Smart beaks: non-visual senses in birds
Most birds, like most people, have excellent vision. This is part of what makes birds such an appealing group to study. However birds navigate their world using different senses as well. In this project, we investigate the non-visual senses of birds with a focus on tactile senses in their beaks. The main theme of the project is to understand the links between bill-tip anatomy and foraging ecology of the three species of southern African ibises: Hadeda Bostrychia hagedash, Sacred Ibis Threskiornis aethiopicus and Glossy Ibis Plegadis falcinellus. These species all 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”: detection of small vibrations made by prey as they burrow or swim through the foraging substrate (soil, mud or water)
PhD student Carla du Toit started this project as an MSc student at the beginning of 2017, and upgraded to PhD at the start of 2018. Carla’s research focuses on the anatomy of the bill-tip organ in probe-foraging birds, both 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 at the tip of the bill. Although the general structure of the bill-tip organ is similar across all probe foraging species that possess it, there is interspecific variation in the shape and orientation of the pits and the receptors within them. The overall aim of Carla’s thesis is to use ibises as a model to investigate the link between the morphology of the bill-tip organ and the birds’ foraging ecology, and to explore whether these patterns can be extrapolated to infer information about the foraging ecology of extinct bird species.
In 2018, Carla spent three months in the field, observing the specific foraging behaviour and sediment usage of Hadeda, Sacred and Glossy Ibises, to assess links between the morphology and histology of the bill-tip organ and the foraging ecology of the birds. She carried out 15 minute focal observations of individual birds and collected sediment samples where they were observed foraging. Carla discovered that the three species exhibit different foraging strategies and use of different sediments, and these differences correlate strongly with the changes in the structure of the bill-tip organ. For example, Glossy Ibises, which Carla previously found to have higher pitting than the other two species on the inside surfaces of their beaks, were observed to spend the most time probing with their beaks held open. This indicates that the location of the pits on the beaks’ surfaces are a good indicator of specific foraging behaviours. A similar correlation was found for the depth of probing and length of beak pitting. Furthermore, the Glossy Ibises foraged in substrates with higher moisture content and greater penetrability than the other two species (with Hadedas probing in the hardest, driest substrates, and Sacred Ibises falling in between the other two species). This fits our hypothesis that an increase in extent of pitting in the bill-tip linked to more aquatic habitat use is, at least in part, due to the transmission of vibrations in the sediment in which the birds forage. This pattern will be further investigated by Carla in 2019, using captive birds.
Carla presented her results at two conferences in 2018. Her initial morphological results were presented at the BirdLife South Africa Learn About Birds meeting in Langebaan in March, 2018. In August, she travelled to Vancouver, Canada to present the preliminary results of her field work as a speed talk at the International Ornithological Congress.
The strong link between the morphology of the bony parts of the bill-tip organ and birds’ foraging behaviour suggests we can use the structure of fossil beaks to infer information about the palaeoecology of extinct birds. In November 2018, Carla travelled to the USA to examine fossil specimens, as well as to compile a review of the mechano-sensory structures in the beaks of all modern bird families. The review will allow us to better understand the occurrence of these structures in modern birds. Furthermore, based on the phylogenetic position of some of the fossil specimens that Carla is studying, we may be able to make some very significant conclusions about the foraging ecology of some of the most basal members of the avian tree, shedding light on some large and contentious questions regarding the evolution of modern birds. This work is still ongoing, with the aim to publish in the next few months, completing one of Carla’s thesis chapters.
- Carla upgraded to PhD level in January 2018.
- A clear link was found between the morphology of the beaks and the specific foraging behaviours and sediment usage of the three ibis species, indicating that examining the bill-tip organ structure can be a useful proxy for understanding very fine scale changes in the foraging ecology of the birds.
- Carla presented her results at the International Ornithological Congress in Vancouver, Canada.
- Carla visited the Smithsonian Natural History Museum in Washington, DC and the American Museum of Natural History in New York. This enabled her to successfully capture data on the beaks of all modern bird families to perform a review. Excellently preserved material of fossil beaks at the Smithsonian were also examined and data collected. Alcohol-preserved material was also acquired from the Smithsonian to perform histological analyses on some hard to acquire species.
Impact of the project
This work will help us better understand the links between anatomy, morphology and behaviour in birds. From a conservation and global change perspective, it will allow a better understanding of the substrate conditions under which ibises are best equipped to forage successfully, improving understanding of potential mechanisms underlying the expansion of Glossy Ibises and Hadedas into the south and west of South Africa, and the likely impact of the current drought and ongoing climate drying on the foraging success of these species. Furthermore, the results could potentially be applied to other species of ibises, particularly where behavioural data is difficult to acquire, in order to better understand their requirements for foraging habitat, simply by studying specimens of their beaks, which can be obtained from museum collections. The comparative work on paleontological specimens will improve our understanding of the ecology of extinct birds, and shed light on both the evolution of this unique behaviour in modern birds, and potentially alter our understanding of the morphology and behaviour of some of the earliest ancestors of large clades of modern birds.
DST-NRF CoE grant; DST-NRF CoE in Paleosciences.
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)