I've been busy in Florida, working at the Space Coast Birding and Wildlife Festival. The weather's been sunny and in the 70's. Liam's with me. We're in heaven.
This afternoon we raced over to Canaveral National Seashore and watched birds.
I watched a willet looking a little full in the crop. I don't have time to crop or edit these photos, but I wanted to toss them out to you. Maybe someone out there knows what's going on here.
It began gagging
and it brought up a disorganized bolus
which dropped on the sand. The willet contemplated this
and then began re-ingesting the bolus, bit by bit.
It ate pretty much the entire thing, in little pieces.
I got the feeling that this is something willets do, because I saw another bird do exactly the same thing on down the beach.
Hmm. I wondered if it was like rabbits, who eat the droppings they excrete soon after a meal, and essentially digest the meal twice. Yuck, but it works for rabbits.
So the next bird I saw doing this, I apologized to and interrupted. I ran over and picked up the bolus.
I would love to know what's going on here. How does a bird crush and digest these things in its gizzard? It would seem to me to take an enormous amount of pressure to crack and extract the meat from such tiny closed bivalves. Does regurgitating them and re-ingesting them somehow help the process? Is it regurgitating the things it found and ingested blindly in deep sand, then coughing them up to pick the real food (clamlets) out and leave the indigestible rocks and sand behind?
(I kind of like the feel of this theory in my brain).
For that matter, how does it find all those perfect baby bivalves? It probes blindly deep in the sand, picking up what it can feel. It must be able to tell a clam from a rock with the tip of its sensitive bill. How?
The willets aren't talking.
10 comments:
Donax. You can find them pretty easily on the beach by scooping a small hole in the wet sand as a wave wash runs back to the sea.
You should see mole crabs (sand fleas) in the same hole..
Everyone will be scurrying to rebury.
In a quick Google check I found several other mentions of the phenomena you observed, without explanation. Perhaps one the authors of this paper (if still around!) know the purpose:
http://tinyurl.com/mwsxvyk
Curiouser and curiouser!
(many) Shorebirds have pressure sensitive bills - I've forgotten the great book that describes how the upturned bills (vs decurved) use pressure-sensitivity and angle of attack to hone in on their underground prey and to distinguish rocks from "clamlets". Clamlets it turns out have different pressure gradients then rocks - and it can be detected by the bill.
Here's one link on the web:
http://www.mapoflife.org/topics/topic_454_Pressure-sensitivity-and-the-tactile-sense-(excluding-the-lateral-line)/
The Scolopacidae, a family of shorebirds that includes the sandpipers, possess elongated bills, which they use to probe coastal mudflats and sands for buried invertebrates. Intriguingly, most species do not need to touch their prey directly but use a sensory structure in the bill tip that provides a remote touch sense. This bill-tip organ consists of a set of characteristic bony pits housing clusters of mechanosensory cells known as Herbst corpuscles. These corpuscles are strongly convergent with the Pacinian corpuscles of mammals and evidently serve to detect pressure gradients within the sediment. The precise mechanism of prey localisation, however, is still unknown.
http://people.eku.edu/ritchisong/birdbrain.html
Birds "Feel" Their Prey Under the Sand - Red Knots (Calidris canutus) can locate their favorite food (shellfish) in wet sand by inserting their beak half a centimeter into the sand for a few seconds (Piersma et al. 1998) This ability was demonstrated in experiments in which researchers hid small stones in the sand. Because stones do not send out any signals, the ability of Knots to detect them must be based on the sensitivity of their beaks to differences in currents in the water in wet sand between the individual grains, stones, or shells. Knots used in the experiments were unable to find hidden stones in dry sand. At the end of their beak, knots have clusters of 10 to 20 Herbst corpuscles that are sensitive to differences in pressure. When the bird sticks its sensitive beak into the sand at low tide, it produces a pressure wave because of the inertia of the water in the interstices between the particles. The pattern thus created betrays the presence of objects larger than the grains of sand. The rapid up-and-down movements of the bird's beak loosen the grains of sand, which then become packed together more tightly, displace the interstitial water, & cause the residual pressure around the object to increase. The nature of their localizing ability means that knots cannot distinguish between stones & shellfish in the sand, which is why they rarely look for food in areas where the sand contains stones, no matter how much shellfish could be found there.
http://people.eku.edu/ritchisong/birdbrain.html
Birds "Feel" Their Prey Under the Sand - Red Knots (Calidris canutus) can locate their favorite food (shellfish) in wet sand by inserting their beak half a centimeter into the sand for a few seconds (Piersma et al. 1998) This ability was demonstrated in experiments in which researchers hid small stones in the sand. Because stones do not send out any signals, the ability of Knots to detect them must be based on the sensitivity of their beaks to differences in currents in the water in wet sand between the individual grains, stones, or shells. Knots used in the experiments were unable to find hidden stones in dry sand. At the end of their beak, knots have clusters of 10 to 20 Herbst corpuscles that are sensitive to differences in pressure. When the bird sticks its sensitive beak into the sand at low tide, it produces a pressure wave because of the inertia of the water in the interstices between the particles. The pattern thus created betrays the presence of objects larger than the grains of sand. The rapid up-and-down movements of the bird's beak loosen the grains of sand, which then become packed together more tightly, displace the interstitial water, & cause the residual pressure around the object to increase. The nature of their localizing ability means that knots cannot distinguish between stones & shellfish in the sand, which is why they rarely look for food in areas where the sand contains stones, no matter how much shellfish could be found there.
Swallow whole, upchuck, nibble, makes perfect sense to me.. In the bird the Coquinas expire, their strong little adductor muscles relax, the shells open and the willet nibbles the meat.
Anyhow, thats how I would do it if I were a willet.
I'm no bird expert, but wouldn't the shells act like grit in the gizzard and help with digestion?
Might be interesting to see what comes out the other end...concerning the shells and their condition I mean. This blog never ceases to amaze me!Love it!!
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