New paper on "African alarm clocks", Hadeda Ibis

Hadeda Ibises (Bostrychia hagedash), and their characteristically loud calls, are a common feature in and around many human cities and settlements across South Africa. Historically, however, they only occurred in the eastern regions of the country. Their range expansion has previously been shown to follow the geographic pattern of increased soil irrigation by humans in suburban and agricultural areas. A new study from the FitzPatrick Institute of African Ornithology has shown that one factor that may have helped drive this range expansion in Hadedas is that it is easier for them to locate their prey in wetter soils. 

Birds like ibises (including the “African alarm clocks” Hadeda Ibis in southern Africa and the “bin chickens” Australian White Ibis) that feed on buried invertebrate prey items (such as earthworms) often do so blindly – without the aid of visual cues to help them locate their prey. They capture their prey by probing their long beaks into the substrate and pulling out the prey, a feeding method known as probe-foraging. Some probe-foraging birds (including ibises) have a “sixth sense” called remote-touch, which enables them to locate their buried prey before it comes into direct contact with their beaks. Remote-touch refers to their ability to detect vibrations from their prey items in the substrate, using a specialized organ at the tip of their beak. This sense can be understood as something between touch, hearing and echolocation – the sense bats use to find insects in pitch darkness. 

As vibrations tend to travel better in wetter soils, the new study aimed to investigate whether Hadeda Ibises were able to locate prey faster using remote-touch when there is more water in the soil.

“Remote-touch is a remarkable sensory adaptation found in a diverse number of birds, from our Hadeda Ibises, to the illusive Kiwi in New Zealand, as well as various shorebirds, including some of the most endangered birds on the planet (e.g., Giant Ibises in Cambodia and Spoon-billed Sandpipers in Eastern Asia),” says author, Dr Carla du Toit, whose doctoral thesis this study formed a part of. “Despite this, there is very little research into how remote-touch works under different environmental conditions, and many people are still unaware that so many wetland and shorebirds rely on it to find their food.”

Using a group of captive Hadeda Ibises at the World of Birds sanctuary in Hout Bay, the UCT researchers tested under controlled conditions how adding more or less water to the soil affected the birds’ ability to locate buried prey items (superworms). When the worms were dead, and not producing any vibrations for the birds to detect (and could only be found in the soil by random chance – the birds were shown to be unable to locate the worms by smell or hearing), the amount of water in the soil had no effect on how quickly the birds located them. When the worms were alive and wriggling around, the birds were able to locate them more quickly than the dead worms as they could make use of their sense of remote-touch. When more water was added to the soil, the birds were able to locate the live worms faster using remote-touch. Furthermore, when the soil was too dry, the birds were unable to locate the living worms faster than the dead worms, indicating that they were unable to use their remote-touch “superpower” if the soil was too dry. 

“It was surprising how quickly the birds took to probing in the trays of soil. We’d barely put them down near their usual food trays before the Hadedas started investigating and probing their beaks into them – they didn’t even know there were worms in there yet when we started training them!” says du Toit. “Once they realized there were fat, juicy superworms hidden in the trays, they couldn’t get enough. They’d fly over to me and start calling when I came in every morning and would follow me around the aviary and peck at my shoelaces if I took too long to set up the daily experiments.” And the birds weren’t the only ones who seemed to enjoy the trials: “As the experiments were put on hold due to the Covid-19 lockdowns, when they resumed, the Hadedas were the only ‘colleagues’ I was allowed to see in person!” jokes du Toit.

The results of this study indicate that Hadeda Ibises have “followed” the path of human development across South Africa, as our irrigation of soils in gardens, parks, golf courses and farms has made it easier for them to find their buried prey. There are other factors, such as introduced prey items and availability of suitable nesting trees, which have likely also contributed to this pattern. But if the birds cannot locate prey (i.e., if soils are too dry) then it doesn’t matter how much prey is present in the soil, the birds would still not be able to forage successfully or survive in that area. Therefore, it is important when studying habitat suitability to understand and include animals’ sensory requirements and limitations in a particular habitat, instead of simply looking at presence/absence of prey items. 

“While our study with Hadedas highlights the importance of water levels of the substrate in which remote-touch probing birds forage, there is still much work that needs to be done,” says du Toit. “We know from the anatomy of their sensory organs and their use of diverse habitats that different species likely have different levels of soil water that they require in order to use remote-touch to find prey. But we don’t know what those thresholds are yet for other species. This knowledge would be particularly valuable in species which rely on fragmentary, specialized habitats, many of which are increasingly threatened by climate change and human activities.”