ECR Feature: Audrey M. Prasetya on environmental correlates of carotenoid plumage colouration in birds

Audrey Miranda Prasetya is a PhD student at the Australian National University. She is a biogeographer interested in studying the evolutionary history of birds. Audrey shares her recent work on environmental factors that potentially drive the distribution of carotenoid-based plumage colouration in Australian passerine birds.

Presenting my honours research at the International Conference for Undergraduate Research 2018. Photo by the International Conference for Undergraduate Research.

Personal links. Twitter | LinkedIn

Institute. The Australian National University

Academic life stage. PhD candidate

Major research interests. Biogeography, Birds

Current study system. The birds found in the Indo-Australian Archipelago make up almost 10% of all bird species found globally – impressive considering the geographic area comprises just 8% of terrestrial land in the world. Many of these bird species are endemic. The unique faunal (and floral!) diversity of the Indo-Australian Archipelago can be attributed to its very dynamic geological and environmental history that resulted from massive continental plate shifts through time. Birds in the area range from beautiful tiny blue-fairy wrens, giant cassowaries, and one of my personal favourites of the region – the famous Greater Bird of Paradise.

Recent paper in JBI. Prasetya, A. M., Peters, A., Delhey K. (2020). Carotenoid-based plumage colour saturation increases with temperature in Australian passerines. Journal of Biogeography,47, 2671-2683. https://doi/10.1111/jbi.13968

Motivation behind this paper. In the early 1800’s, C. L. Gloger described patterns of geographic variation in the colour of birds. He suggested that bird colouration tends to be more intense in warmer climatic regions, particularly the tropics. This pattern has been supported for colour variation due to the deposition of melanin pigments, but, less work has been done on other types of colouration. Here we focused on colouration that depends on carotenoids – plant pigments that cannot be produced by birds themselves and must be obtained through their diet. It is therefore possible that the amount of carotenoids available in the environment determines the intensity of carotenoid-based colouration, and that this explains geographic patterns of variation in bird colour.

Live forms of the specimens showing off their colours! (left) The Scarlet Robin (Petroica boodang), and (right) the Crimson Finch (Neochmia phaeton) and one of my personal favourites. Photos by Kaspar Delhey.

Key methodologies. We systematically tested whether environmental factors correlate to patterns of intraspecific geographic variation in carotenoid-based colouration in a wide range of species. In our study, we measured 2000+ specimens from forty-nine selected species of Australian passerines with carotenoid-based plumage colouration. Unlike previous studies, we attempted to establish consistent patterns of intraspecific geographic variation of carotenoid-based plumage colouration by measuring a wide range of species and randomly selecting them to minimize taxon selection bias. Plumage colouration was measured using reflectance spectrometry and then analysed using linear models to determine whether geographic variation was correlated with latitude, temperature, rainfall, or primary productivity. These environmental variables were selected to include factors identified by Gloger (latitude, temperature) and those that might indicate carotenoid availability in the wild (precipitation, plant productivity). We the used meta-analytical techniques to combine the effects across the different species and test for general patterns.

Museum specimens, lined up for colour measurements.

Major results. Although we did find an overall latitudinal and temperature effect in carotenoid-based colouration (more intense colours at lower latitudes and higher temperatures), we did not detect an effect of precipitation and plant productivity, which we hypothesised to be contributors to spatial variation in carotenoid-based colouration. A possible reason behind this is that perhaps the intensity of carotenoid-based colours is not as limited by carotenoid availability as we first thought but by other factors. Another potential explanation for the correlation with temperature could be the newly discovered close links between carotenoid and cellular metabolism in birds. We also predicted that the effect we found would be stronger in yellow then red carotenoid-based colours due to previous research – but we found the opposite! The spatial variation we found could match latitudinal variation of other selection pressures such as predation risk, sexual selection, and parasite prevalence.

Measuring carotenoid colour patches using a spectrometer in the Melbourne Museum.

Challenges you faced on the way. At the start of the project, we randomly chose fifty passerine species. However, we needed enough specimens per species to get a good enough average of the species’ colour reflectance. I had to check that each species had at least twenty male specimens listed in Museums Victoria and Australian National Wildlife Collection specimen database. Unfortunately, the problem did not end there! It became clear after my first visit that I was unable to obtain decent measurements from some of the specimens from our original list of species. Some specimens had unclear labels without a location, or I could not determine its sex. Some were moulting so the feathers were not showing carotenoid colours. Other birds had missing feathers or no carotenoid patch due to the taxidermy process. In the end, I had to drop one of the species as there were not enough specimens to get a good sample size. Not everything will happen as planned in an initial research proposal!

Next steps? Although we observed a latitudinal gradient in carotenoid colouration, we still do not know what the underlying mechanism for this pattern is. Therefore, the next steps for this research are to test for other possible drivers of geographic variation that may have correlated with the latitudinal effect we found. This may include testing for the effects of predation risk, sexual selection pressure, parasite prevalence or variation in cellular metabolism. Furthermore, it would be interesting to see if the pattern we found holds true to birds in other continents and regions.

If you could study any organism on Earth, what would it be? As an Indonesian I would love to study our amazing Indonesian fauna – which I am technically doing! There is so much amazing wildlife that deserves research love and attention. Other than that, as a self-proclaimed ‘museum nerd’ it would be a great pleasure to work on specimens collected and handled by the great naturalists of our time such as Darwin, Wallace, John Gould, … the list goes on!

Anything else? A lot of people think that I travelled a lot to collect all my data as it had to encompass a large amount of area – both during Honours and now for my PhD. Although I would have loved to go around my study area, I have found an immense passion for natural history collections in museums. Not only is it incredibly surreal to be able to meet all the different species from around the world in a single location, but there is also something really special about reading little handwritten tags of collectors in the past (and present!) and the care put into each specimen during capture, preparation, and conservation. There is so much story behind a single specimen and to be able to be take part in unravelling a portion of this story through research is an incredible honour.

Published by jbiogeography

Contributing to the growth and societal relevance of the discipline of biogeography through dissemination of biogeographical research.

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