It’s easy to add pre-trip postings here from my desk in Virginia, less than 2 weeks out from the trip, so I will say a few things now that I’ll probably be too busy or too tired to write about from Uganda. Besides, once I’m there all this pre-study musing may be obsolete quickly, the new material intrinsically more interesting.
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Let me leap back nearly a half-century to 1973/4, when I spent two summers in Mexico (San Blas, Nayarit…a sleepy shrimping village on the Pacific coast about 200 miles south of Mazatlan). I was doing a side (non-thesis) project on a curious species, the boat-billed heron, Cochlearius cochlearius. Boatbills were once placed in their own monotypic neotropical family, Cochleariidae, because of their remarkably odd bill, which is about 3-4x wider than the narrow bills of other herons, but its post-cranial skeleton is essentially indistinguishable from that of the cosmopolitan black-crowned night-heron. The skull itself is a bit weird, but that’s related to supporting the bill.
Boat-billed heron
Black-crowned night-heron
I bring up that old project in Mexico for a couple reasons. First, it was my first adventure-travel research experience, my first time living outside the U.S., etc. The work was worthwhile and helped me build up my early-career publication list. I came to appreciate the value of living abroad…the perspective of distance on how one regards ‘home.’ The end of U.S. involvement in Vietnam was underway and that had been a dominant backdrop for my generation, but I had no idea how the rest of the world viewed America. During my second summer in San Blas, Richard Nixon resigned the presidency over the Watergate scandal and my home country went into all sorts of spasms, but word of that did not even reach the outpost of San Blas for nearly two weeks! Even more surprising, when that news did arrive, the locals were so delighted that they threw a spontaneous street party! It had never occurred to me that non-Americans pay far more attention to our politics than we do to theirs!
More to the point of this blog, that boatbill study started me wondering about why the ancestors of such a predatory bird might have benefited from genes favoring an extra-wide bill – the problem I am now pondering with shoebills. A year later, I published a note in a bird journal speculating why boatbills have odd bills, later revising that argument a bit (e.g., in Dan Janzen’s edited Natural History of Costa Rica). In a nutshell, I now think of that heron’s wide bill as most likely an evolutionary response to poor aim.
Let me explain with a sports analogy. In baseball, the biggest mitts are worn by players (outfielders, first basemen, and especially catchers) whose primary concern is catching a fast-moving ball, while other position players (e.g., infielders) use smaller gloves because their challenge is a bit different, namely getting the ball OUT of their gloves for throwing. The biggest mitts of all are worn by catchers who work with screwball pitchers (that’s a type of pitch, not a comment on mental health): nobody knows just where those balls are going!
Boatbills commence nesting at the beginning of the rainy season, when billions of shrimp migrate from the ocean into the brackish mangrove swamps for breeding. These herons feast on the temporary super-abundance of food. I spent most of my time inside a floating blind tied to some red mangrove roots near the herons’ nesting colony, but to reach my observation blind I had to walk through a quarter-mile of warm swamp water in pitch darkness (at about 4 AM). Of course, I used a flashlight to find my way, but when that artificial light was off, the darkness was profound: I literally could not see my own hand in front of my face. A Gossen LunaPro light meter, sensitive to 0.01 foot-candle of ambient light, actually registered zero! (The reason for this is evident: the solid rainy-season cloud cover simply blocks all moon- and star-light.) Sitting in my blind, I could hear shrimp splashing all around me, but I couldn’t see them and when the the first light of dawn arrived the shrimp went silent…no longer feeding at the surface.
From this came what we might call the Catcher’s Mitt Hypothesis for the boat-billed heron’s schnozzola: extra width compensates for imperfect aim. In this context, the constraint is profound darkness that plausibly forces the bird to strike mainly on the basis of sound (triangulating from information received by left-and-right ears). Night-herons in general have enlarged eyeballs and a tapetum layer behind the retina that reflects incoming light thus doubling the light’s power to fire visual receptors; but if there is essentially NO ambient light, doubling it doesn’t help much. My conjecture, then, was that the bill evolved to help boatbills make contact with shrimp/surface-visiting prey largely on the basis of acoustic information (splashes).
