Birds
Wrybill
Anarhynchus frontalis · also called wrybill, ngutu parore
The wrybill is a small, pale grey plover from New Zealand with a feature shared by no other bird on Earth: a bill that curves sideways. It always bends to the right, and the wrybill uses that curve to reach under the smooth, rounded stones of its braided-river home and sweep out the insect larvae hiding beneath.
Overview
The wrybill is endemic to New Zealand, breeding on the braided rivers of Canterbury and Otago and wintering on North Island harbours. It is a small plover, 20–21 cm long, pale grey above and white below, with a thin black breast band when breeding and a short black bill bent to the right. It eats mayfly and caddisfly larvae in summer and estuary invertebrates in winter. The whole species numbers only about 4,000–5,000 birds.
The sideways bill
No other bird has a laterally curved bill. The wrybill’s bends right by 12–26 degrees, and the bird works around each stone in a consistent direction so the curve slides into the gap beneath — a finely tuned adaptation, not a quirk.
Two habitats, two seasons
It nests on open shingle, its eggs and chicks almost invisible against the gravel. After breeding, most of the population moves to a few North Island harbours, chiefly the Firth of Thames and Manukau Harbour, forming large flocks that wheel over the mudflats in tight formation.
Status
The wrybill is Vulnerable and declining. Its braided rivers are degraded by introduced predators, invasive lupins and willows, water abstraction, and vehicles, and its concentrated wintering sites are also under pressure.
Wrybill: common questions
Why is the wrybill's bill bent sideways?
It is a feeding adaptation to the wrybill's specific habitat. The bird forages on braided riverbeds carpeted with smooth, rounded stones, and its food, mostly insect larvae, shelters underneath and around them. A sideways-curving bill lets the wrybill slide the tip in under the edge of a stone and sweep it in an arc through the space beneath, scooping out prey that a straight bill could not reach. The bill always curves to the right, and the bird tends to circle each stone anticlockwise so the curve is working into the gap, which suggests the whole behaviour is finely tuned.
Is the wrybill the only bird with an asymmetrical bill?
It is the only bird with a bill that is bent sideways. A few other birds have bills that are unusual in other ways, such as crossbills, whose upper and lower tips cross over each other for prising seeds out of cones, and the wrybill's own left-right curve is sometimes compared to that. But a bill that bends consistently to one side, in the horizontal plane, in every member of the species, is unique to the wrybill among the roughly 11,000 birds in the world.
Where do wrybills go in winter?
They leave their South Island breeding rivers and migrate north, within New Zealand, to spend the non-breeding season on the intertidal flats of a small number of North Island harbours. The Firth of Thames and Manukau Harbour hold the great majority of the world population at that time. There the birds switch from prising larvae out of riverbed stones to probing estuary mud for small invertebrates, and they gather in large flocks that perform tight, synchronised aerial manoeuvres over the water, especially when a falcon or harrier appears.
Why is the wrybill threatened?
Its breeding habitat is in trouble. Braided rivers are naturally shifting, open, gravel-bed systems, and the wrybill nests out on that bare shingle. Introduced mammals such as stoats, cats, ferrets, and hedgehogs eat the eggs and chicks, invasive plants like lupins and willows choke the open riverbed and give predators cover, water is taken for irrigation and hydro schemes, and off-road vehicles crush nests. With only a few thousand birds left and a small number of concentrated wintering sites that are themselves under development pressure, the species is classed as Vulnerable and declining.