Spotlight: Not a spoon, but maybe a hammer, filter, or microphone
It’s a bird with a “spoon” for a bill, but though this bird does use its spoon-bill for eating, we’ll learn in a paper by Kelly and colleagues1 in this issue of Wader Study that this bird does not use its bill like a spoon.
The Spoon-billed Sandpiper is a small wader that breeds in NE Russia and winters in SE Asia. It is one of the world’s best known waders, despite being one of the world’s rarest birds, but it would be pretty plain if not for its distinctive spatulate bill. It is this bill that fascinates birders and bird scientists alike, because the Spoon-billed Sandpiper is the only wader species in the world to have evolved a spoon-shaped bill – and that raises the tantalizing question: Why?
Kelly, Zöckler, Scampion and Syroechkovskiy tackle this mystery through an impressive set of observations collected through all stages of the Spoon-billed Sandpiper’s life cycle. They synthesize these observations into six feeding techniques that explain how these waders use their remarkable bills. Then they use these newly described feeding behaviors to discuss how and why the spoon-shaped bill might have evolved.
To begin, the researchers present a detailed description of the Spoon-billed Sandpiper’s extraordinary bill. Upon close inspection, it is rather more like a spatula than a spoon. Its tip is expanded sideways into two triangles with rounded outer edges that slope downwards slightly.
Though this clearly looks very different from other Calidris sandpiper bills from the outside, the authors point out that when the bird opens its bill it is possible to see the outline of the basic sandpiper bill within (see their Fig. 1). However, the bill of Spoon-billed Sandpiper also differs from other sandpipers in several important respects: (1) the bill is expanded at the base as well as the tip, giving the birds a broader tongue, (2) there are more epidermal papillae (which help to keep food moving backward while the tongue is moving in both directions), (3) there is a larger than usual concentration of Herbst corpuscles (nerve endings that may help birds to sense prey under wet sand or soil2) at the bill tip. These intriguing differences likely relate to how the bill is used.
So how do Spoon-billed Sandpipers use their bill for feeding? Answering this question took years of careful observation, both at breeding areas in Russia and at non-breeding areas in Bangladesh, Thailand, Myanmar and China. During this extensive fieldwork spanning from 2003 to 2017, the researchers also collected more than 1000 photos and videos.
Using this treasure trove of field observations, the authors first define behavioural elements (which are not in themselves feeding techniques): Concertinaing, Grandstanding, Pirouetting, Jumpback and Transitting (the paper provides detailed descriptions in Table 1 and photo examples of birds performing these behaviours). The authors then use these fascinating behavioural elements to describe six specific feeding techniques (nicely summarized in their Table 2):
- Selectively pecking: Steady forward pecking in mud or shallow water.
- Sweep-stitching: A stitching movement along the axis of the bill combined with a sideways motion at the highest point in the “stitch”. This technique is unique to Spoon-billed Sandpipers. Other Calidrids use similar techniques, but no other creates the impression of sideways sweeping.
- Burst-stitching: Similar to sweep-stitching, but with less side to side movement.
- Spear-hunting or ‘hammering’: A technique in which the bird grandstands and then ‘hammers’ their bill into the water or substrate.
- Stabbing: Like burst-stitching, but more forceful and with the bill closed when it is inserted into the substrate.
- Hoovering: A technique where only the bill tip is inserted into the substrate and the bird makes very rapid but shallow stitching movements creating the impression of hoovering/vacuuming the substrate.
The authors then discuss these techniques within the context of previously published hypotheses of how the Spoon-billed Sandpiper’s bill might be used for food intake like a shovel, a hoe, a mousetrap and a biofilm grazer. Then they develop their own metaphors for how the birds use their spoon-shaped bill like a hammer, a filter and a microphone.
The hammer. A hammer is a tool with a heavy head used for jobs like breaking things or driving objects into a surface. Spoon-billed sandpipers use their bill like a hammer to forcefully immobilize prey while spear-hunting or ‘hammering’ and also during stabbing. But other species do this too and the authors argue that there is no firm link between the spoon-billed structure and better success in direct hits of larger prey.
