Until the discovery of this species, it was generally assumed that crakes are not, in an evolutionary sense, malleable. The only significant differences between most island crakes and those of mainlands are the island forms’ flightlessness and their unwariness. The Great-billed Crake of Sandy Island is the only crake known thus far to have an obvious physical modification coming from island evolution. Specifically, its bill is proportionally larger and thicker than those of other crakes. It is not truly wedge-shaped like those of the Purple Swamphen and Takahe; instead, it is conical and evenly tapered. This almost toucan-like beak is attached to a large head for a crake; it is not however otherwise a robust bird. The legs are longer and slenderer than those of other island crakes, approaching the proportions of the mysterious Ua Huka Rail. The coloration is similar to Baillon’s Crake, from which it doubtless evolved; the underparts are a warmer shade of gray, and the flank barring is not reduced as in the Laysan Crake. The bill is relatively bright green, with a dark ring around it near the tip. Though its wings are much shorter than in mainland crakes, the Great-billed is not flightless. The nesting of the Great-billed Crake has yet to be studied in detail; it does not seem to differ from other crakes in this regard. Chicks have been seen walking with adults in typical crake fashion.
With its outsized mandibles, the Great-billed Crake covers a very different niche from the Spotless Crake, which is also found on Sandy Island. The Great-billed’s principal prey is the robber crab. A full-grown robber crab is several times the size of a crake, so the crakes generally attack young crabs. Above a certain size of crab, crakes may hunt in packs. As many as 6 or 7 crakes will endeavor to seize a crab’s legs, avoiding the claws, and pull away from each other so that the crab is essentially drawn and quartered. Alternatively, crakes may ride crabs’ backs and attack the interstices between the carapace joints. (The robber crab is really a shelled hermit crab, and its carapace is jointed where a hermit crab’s abdomen is soft.) Robber crabs form only one part of the crakes’ diet. Another important element is bird eggs, which they can easily crack with their outsized bills - even the thick-shelled Giant Pigeon’s. The crakes are the scourge of Dwarf Du during their “bornge”, though not quite as depredaceous as the Robber Starlings. As megapodes’ eggs have especially thin shells, it is an entertaining sight to see the crakes crush Dwarf Du eggs wetly like boba. Due to their breeding in the austral summer, Great-billed Crakes are in turn the victims of egg-hungry migratory Bristle-thighed Curlews.
The Great-billed Crake is a noisy bird. Its piercing screams are one of the most frequently heard sounds on Sandy Island. The Warburton expedition likened the crake’s vocalization to the tenor saxophone extended techniques of Illinois Jacquet, but more recent accounts have contended that they are much closer to those of Sam Weinberg. All manner of courtship and aggressive displays are exhibited by the Great-billed Crake after the fashion of other crakes, with a great deal of “deep knee bends” that seem to strengthen the pair bond.
It has been posited that Cacroenis inornatus, a mysterious name for a Tuamotu rail used by Bruner in his bizarre book of French Polynesian birds, actually could apply to the Great-billed Crake. The genus name could be loosely translated as “crab-dove”, and it is certainly possible that Bruner wound up on Sandy Island thinking it was part of the Tuamotu archipelago, as many land features on Sandy are very similar to those on some of the Tuamotu islands. The kind of storm described at the beginning of The Cruise Of The Kawa would lead to this level of South Pacific confusion.
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