Sanderlings, Calidris alba

Wednesday, March 3, 2010

Nature Notes
by Bob Thomas

Among the most entertaining beach birds are sanderlings, Calidris alba. They are often referred to as a “wind-up toy” due to their rapid movements up, down, and across the beach, their legs moving so fast that they are a blur. They belong to a group of small sandpipers that birdwatchers affectionally (or frustratingly) call peeps.

Most beach lovers along the Gulf of Mexico coast probably consider them “our” birds since they are abundant along our beaches during late summer vacation season and throughout the winter. Actually, the migrants are just arriving at that time. They remain here until late spring when they return to their northern breeding grounds. Juveniles tend to spend their entire first year in the south, so the species is here year round.

But what about the “our birds” concept? Sanderlings spend their winter months here, the Caribbean, South America, Australia, and many other coastal areas around the world. By late spring, they are making their way to the circumpolar tundra regions to breed in moist marshes. They build flimsy nests, just a depression lined with leaves and grass, and lay three or four eggs in each of one or two clutches per year. The eggs hatch after about one month incubation (by both male and female, and sometimes a second male), and are scurrying about feeding themselves within 12 hours of hatching. The babies fledge in two weeks. The adults fly south soon thereafter, and the young follow a month or so later. Amazingly, they innately find their parents’ home beaches.

When breeding in the far north, their colors are reddish-brown. When they densely populate our beaches, arriving in throngs in late summer, they are rather drab with white bellies, gray backs, and obvious white wingbars in flight.

Sanderlings are unique among our local sandpipers by having three forward pointing toes and no hind toe. This is an adaptation that allows them to run fast as they move about the beach searching for food.

Food items of sanderlings vary with their habitat. While nesting in Canada in marshes, they feed primarily on insects. On beaches, sanderlings feed on polychete worms, amphipods, small bivalves like coquinas, mole crabs, etc., taking advantage of thixotropy (the topic of a previous Nature Notes) by easily poking their beaks into the normally packed sand grains when water rushes over and through the swash zone. This is why one sees sanderlings run toward the receding waves and frantically poke about in the sand, only to flee the next incoming wave. They have to be fast and with a strong work ethic to be successful in this rather narrow feeding niche.

I’ve spent many hours watching sanderlings feed on coquina bivalve mollusks (Donax variabilis) on beaches in Alabama and Florida. I’ve never been able to discern with certainty whether they swallow the entire mollusk or instantly pop the shell open and consume only the animal tissue. I’ve never found the discarded shells where a bird was actively feeding, and I am assured by Drs. John Dindo, Dauphin Island Sea Lab, and María de los Ángeles Hernández, Centro Nacional Patagónico, that they swallow the entire bivalve.

As mentioned, sanderlings’ food is often encased in coverings difficult to digest (shells of coquinas and exoskeletons of crabs), and they accidentally engulf a considerable amount of sand as they feed. As a result, they regurgitate pellets composed of these indigestible pieces-parts.

It is known that sanderlings that migrate to South America each summer have excellent timing for arriving on Delaware beaches during horseshoe crab spawning. Studies show that a sanderling can consume 9,000 horseshoe crab eggs per day for several weeks before taking flight for their long journey to the coast of South America.

The next time you truly want to relax, go to a beach, place a chair near the swash line, and watch the avian wind-up toys doing their thing.

                                            

As a wave runs down the beach face back                                         Between waves, while the sand is still wet,
into the gulf, a sanderling quickly runs to the                                    the sanderling enjoys its moment to search
middle of the swash zone to search for food.                                     for food.
Photo by Bob Thomas.                                                                   Photo by Shannon Fortenberry.

                                            

Pecking marks left by the sanderling.                                                As the next wave runs up the beach, the
Photo by Bob Thomas.                                                                 sanderling retreats, only to immediately
                                                                                                     follow it down the beach face to search for
                                                                                                     more food.
                                                                                                     Photo by Bob Thomas.

Foot print of a sanderling. Note there is
no fourth toe pointing backward. The small
holes are air holes that are common on
beaches when the swash has receded.
Photo by Bob Thomas.