Adapted from Jim Conrad's online book A Birding Trip through Mexico, This excerpt from "The Beach at Mazatlán" in southern Sinaloa state
These Sanderlings are in the same family as the Spotted Sandpiper seen at Témoris. Like Spotted Sandpipers with their polyandrous nesting practices (one female nesting with two or more males at the same time), Sanderlings also have a nonstandard mating system. Both male and female Sanderlings incubate their own separate clutches, and sometimes they even enter into polyandrous relationships like Spotted Sandpipers.
In evolutionary terms, the Sanderlings' one-nest-for-each-mate system is thought of as the probable "first evolutionary step" toward polyandry. In fact, the Sanderlings' relationship is technically referred to as protopolyandry. Protopolyandry makes sense in the Arctic region where Sanderlings breed, for Arctic summers are so brief and unpredictable that if a nest is lost to a predator or a storm, renesting is impossible. Having two nests reduces the risk of total loss of young.
Sanderlings are sometimes seen doing something else that at first glance seems unlikely. Down the beach we just saw a Willet. Sometimes Willets, which are much larger than Sanderlings, will be seen tearing apart prey such as large sand crabs and a Sanderling will draw near the Willet and begin running off other Sanderlings in the Willet's vicinity. It looks as if a turncoat Sanderling is protecting a Willet from the Sanderling's brothers and sisters. What's really happening is that crabs are too large for Sanderlings to rip apart themselves, so the Willet-protecting Sanderling is actually just hoping that as the big Willet tears into its crab a few fragments will fly to the side for the Sanderling, and that Sanderling doesn't want any Sanderling competition for the scraps.