David Sibley, of the Sibley Guides to Birds fame, recorded the above video of the critically endangered Spoon-billed Sandpiper, Eurynorhynchus pygmeus, foraging in the mudflats of Thailand where they winter. In an expanding upon the observations annotating the above video, Sibley proposes a hypothesis about how these lovely little birds use their odd bills more like shovels than spoons:
Before seeing the birds, most people assume that they use their bills to swipe sideways through the water, in the manner of the true spoonbills (genus Platalea), sensing and grabbing food items as they pass between the flattened tips of the mandibles. But in reality these sandpipers use very little sideways motion in their feeding. There does seem to be a bit more sideways movement of the bill than in other small sandpipers such as Red-necked Stint, but these are subtle, irregular, and tiny movements and nothing like the rhythmic sideways swiping of true spoonbills.
Coming up with a new hypothesis proved difficult. At first I couldn’t detect any difference in the way these sandpipers fed compared to the stints. They do tend to keep their head down and their bill in the water for longer stretches than the Red-necked Stints, which have a more frenetic foraging action dipping their bill briefly into the water and mud and then raising it again, over and over. Also, the Spoon-bills seemed to feed exclusively in water – I never saw one feeding on open mudflats.
After several days of observation I noticed that while their bills were in the water the Spoon-billed Sandpipers were pushing lumps of mud and algae ahead of them, using their bills as shovels to move mud around. They always look a bit “husky” and thick-necked, which comes in part from this habit of pushing the bill through the mud, as they use their body for leverage and push with their legs. It’s not unusual to see one of their feet suddenly slip backwards under the effort of pushing. Once some mud or algae has been lifted the bird very quickly works the bill tip around underneath it, then moves on. This video shows the shoveling motion clearly in the last scene. (The video will be a little sharper if you click here to open it in YouTube and select 480p).
This seems like a plausible hypothesis to explain the unusual bill shape. The broad bill tip could be used as a shovel to get under and lift up loose substrates, and then would make an effective tool for finding and grabbing any small invertebrates that were in the slurry of mud and water flowing in behind the lifted material. This could also explain why they cover so much ground on the mudflats. If they are looking for loose bits of mud/algae/etc. that they can lift to search for prey, these might be scattered across a wide area, forcing them to walk in search of these foraging opportunities.
Have you ever seen these birds forage? Are you in a position to make more observations in other locations to see if they do the same thing? I am not, much to my regret while watching the above video... Given the rapidly declining populations and our ignorance about even their basic biology, it is clear that the spoons these birds are born holding in their mouths are far from silver ones! Can we at least find out how this marvel of evolution, this wonderful spoon-bill, works before we are forced to bid adieu to the Spoon-billed Sandpiper?