Why do Sandpipers and Plovers "hang out" together?
I walk on Del Monte beach in Monterey, California quite often.
I see LOTS of Sandpipers there; and always with them are Plovers (which remind me of little frenetic Killdeer).
The Sandpipers dig into the sand when the tide rolls back, seeking, I am told, sandfleas and perhaps tiny crustaceans. The Plovers are there "picking up the slack" or "eating the crumbs that fall from the tables" of the Sandpipers, it would seem.
Am I right - are they simply after the same sort of food? If so, why do the Sandpipers (larger and more numerous) tolerate the Plovers? Or is there some sort of symbiotic interchange occurring here?
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You're right that the various species of shorebirds, including in your area, like most of the same types of foods. The diet is primarily comprised of invertebrates such as crustaceans, worms, mollusks, and insects.
Although some species are indeed bigger, they tend to co-exist peacefully. One reason is that they have different types of bodies and bills, and are designed to eat at different depths, areas and locations. There's plenty of food to go around, and the birds know how to find it.
Sandpipers have long thin bills, which have what are called "tactile receptors" in the tip. This means that they feel their prey before seeing it. They open their bill a little before poking the water, effectively turning it into a forceps, which they use for pulling up food that's below the surface. Less frequently, they open their bill and run in shallow water, catching little fish along the way. Many shorebirds, including some sandpipers, have an ability to curl up only the front section of the top bill. This fascinating feature called Rhynchokinesis enables them to open wider to catch larger prey. Birds with varying degrees of it dig at deeper depths, which is one reason why there's no need to fight for food.
Plovers have shorter bills and better vision, so they use a run-stop-hunt feeding method similar to that seen in robins and other land birds. Plovers prefer to eat higher up on the beach. They enjoy finding small crustaceans tucked into pockets of seaweed which wash up with the tide, and are built to easily maneuver their way in and out of that. They also feed at the edge of dunes, tide pools, other sandy areas ignored by other shore birds, and even at the base of low-growing plants.
Although considered a shorebird, killdeer are the least frequent bird found at the water's edge, and don't get in the way of others who are eating. In fact, if you think you see a shore bird on the golf course or in an open field, it's likely to be a killdeer.
Source materials and further research:
Western Snowy Plover Natural History
Sandpipers in Monterey California
Shorebirds in Northern California
Distal rhynchokinesis in Purple Sandpipers
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"hanging out" together happens because different species eat the same food, require the same nesting areas, migrate in the same flyways, etc. Many species compete/become territorial for nesting areas, but less so for food sources.
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This is really just a hypothesis, but here in the UK we also get mixed wader flocks. A significant reason for large/mixed flocks to occur is predation. Watch out for what happens when a large flock of waders (almost by definition very exposed on the ground) spots a falcon. They scatter in every direction, not maintaining a straight line for any time at all. This makes it hard for the falcon to pick a target and strike. Add more birds (of any species) to the mix and it increases the confusion. It also increases the number of targets, reducing the chances of any individual bring picked off, and even the number of eyes to spot a predator.
Of course, any costs of competition must be taken into account, and if the sandpipers' and plovers' food sources overlap this will be real. But there's also a cost (energy, feeding time, not looking out for predators) to chasing other birds away. This must be weighed up against the potential loss of food, especially if the smaller species may be quite persistent due to a lack of anywhere else to go.
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