  • After being culled because considered wrongly as an agricultural pest, especially in the 50s, it was protected under the law in 2010.
  • It went extinct from the mainland at the same time as the Tasmanian devil and the Tasmanian tiger about 3,500 years ago, probably due to the arrival of dingos, a cultural intensification by the Aborigines, and a climate change. It was the only bird to go extinct at the time.
  • It is a very rare and not fully understood case of the evolution of flightlessness on continent (mainland Australia), in the presence of predators.

 

References

Goldizen, A.W., Putland, D.A. & Goldizen, A.R. (1998) Variable mating patterns in Tasmanian native hens (Gallinula mortierii): correlates of reproductive success. Journal of Animal Ecology, 67, 307-317.

Ingram, J. (2018) An adaptive management case study for managing macropods on Maria Island National Park, Tasmania, Australia: adding devils to the detail. Pacific Conservation Biology, 24.

Steadman, D.W. (2006) Extinction and biogeography of tropical Pacific birds. University of Chicago Press.

 

 

 

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