Most birds have excellent vision, which is part of what makes birds such an appealing group to study. However, birds also use other senses to navigate their world. This project investigates the tactile senses in their beaks to understand the links between bill-tip anatomy and foraging ecology. The main focus is on three species of southern African ibises that forage in different habitats. All three have a honeycomb pattern of pitting in the bones of the bill tips, which suggests they should be able to forage using the sixth sense “remote touch”, detecting small vibrations made by prey as they burrow or swim through the foraging substrate (soil, mud or water).
Carla du Toit started this project as an MSc student in 2017, upgrading to a PhD in 2018 which she completed and submitted at the end of 2021. Her research focuses on the anatomy of the bill-tip organ in probe-foraging birds, including modern ibises and extinct species in the paleontological record. The bill-tip organ of probe-foraging birds is made up of mechano-sensory receptors embedded in densely clustered pits in the bone of the bill tip. Although the general structure of the bill-tip organ is similar across all probe foraging species that possess it, there is variation in the shape and orientation of receptors among species. Carla uses three ibises as a model to investigate the link between the morphology of the bill-tip organ and the birds’ foraging ecology: Hadeda Ibis Bostrychia hagedash, Sacred Ibis Threskiornis aethiopicus and Glossy Ibis Plegadis falcinellus.
Carla is also exploring whether these patterns can be extrapolated to infer information about the foraging ecology of fossil birds. If there is a strong link between the morphology of the bony parts of the bill-tip organ and foraging behaviour, we can use the structure of fossil beaks to infer information about the palaeoecology of extinct birds. In 2020, we published an extensive review of the bone structures of the beaks of over 500 species from all orders of extant birds. This established that we can determine the presence of a remote-touch-capable bill-tip organ in modern birds solely from the structures of their beak bones. Using this knowledge, we have been able to infer the foraging ecology of some of the most basal members of the avian family tree, shedding light on some contentious questions regarding the evolution of modern birds.
We have shown how some of the earliest known modern birds from the fossil record, the lithornithids (which evolved during the end of the Cretaceous Period), possessed remote-touch bill-tip organs. This shows that this remarkable sense evolved very early in the clade of modern birds. We have also shown how the organ is plesiomorphic in all modern paleognathous birds, including the extinct elephant birds and moas, and is vestigial in all except kiwi.
Activities in 2021
- On completion of her experimental work assessing the tactile foraging behaviour of Hadeda ibises at the end of 2020, Carla spent the year writing up the remaining chapters of her doctoral dissertation. She has submitted it for examination, and we await the results from the examiners.
- Carla completed her analyses of the behavioural data on Hadeda ibis foraging, and her results indicate that when using remote-touch to locate prey, the birds are more successful at foraging in wet soils than dry ones. However, if the birds are unable to use remote-touch (and simply rely on direct contact with prey), the water content of the soil had no effect on foraging success. Thus, her study indicates that the higher foraging success rate of Hadeda Ibises in wetter soils is not due simply to “ease” of probing, but rather the birds find it easier to actually detect their prey using remote-touch when there is more water in the soil because vibration cues are better transmitted through saturated substrates. This pattern could have important implications when understanding the habitat requirements of remote-touch probe-foraging birds (such as ibises and shorebirds). Furthermore, this may in part explain why the recent range expansion of Hadeda Ibises is correlated with increased irrigation of soils.
- Carla completed and submitted her PhD thesis for examination in November 2021.
- Overall, her results indicated that there are clear ecomorphological links between the structure of the remote-touch bill-tip organ and interspecific differences in the foraging ecology of ibises (and consequently, likely other remote-touch probe-foraging birds). Her results have shown links between both bone morphology and soft tissue anatomy and the different species foraging behaviour and use of different substrates for foraging. She has also shown how tactile bill-tip organs can be identified looking at the bones of birds’ beaks in the absence of soft tissue structures, which is of great use to palaeontologists studying the sensory ecology of extinct birds.
- Our article concerning the evolution of the remote-touch bill-tip organ in paleognathous birds (the clade of birds containing ostriches, emu and kiwi) continues to rank very highly, remaining in the top 5% of all research outputs scored by Altmetric (a measure of how much people and the media have discussed the article online), and has been favourably cited multiple times since it was published a year ago.
Impact of the project
This project helps us to better understand the links between anatomy, morphology and behaviour in birds. From a conservation and global change perspective, it allows a better understanding of the substrate conditions under which ibises are best equipped to forage, improving our understanding of potential mechanisms underlying the range expansion of Hadedas into the south and west of South Africa, and the likely impact of drought and ongoing climate drying on the foraging success of this and other ibis species.
The comparative work on palaeontological specimens has improved our understanding of the ecology of extinct birds, and shed light on the evolution of this unique sensory system. By changing our understanding of the morphology and behaviour of some of the earliest ancestors of large clades of modern birds, our findings have shed light on a major sensory ecological trend in the evolution of modern birds and dinosaurs.
DSI-NRF CoE grant; DSI-NRF CoE in Paleosciences.
Research team 2021
Dr Susan Cunningham (FIAO, UCT)
Prof. Anusuya Chinsamy-Turan (Biological Sciences, UCT)
Dr Steve Portugal (Royal Holloway, U. London)
Dr Anton du Plessis (U. Stellenbosch)
Student: Carla du Toit (PhD, UCT)