From there, you can see my thought process about shoebills. But…uh-oh…shoebills are DIURNAL and avian eyes are terrific at daytime vision. So why would an ancestral shoebill have needed a catcher’s mitt?
Please understand that this is all playful speculation, but here’s how I am approaching the project. According to what I can read about shoebill foraging habitat, the water is VERY MURKY in addition to being warm and low in dissolved oxygen. Air-gulping prey (lungfish, bichirs, catfish, baby crocodiles, the occasional turtle...) that live in these shallows must rise to the surface 10-12x/hour to get their oxygen, but the turbid water’s opacity means that the fish can’t look up to see potential danger (predator) and the predator can’t see them, either. At least, the shoebill probably can’t see them until they get very close to the surface and even then probably can't see more than a shadowy shape. In the last few cm (where prey can’t breathe yet) the predator should have a brief visual advantage. Aquatic animals living in silty water tend to have small eyes and poor vision, simply because vision is of reduced value to them. Lungfish, for example, have small, weak eyes, finding their own prey (worms, crustaceans, etc.) primarily by touch and smell. Unsurprisingly, air-gulping aquatic creatures do not tarry at the surface (obvious danger), but gulp and go. However, as they rise to breathe, they must first exhale the old, oxygen-depleted air from their lungs producing bubbles that indicate an imminent breathing event.
Two fairly reasonable and testable predictions can now be proposed. (1) Foraging shoebills are probably very attuned to bubbles per se. And (2) the predator’s strike may be done just before its target actually breaks the surface. If the fish is still a few cm below the surface of semi-opaque water when the bird strikes, the predator may see it before it can see predator. But the shoebill may not be able to see clearly which part of the body it is attacking. With the strike, the big terminal hook at the tip of the shoebill’s beak sinks into muscle for a firm grip, and the wide bill may serve to get an imperfect grip on the fish if/when the hook misses. The bill’s width may also provide leverage when wrestling the slimy/mucus-covered fish’s body free of the floating macrophytes.
One anecdotal clue fits nicely with this second prediction. Many/most of the photographs showing shoebills with freshly seized prey also show considerable plant material that was nabbed along with the fish.
Kenneth Tumusiime and I will be watching closely (and videotaping, whenever possible) for details. Does the strike typically precede the fish surfacing? What part of the fish’s body is grabbed? Are the edges of the bill used (e.g., for leverage when wrenching the unhappy fish from its weed-choked water)? Incidentally, the edges of the bill (“tomia”) are said to be so razor-sharp that the bird can literally bite the heads off fish (I’ll have to see this to believe it)!
And maybe all this pre-study speculation will turn out to be sheer nonsense? Maybe the evidence will take us in some totally different direction.
In any case, let me hasten to say that I’m not thinking the wide bill puzzle has a univariate explanation. The wide bill is (again anecdotally) said also to be used for digging estivating lungfish out of soft mud during the dry season (…the Shovel Hypothesis?) and good video can be found online of the wide bill used for TRANSPORTING WATER to the chicks (both for hydration and for cooling), so it serves a bucket as well. But prey-capture is generally the first task one considers when contemplating peculiar bills on predatory birds.
Finally, the main reason we’re doing this fieldwork in October is that this is when shoebills hatch their eggs at Murchison Falls. Reproduction is often when the Darwinian stakes are highest, as breeding age adults must forage efficiently enough to create offspring and provide them with expensive parental care. Avian parents typically time their nesting so that their offspring will have enough food for rapid growth and then survive the transition to full independence. It thus seems best to focus on adult hunting behavior during this period, when they’ll be using all their best tricks. And, of course, we also hope to find nests to videotape the chicks’ siblicidal behavior.
Okay, now you’re all caught up with virtually everything I know or suspect about this strange bird and its strange bill. A month from now I hope we’ll all know a good bit more.
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