Shorebird migration is in full swing. Many of these migrants have nested on the arctic tundra and are heading for Central or South America to overwinter. These trips demand lots of fuel so our shorebirds must feed voraciously.
Most shorebirds are fairly confiding birds so it’s easy to get close enough to watch their feeding behaviors. Our shorebirds show a diversity of foraging techniques.
Let’s start with the plovers. Semipalmated Plovers, Black-bellied Plovers and American Golden-Plovers, all common fall migrants in Maine. Plovers rely on their eyes to find food on intertidal mudflats. The plovers practice a type of feeding that animal behaviorists called run-and-peck. A plover will stand in one spot on the intertidal surface and keep an eye out for movement at the surface of the sediment. The plover will run over and grab the unsuspecting arthropod or marine worm.
Keep an eye out for a clever behavior by these plovers. Several species of marine worms called polychaetes maintain burrows in the sediment. Two of these, the sand worm and the blood worm, are dug for bait. When a small crustacean or other potential prey organism walks across the surface of the sediment, vibrations are created and the worm will shoot its proboscis out to capture lunch in an ambush. Plovers turn the table on these worms. A plover will place one foot right above at surface of the mud and then cause the foot to tremble. The vibrations from the trembling foot mimic the vibrations of a small invertebrate, causing a sand worm or blood worm to come to the sediment surface in search of a meal. That marine worm becomes a meal for the plover. Foot-trembling is common in plovers so keep an eye out for it.
Many of our sandpipers probe the sediment for food with rapid bill movements called stitching. Examples include Semipalmated Sandpipers, Least Sandpipers, Short-billed Dowitchers and Stilt Sandpipers. These birds use touch to locate prey organisms beneath the surface of the sediment. We also know that taste is important. If a sandpiper cannot taste the presence of a preferred prey after a few probes into the mud, it will take a few steps and try a new spot.
Sandpipers use their bill as forceps to capture invertebrates beneath the sediment surface. Now, thanks to the work of Margaret Rubega at the University of Connecticut, we know how some sandpipers use the surface tension of water to get a food item into the mouth.
Sandpipers are able to spread the tips of their bill while keeping the rest of the bill tightly shut, a neat little trick called rhynchokinesis. In Least Sandpipers and Western Sandpipers, Rubega used high-speed video to show that a sandpiper will capture a prey organism at the tip of its bill along with a droplet of water. The water sticks to the upper and lower part of the opened tip of the bill. Then the sandpiper slowly opens the rest of the bill. The water droplet sticks to the bill by surface tension and is stretched out with the prey organism inside. Essentially, surface tension provides an escalator to get the prey item into the mouth.
In the Bay of Fundy, Semipalmated Sandpipers feed on a small crustacean called Corophium. I found that I could see the struggling Corophium extracted when a sandpiper made a successful probe in the mud. The sandpiper also visibly gulped when it swallowed the Corophium.
I used these observations to publish a paper on the success rate of probing by the sandpipers in different areas of the mudflat. You should be able to see prey organisms in the bills of sandpipers too.
The stout, slightly upturned bill of a Ruddy Turnstone is used to flip over shells and other debris to expose the invertebrates hiding underneath.
[First published on August 20, 2017]