Before coming to Palmer, we knew that sheathbills are ingenious seekers/thieves of every possible scrap of nourishment, and that seals are one source of opportunity for them. We'd read of sheathbills eating bits out of seal poop, snipping off scabs and dead tissue from seal wounds, and even of sheathbills snatching milk from the nipple of a nursing seal.
Thus no surprise when, during a period when Southern Elephant Seals suddenly decided that the station's boat ramp was the ideal place to relax, practically a spa, sheathbills gravitated to them and were seen plucking at them in a way the seals did not appear to appreciate.
But it began to seem like more of a discrete thing, a phenomenon worthy of naming and studying. It began to seem as if sheathbills and elephant seals had a relationship that was somewhat similar to the relationships corvids – ravens, crows, magpies, jackdaws, jays, etc. – have with larger animals. The classic example is ravens tweaking the tails of wolves at a kill. This may make an angry wolf spin around, giving another raven a chance to run in and grab something.
All kinds of corvids do this, to all kinds of animals, as in this Youtube compilation. (Other videos and photos in this post are by me.)
Corvids are a famously smart group of birds. Other kinds of birds, including parrots – also famously smart – don't seem to. (I may be wrong about this, but my quick scientific survey of Youtube videos doesn't reveal parrots using their bills on tails of other animals.)
But the sheathbills of Palmer Station do. (I'll bet other sheathbills who are around seals also do.)
What good does this do them? I'll start with the corvids. This is a behavior where I think it's useful to separate ultimate and proximate causes. The ultimate cause is the one that benefits the genes and causes natural selection to benefit the possessors of the genes: ravens who tease wolves get more food. More of their babies survive.
Ultimately, when a raven pulls or tweaks a predator's tail, there's a significant chance the predator will move. Maybe it'll turn and go after the raven, and that raven's mate will be able to steal food. Basically, if they create a disturbance, there may be opportunity.
The proximate reason is the one that actually motivates an individual raven. Which might be trying to get more food. Or showing other ravens how brave and funny you are. Or just seeing the wolf jump.
It may be that on a day to day basis, a raven tweaks a wolf's tail, or a magpie pecks a dog's tail because it's hilarious. This is the same reason you might surreptitiously sneak up behind a person sitting in the galley and zip-tie their ankle to a chair leg.
Why do sheathbills do it? The elephant seals aren't crouching over kills. Sometimes it seems as if a sheathbill is actually nipping something off an elephant seal. Something it will eat. A bit of peeling skin, a scab. Nutritious things. (Don't knock it if you haven't tried it.) Similarly, if a sheathbill tweaks a seal's tail and makes it swivel around, the move may expose something the sheathbill can eat – seal poop, or bloody snow if the seal has wounds from fighting.
I suppose it's possible, although I am not convinced of it, that a sheathbill also might tweak a seal's tail in order to impress another sheathbill. Seriously, that's the first thing some ethologists would look at. Although they would call it displaying fitness.
Okay, so. If sheathbills have a twisted sense of humor, and super-smart birds like corvids have a twisted sense of humor, does that mean sheathbills are smart? Not necessarily. But I think it's a possibility worth looking at.
Addendum: An intelligent reader (Hi, Anita!) mentions a duck she once had who used to sneak up and pinch her dog's tail.
Interesting. Why might a duck have such a behavior in its repertoire? Not to scavenge on carcasses, or make an elephant seal shift position.
I'm guessing the answer is nest defense. All kinds of birds get bold when their nests are threatened, or they think their nests might be threatened. Sometimes crazily bold. They'll chase larger birds, dive-bomb them, pluck at them. I once saw a pair of Western Kingbirds chase a Great Horned Owl. One landed on the flying owl's back, grabbing feathers with both feet, before wisely peeling off.
An evil-minded duck might well decide to apply that behavior to a dog even if the duck had no nest.