The sun dips low over the bay, its fading rays gilding the avocets as they swish their heads through the water. The egrets eye their own reflections as if in profound self-contemplation. A willet flashes past, its black-and-white wings an exclamation in the dusk.
Faced with such beauty, two words come irrepressibly to mind: niche partitioning.
Okay, it’s not a particularly lovely phrase in itself. But niche partitioning is why we can have so many different kinds of long-legged, long-billed shorebirds stalking through the same water, apparently hunting the same food, and yet all coexisting.
Natural selection favors the fittest. This should lead you to expect that one shorebird species would be the fittest—the best at hunting and surviving and reproducing in this shallow-water-and-mud habitat—and, out-competing all other shorebirds, eventually become the only shorebird around. Why doesn’t this happen? Niche partitioning.
A niche is a set of biological conditions: temperature, habitat, types of food, etc. A species’ niche is the set of conditions in which it thrives. Some species have broad niches, being able to do well under a large range of conditions; think of a cockroach, or a pigeon, or the common house mouse. My little buddies the juncos have a fairly broad niche, which is why I was able to study them under a variety of different environmental conditions. Still, there are many things that are outside their niche: juncos could not survive in a punishing desert, or Antarctica, or by trying to live off of nectar from flowers. Other species have narrow niches, specializing in very specific circumstances: cave salamanders, or tubeworms found only next to deep-sea hydrothermal vents.
The niche of shorebirds is, well, the shore. They feed in the same areas and with the same basic strategy: stick your bill in the water (or mud), pull out food, eat, repeat.
Except that this isn’t the whole story. The reason that they can all coexist is that they don’t occupy the same niche; they partition that broad shore niche into narrower, more specific niches, and each species specializes on its own smaller niche. You can see evidence of this in the shapes of their bodies. The shorter-billed Willet won’t be able to reach deep enough into the mud to catch the prey that the Long-billed Curlew can.
The upcurved bill of the avocet is well-suited to its characteristic scything behavior, where it swishes the bill through the water just above the surface of the mud, capturing anything the bill touches as it sweeps. The straight-billed stilt is instead adapted for pecking and grabbing at organisms it sees.
Some of these variations are quite subtle. The bill of the Greater Yellowlegs looks very similar to that of the Willet, but look closely and you’ll see a slight upward curve to the yellowlegs’ bill that the willet’s bill lacks.
It’s not always about the bill. Great and Snowy Egrets have basically identical bills, body plans, and hunting strategies; the difference between them is simply that of size.
Size matters. Here is our snowy, stalking along in the water, looking for prey and then stabbing down to seize it. What kind of prey is she getting?
Worms. Yum! I watched this Snowy Egret catch half a dozen worms in a few minutes, barely taking more than a few steps between each capture. On the same evening, in the same place, I watch a Great Egret stand very still for a long while, barely twitching, then plunge down to make his kill.
It isn’t that the Snowy Egret couldn’t have caught that fish, or that the Great Egret couldn’t catch worms. There are some large prey that the Great Egret can take which are out of the snowy’s repertoire—I have seen a Great Egret eat a vole, which I don’t think a snowy could subdue or fit down its throat—but that little fish is fair game for both of them. The difference here isn’t one of ability but of costs and benefits. For the smaller snowy, the worms are big enough to be a good snack, worth the work of catching them. For the larger Great Egret, the worms are so small relative to his body size that it’s better, instead, to ignore them and wait for something larger. The end result is that these two near-identical species can forage side-by-side, eating different things.
This is niche partitioning: a bay-ful of prey animals divided among predators large and small, visual and tactile hunters, long- and short-billed.
The divisions among the niches aren’t perfect. The Snowy and Great Egrets would both happily eat a small fish, as would pretty much any other bird mentioned in this post. Animals can adjust their behavior, too: stilts usually forage visually, spotting prey and then pecking at it; but at night, or when the wind is so strong that it disturbs the surface of the water and makes it hard to see, stilts switch to scything through the water like avocets. Still, even slight differences in niche allow all of these species to pursue their own specialties most of the time—and let us humans enjoy diverse shores.