VU
Bower's Shrike-thrush Colluricincla boweri



Justification

Justification of Red List Category
Recent monitoring data indicate recent rapid declines of this species in response to climate change. For this reason, it is assessed as Vulnerable.

Population justification
The abundance of Bower's Shrike-thrushes is calculated from the density and distribution of birds using data from standardised transect surveys along elevational gradients and the area of climatically suitable habitat at different altitudes in 2016 (Williams et al. 2010a, 2021); the population thus numbers 250,000-490,000 mature individuals, with a best estimate of 350,000.

Trend justification
A substantial decline over the past three generations is highly likely, but not all data are consistent. Annual monitoring undertaken between 2000–2016 (1,970 plots, 62 different locations, at 0–1500 m) suggests a 71.6% decline in the total population over the three generations to 2016 from an estimated 1.22 million to 350,000 mature individuals (Williams & de la Fuente 2021). The decline was most marked at mid-altitudes (450–850 m) whereas upland declines were slower and less consistent, which is congruent with climate change modelling (Williams et al. 2003). 

This rate of decline is consistent with a 71% decline between 2000–2007 and 2013–2019 in the proportion of weeks in which the species was recorded (from 42%–12%) at the School for Field Studies Centre near Danbulla (740–780 m; A. Freeman, M. Craig unpublished, in Williams et al. 2021). Barnes et al. (2015), however, concluded that the prevalence of Bower's Shrike-thrush had increased on lists collected between 1996–2005, and there was no significant change in reporting rates for 2-ha 20-min surveys and 500-m radius area searches for the period of 1999–2018. 

The discrepancy between analyses is thought to be because populations at lower elevations have been particularly strongly depleted, and the extent of forest and population size at those elevations is far greater than higher up the mountains where more birdwatching occurs. Williams et al. (2021) evaluated these trends and concluded that the best estimation of decline across the species’ range is 30%–49% in three generations, which is followed here.

Distribution and population

Bower's Shrike-thrushes are endemic to the Wet Tropics of north Queensland, Australia from the Paluma and Bluewater Ranges north to Mt Amos and the Windsor Tablelands, extending west to the Herberton Range (Higgins and Peter 2002).

Ecology

Bower's Shrike-thrushes occur in upland rainforest, formerly attaining their highest densities at 800–1000 m (Williams et al. 2010b), feeding on insects caught in and below the canopy (Frith 1984). They usually lay two eggs in a cup nest built in a tree fork or vine tangle (Higgins and Peter 2002).

Threats

Climate change is the only known threat and could be having both direct effects as a result of increased mortality during heat waves, which have been longer and hotter in the last two decades, and an indirect effect because the dry season has been longer, drier and hotter, which is likely to have reduced resource availability (Williams et al. 2010b, Williams and de la Fuente 2021).

Conservation actions

Conservation Actions Underway
Almost the entire species's range is within protected areas.

Conservation Actions Proposed
Understand constraints on population size and survival. Identify reasons for sensitivity to climate change and options for countering them. Develop strategies for maintaining remaining populations. Apply adaptation strategies as required

Acknowledgements

Text account compilers
Vine, J., Berryman, A.

Contributors
Butchart, S., Ekstrom, J., Fisher, S. & Harding, M.


Recommended citation
BirdLife International (2022) Species factsheet: Colluricincla boweri. Downloaded from http://www.birdlife.org on 01/10/2022. Recommended citation for factsheets for more than one species: BirdLife International (2022) IUCN Red List for birds. Downloaded from http://www.birdlife.org on 01/10/2022.