THE SANDY ISLAND GIANT PIGEON (Neoraphus immobilis)
The Giant Pigeon is arguably the most unique and bizarre inhabitant of Sandy Island. Considering that flightlessness and gigantism in pigeons has only occurred in a very few other places - the Mascarenes, St. Helena, and Fiji - the Sandy Island species offers a great comparison with the others. This species is usually slightly smaller than the Mauritius Dodo but still an enormous pigeon at around 26 to 29 inches in length. Like the Dodo, the Sandy Island Giant Pigeon has very small wings. Its legs, however, are rather shorter and slenderer than those of other giant flightless pigeons. The neck and bill are also not quite as long and thick as the Dodo’s; the gape is just as wide. The head is surmounted by a brushy tuft of filoplumes like an egret or a silkie rooster. The tail is short and tapered, not plumous as in the Dodo. This species’ general coloration is gray, like the Dodo; similarly, the face is unfeathered and dark grayish. Rather unusually for a pigeon, the eyes are quite large.
The most remarkable characteristic of the Sandy Island Giant Pigeon is related to its seasonal fat cycle. This species times its breeding with the fruiting of the wattle palm, and as the palm’s fruits fall they become the pigeon’s only diet. The pigeons gorge on the fatty palm fruits, which makes them the fattest of all wild birds - and at that time they are not only flightless, but walkless. The Giant Pigeon is the only species of bird that is (at least temporarily) incapable of locomotion both on wing and on foot. A fully fattened Giant Pigeon moves by rolling on the ground, usually sideways but occasionally end over end. The motion is similar to the trajectory of a thrown balloon full of yogurt, or a closed plastic garbage bag full of milk on a sloping Slip ’n’ Slide. For obvious reasons, Giant Pigeons mate before they get quite to that stage; only once the eggs are laid do they become truly walkless. The nest is a mere scrape in the ground, if that - the island’s lack of terrestrial predators means that Giant Pigeons can just lay their two white eggs (rarely three) on the ground. These eggs are too large and thick-shelled for a Robber Starling to crack, though Great-billed Crakes occasionally crake them; it should also be noted that the Giant Pigeon’s (and Dwarf Du’s) breeding season is in the northern hemisphere summer, when migratory Bristle-thighed Curlews, also notorious egg thieves, are elsewhere.
The Giant Pigeons have a unique incubation system brought about by their walklessness. Their circadian rhythms during the fat season are essentially arbitrary; they eat until they tire of eating, regardless of the time of day. When they can no longer eat they roll to the nearest available nest and sleep on the eggs. (This is surely why the eggshells are so thick!) The nests are close enough together that this takes little effort; when a pigeon wakes it simply rolls off the nest in search of more fruit. The bird’s large eyes are a testament to its occasional nocturnal wakings. Whenever one wakes, it gives the eggs a turn so that they will be properly incubated by the next sleeper. With this system of random incubation (termed by some wags “incubus incubation” but really a modified creche system), there is no guarantee that a Giant Pigeon will ever incubate the eggs that will hatch its own offspring. Once the chicks hatch, the adults continue the same behavior, with some unique twists. The chemical digestive process that in other pigeons creates “pigeon milk” has been repurposed for a different way of feeding the young. It is not entirely understood, but the ultimate effect is that it causes wattle palm fruits to be indigestible, and the adults must regurgitate them - into the chicks’ mouths. This is also how the adults lose weight. By the time the young leave the nest (quite early for a pigeon), the wattle palms have stopped fruiting and the adults begin to regain their walking abilities.
At the start of the nonbreeding season, Giant Pigeons frequent beaches, where they take advantage of the newly arrived Blue Noddy breeding colonies, eating eggs when they can and sometimes even noddy chicks. The pigeons kill noddy chicks with a technique that is unknown elsewhere in the bird world: despite their lesser weight than during the breeding season, the pigeons are still hefty enough to catch downy young noddies by lying down on them and squashing them. This is a decidedly rare occurrence. When there are no more noddy eggs, the pigeons do not risk squashing a lot of noddy chicks. Instead, they go after robber crabs, attempting to crack the shells of small individuals and scavenging crab carcasses when they find them. Success in this endeavor is limited; there are usually enough fallen fruits and seeds from other trees to tide the pigeons over until the wattle palms fruit again.
The Sandy Island Giant Pigeon is not a noisy species. In the nonbreeding season it occasionally utters pig-like grunts and squeals. The same sounds are adapted into a chattery call frequently uttered when rolling to and from the nest - the Warburton expedition thought that if they were ever slaughtered by colonizing humans, the chattering and squealing of the dying birds would resemble the group laughter in the Beatles’ “Revolution 9”, reversed. Breeding males call with a low, blown-bottle-like “brooowlt-brooowlt-brooowlt-brooowlt”.
