This species, like the Great-billed Crake, is an example of evolutionary malleability in a genus that was not believed to be so variable prior to the species’ discovery. As its name implies, the Giant Fantail is by far the largest of all fantails. A foot long, it is proportionally shorter-tailed but just as fat-headed and short-billed as most other fantails; the combination of size and proportion recalls a cuckoo or nightjar. The latter comparison, however far-flung, is accurate; the Giant Fantail’s gape bristles are certainly luxurious, and the species does appear to be crepuscular from time to time. The fantail’s coloration is very variable, with well-defined forms but many intermediate individuals. There is a pied form, black-and-white very similar to a Willie Wagtail but with less white on the belly, and a barred form with the same pied head pattern in dark brown instead of black, the underparts also dark brown heavily barred with buff. A similar form exists with streaked rather than barred underparts, which can also be black and white instead of brown and buff. Yet another form is entirely rufous. All of these forms can also have a white or buff throat, rump, and/or wing bars. The bill and legs are often black, but especially rufous-form birds can have a horn-colored bill. This polymorphism appears to be mostly unrelated to sex, as several combinations of two forms have been observed mating; however, all barred birds observed have proven to be males. Juveniles are generally gray with a faint head pattern and wing bars.
The Giant Fantail approaches the cuckoos, none of which breed on Sandy Island, in both appearance and niche. It has developed a unique system of brood parasitism that is ultimately cuckooish but with several kinks. The plumage polymorphism is an adaptation related to the broad range of host species, compensating for the absence of egg color polymorphism. Females appear to parasitize the host species they resemble the most: pied ones lay in the nests of White-breasted Woodswallows or Long-tailed Trillers, buff-bellied ones choose Melanesian Whistlers, rufous ones Island Thrushes, streaked ones Robber Starlings, and one all-black female was observed around the nest of a Melanesian Cuckooshrike. Fantail chicks dominate by outcompeting, rather than throwing host eggs out of the nest as cuckoos do. A female fantail probably identifies the male of a host pair by his song; she then spies and sneaks by skulking, waiting for him to replace his mate on the eggs. In the middle of his shift, the fantail spells him; they appear to know the host species’ contact calls and fool them thus despite the difference of size. This vocal mimicry of the host extends to the male song, as it does in those charitable parasites, the whydahs. Here, however, it is rather unusually adapted as the song’s quality of mimicry is rather poor. For example, the triller imitation is usually at only half the speed. The whistler impression is an octave lower, frequently with three whiplashed phrases instead of one. As for the thrush, the fantail’s version sounds like a thrush run through a highpass filter and a distortion pedal. The most remarkable quality of the fantail’s song is its double mimicry; as the woodswallow is already a mimic, a fantail will occasionally imitate other birds in a woodswallow’s typical order, but much more sloppily!
But the fantail is only a sloppy mimic when singing. Not only is it a brood parasite, but it is also an incorrigible kleptoparasite. Like its cousins the drongos, it has a wide variety of alarm-type calls with which to fool other species into dropping the yummy grubs they may have found. Several non-passerine calls are imitated for this purpose, including the “krawk” of the Dwarf Du. It is interesting to note that all the endemic Sandy Island birds essentially lack an alarm call; that “krawk” is mostly used to summon other Dwarf Du. The only bird predator on Sandy Island is the Barn Owl, which eats a lot more lizards than birds on small islands.
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| Color variations of the Giant Fantail. |
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