Shorebirds are some of the longest distance migrants among birds, moving back and forth annually from breeding areas in the far northern hemispheres to wintering habitat in the far southern hemisphere.
Flocks of western sandpipers and dunlin may be in transit to their breeding areas in northern Canada and Alaska. The mudflats of the San Francisco bay delta (at Alviso marina county park) provide the perfect stopover to refuel for the continuation of their journey.
Wherever they are, there is competition from resident species that push both migrant and resident species to diversify the way they exploit the food resources. Some of that diversity is reflected in the unique structure of their beak.
Two Western Sandpipers (center) are dwarfed by the much larger Dunlin (another type of sandpiper). Their bills are half as long, and they use them to skim the surface of wet sand picking up minute invertebrates, rather than probing more deeply for worms and crustaceans like the Dunlin do.
Long-billed Curlews are uniquely suited to probe deeply into the mud and sand to extract clams, worms, or crabs. Photo from Wikipedia (by Mike Baird)
American Avocets swish their slender, uptured bill back and forth in the water to stir up invertebrates, which they then grab in the pincer-like tip. Clearly, this is not a bill designed for probing deeply into mud, despite its length.
But even in birds having similar shapes or sizes of beak, minute differences in structure or the way they use it (specific foraging behaviors) might be enough to reduce the competition.
Black-necked Stilts and Greater Yellowlegs are about the same size, and have about the same length bills. Stilts are very visual hunters, often herding small fish or crustaceans to a site where they can grab them. Their bill is a bit more slender and they use it like a pincer. Greater Yellowlegs are also fond of fish and small crustaceans, but they hunt by feel using the very sensitive tip of their bill to detect prey. Sweeping the bill side-t0-side through the water, they grab at any prey they detect.
Greater Yellowlegs are called “greater” because their bill tends to be about 1.5 times the length of their head. Closely related Lesser Yellowlegs have a smaller body relative to the length of the legs and a shorter bill (about the same length as the head).
Willets are another, large-bodied shorebird with a short, stout bill they use to probe into the mud for crustaceans and worms. Unlike the stilt or the yellowlegs, willets can hunt both by feel at the tip of the bill and by sight, and are able to feed bot day and night — a handy trait when there is a lot of competition for food.
A really excellent video produced by the Cornell bird lab illustrates the way that shorebirds specialize in harvesting their food, by variation in their bill structure, foraging behaviors, vision, or micro-habitat preferences.