The Filter. A filter is a porous device that serves to remove solid particles from a liquid (or gas) that is passed through it. The bill may act like a filter to harvest small food items, especially when feeding on biofilm. The idea, proposed by Danny Rogers in 2007, is that the sandpipers use their bill like a Dutch hoe (a garden tool with a blade that is similar in shape to the bill) to liquefy the substrate and immobilizes prey. Prey are then filtered from the liquid using the epidermal papillae and the large tongue as a pump. The authors conclude that this hypothesis has traction and might have contributed to the evolution of the bill shape, but perhaps in combination with the bill first being used as a microphone (sensor) to detect prey.
The Microphone. A sound microphone detects changes in pressure (sound waves) and turns them into something else (electrical signals), these signals then move to an amplifier or a recording device. The Spoon-billed Sandpiper’s bill does not detect sound waves, but rather it acts as a microphone by acting as a sensor to detect changes in pressure. The Herbst corpuscles are used to detect pressure gradients at various locations on the bill surface. The bird then detects obstructing objects (prey) that have caused a disturbance in this pressure map.
The fact that Spoon-billed Sandpipers have much higher numbers of Herbst corpuscles than related sandpiper species is circumstantial evidence that pressure sensing might have been involved in the evolution of their bill shape. In addition, the part of the bill containing the Herbst corpuscles is thicker than the rest of the bill, which might enhance the pulse and amplify the signal. Also the canted angle of the front edges and the soft corners of the bill might reduce turbulence.
Examined in this context, feeding techniques such as pecking, stitching and stabbing could be used to generate a disturbance in the pressure field. This disturbance would make it possible for birds to detect prey based on directional cues enhanced by the laterally expanded bill. Hoovering might also be a method of pressure sensing near surface level with lighter impact pecks generating smaller pressure waves to detect items at close range. Using the bill like a pressure sensor might be particularly useful in muddy substrates where the technique would allow effective spear-hunting without the bird needing to visually see the prey.
The authors conclude that the pressure-sensing hypothesis (the bill as a microphone) seems to be the most compelling as an explanation for how small but incremental lateral enlargements of the bill might have provided a selective advantage. Using their bills like a microphone, birds with broader bills might have possessed greater ability to detect and capture prey, enjoyed better survival, and produced more chicks with spoon-bills.
Of course all of these arguments make the reasonable assumption that the bill shape is a specific adaptation for feeding. The authors do not discuss the possibility that the Spoon-bill Sandpiper’s bill might have some other adaptive value, such as for thermoregulation3. Or that the bill might not be adaptive at all. Odd things happen in nature all the time. Take for example an astonishingly long Eurasian Oystercatcher bill also described in this issue of Wader Study4. Is this adaptive? What would happen if females started to find this male incredibly attractive and he produced many offspring? In a recent book5, Richard Prum –building on Darwin’s ideas about mate choice – proposed that some traits might have nothing to do with functionality, but rather evolved through pure aesthetics. If the Spoon-billed Sandpiper’s bill serves only to attract mates then it might even contribute to the species’ small population size; sexual selection and extinction risk can be related6. Another possibility is that the bill has no real benefit, but is not sufficiently maladaptive to be removed by natural selection. Such evolutionary oddities have the greatest chance of arising and persisting in small populations, which may explain other traits of unclear function, such as the equally peculiar apparatus of the Wrybill. The truth is we just don’t know, but Kelly and colleagues give us much to think about.
- Kelly, C., C. Zöckler, B. Scampion & E. E. Syroechkovskiy. 2017. Hammer, Filter or Microphone – How does the Spoon-billed Sandpiper Calidris pygmaea use its bill to feed? Wader Study 124(2): 99–104.
- Piersma, T., R. van Aelst, K. Kurk, H. Berkhoudt, & L. R. M. Maas, Leo R. M. 1998. A New Pressure Sensory Mechanism for Prey Detection in Birds: The Use of Principles of Seabed Dynamics? Proceedings: Biological Sciences. 265(1404): 1377–1383.
- Tattersall, G.J., B. Arnaout & M.R. Symonds. 2017. The evolution of the avian bill as a thermoregulatory organ. Biological Reviews 92: 1630–1656.
- Cooney T. 2017. Extraordinary bill abnormality in a Eurasian Oystercatcher. Wader Study 124(1): 155–156.
- Prum, R. O. 2017. The Evolution of Beauty: How Darwin’s forgotten theory of mate choice shapes the animal world – and us. New York: Doubleday.
- Kokko, H., & R. Brooks. 2003. Sexy to die for? Sexual selection and the risk of extinction. Annales Zoologici Fennici 40: 207–219.
PDF is available here.