We’ve all watched birds pecking at trees working their way around the trunk and sticking their bills under the bark. These are the insectivore probers, looking to extract an invertebrate
morsel. The penduline-tits of Eurasia, so named because of their pendulum-shaped nests, have conical, sharply pointed bills which they use to open the stems of reeds to expose insects like the larvae of flies and moths as well as adult wasps and spiders. The Brown Creeper of North America is a small five inch bird with a sharp downcurved bill and spiky tail feathers. It flies to the base of a tree and works its way upward, spiraling around the trunk as it searches under and around the bark for insects, their larvae or eggs, and other small creatures. Its long claws and stiff tail move the bird upward as it clings close to the tree looking like a piece of bark. Nuthatches work in a similar manner but with a sharp straight bill, short tail and the habit of starting at the top of a tree and working their way downward. Although mainly insectivorous, they get their name from “hatching” – actually cracking nuts and seeds open with their sharp bill. Like a few other birds, nuthatches will cache food, both insects and seeds, but typically with only one food item per cache.
The birds which have honed probing to a real science are the shorebirds. Probing into the mud for invertebrates such as worms, insect larva, amphipods, crustaceans, and mollusks, their bills go shallow, deep, or somewhere inbetween, reducing competition for food on and below the surface. Short billed plovers probe from the surface down to a depth of six centimeters while the very long-billed curlews feed at all levels down to 20 cm. The medium-billed stilts feed between eight and 24 cm. So although the actual depth at which a bird species can feed is constrained by its bill length, the birds also reduce competition by sharing the habitat. The Long-billed Curlew would not bother with prey near the surface since other birds can reach those and thus probes deeper than other birds. Avocets have long upcurved bills but although they do some probing, they generally sweep their bill side to side through the water or silt in search of prey or swim to deeper water and dabble, head down, like ducks, for invertebrates.The Sanderling, a name used by many seafood restaurants, is a small shorebird that picks invertebrates off the seashore or probes only shallowly. Breeding in the circumpolar north, they winter on marine shores around the world. Their feeding habits give them away immediately as they typically run in flocks at the edge of the surf, up and back, up and back, picking away at the isopods and mole crabs that live in burrows but rise close to the surface at the water’s edge. The softer sand makes the creatures more vulnerable to the Sandering’s incessant probing.
Ever try probing? Think about putting a long pair of forceps in to the muck; how do you grab something? It would be difficult to open the entire forceps. Long billed shorebirds have figured this out – only the tips of their bill open and close – sort of like one of those picker-upper gadgets one would use to collect litter from the curb. But how do you find food to grab it? Recent research has shown that many shorebirds such as the Western and Least Sandpipers and the Red Knot can detect prey under the sand of the shoreline without seeing, smelling, or touching it. Herbst corpuscles, sensory organs just underneath the surface of the bill are the answer. Sticking their bill only one centimeter into the sand, the birds can detect the presence of a rock or a prey item because the corpuscles can detect differences in the pressure gradient. For many years it was thought that the kiwi locates its worm prey by the sense of smell since the nostrils are located near the tip of the bill and the olfactory lobes of the brain are large, but recent research indicates that kiwis have mechanoreceptors similar to that of shorebirds. Ibises also possess this adaptation and since the birds are not closely related, these mechanoreceptors must have evolved independently in these groups.