For the Birds: Plover and Sandpiper Feeding

Shorebird migration is underway. In the past week, Maine birders have found Greater Yellowlegs, Lesser Yellowlegs, Short-billed Dowitchers, Semipalmated Sandpipers and White-rumped Sandpipers. Most of these species nest in the Arctic. The window of opportunity for nesting in the Arctic is short so it is not surprising that these birds have departed the high latitudes already.

For many Arctic-nesting shorebirds like the Semipalmated Sandpiper, two pulses of migration are seen. The first pulse, the one we are beginning to see now, is almost entirely adult birds. These birds have left their young on the breeding grounds, in many cases before the young have even learned to fly.

Shorebirds do have precocial development; they hatch with feathers and can forage for food shortly after hatching. So, adults departing early is really not a form of child neglect. The young will eventually learn to fly and then depart on their migration. These juvenile birds, arriving in Maine mostly in September, constitute the second pulse of the migration.

Most migratory birds have their migration route hard-wired rather than having to learn a migration route. That becomes obvious with birds like Semipalmated Sandpipers who do not have mom or dad to show them the way. Mistakes do occur, however. The chance of a first-year sandpiper showing up at some unexpected location is greater than the chance of an adult appearing at the same place.

To fuel their migration, shorebirds have to feed gluttonously along the way. I think it is fascinating to watch how different species of shorebirds feed.

Let’s start with the plovers. A careful look at a plover will indicate that its eyes are quite large relative to its head size compared to most sandpipers. These large eyes suggest that vision is of primary importance in finding food. That is indeed true.

The foraging behavior of plovers can be called run-and-peck. A plover on an intertidal flat will look for movements at the sediment surface indicating the presence of a crustacean or polychaete worm. It will then run to the location and attempt to grab the prey item.

Keep an eye out for a behavior of Black-bellied Plovers called foot trembling. A plover will stand on one leg and vibrate the other food right at the sediment surface. This trembling appears to set up vibrations that a bloodworm or other invertebrate predator interprets as a possible prey item. The bloodworm comes to surface looking for dinner and finds that it is on the menu of the plover!

Sandpipers, the other major group of the shorebirds, rely on touch to find their food. The bill of a sandpiper is richly endowed with touch receptors, particularly at the tip. A sandpiper probes the sediment until the touch receptors detect the movement of a small invertebrate and the bill clamps down on the prey item.

Particularly for long-billed sandpipers like dowitchers, a significant amount of energy would be required to open the long bill while it was stuck in several inches of mud. To make opening the bill easier, sandpiper bills are rhynchokinetic. That is, the can open the tip of the bill without having to open the portions of the bill closer to the skull. You can see a nice picture of this behavior at: http://birdblog.merseyblogs.co.uk/archives/2006/07/its_called_rhyn.html

Sandpipers on an intertidal flat occur in characteristic spots. Dowitchers and Stilt Sandpipers with their long bills wade in up to their bill and probe rapidly into the sediment. Their probing is often quite rapid and is referred to as stitching (like a sewing machine). Dunlins with somewhat shorter bills tend to forage right at the water’s edge. Short-billed sandpipers like the various peeps feed above the tidemark.

One advantage sandpipers have of using touch rather than sight to find food is that sandpipers can feed at night. Nocturnal feeding is particularly valuable during migration when sandpipers are trying to pack on fat as quickly as possible.

Some years ago, I determined the predation rate of Semipalmated Sandpipers in the Bay of Fundy on their favored prey, the small crustacean Corophium volutator by videotaping foraging birds. Corophium is about 3/8 inch long. I found that each sandpiper was taking about 17,000 Corophium each day. That goes a long weight toward explaining how these sandpipers double their weight in only two weeks.

Recent research has shown that Western Sandpipers acquire a majority of their energy by consuming biofilms. A biofilm is a thin layer of bacteria, detritus and sediment held together by a glue-like material secreted by microalgae and bacteria. Biofilms may be important in the diets of other shorebirds too.

[Originally published on August 23, 2008]