Wrybills (Anarhynchus frontalis) feed on insects found in braided rivers.
Each bird species on braided rivers has evolved to feed on insects in distinct ways. Specialisation minimises competition for food between the bird species.
Wrybills feed in shallow channels, riffles and the edges of pools. Their bent bill is specially adapted toallow them to reach under stones for mayfly larvae.
In a braided river the availability offood is always unpredictable. During lean times, the birds must range from the riverbed into stableside channels and pond areas to find food.
Breeding on a riverbed is a risky business. Many eggs and chicks do not survive. Riverbed birds have adapted to cope with floods and are able to renest if eggs or chicks are lost. This is an important adaptation for breeding in an unstable environment where floods are common.
Each pair of wrybills, stilts, dotterels and oystercatchers defends a territory and nests alone. These species rely on camouflage to protect their eggs and chicks. They also actively defend their nest using distraction behaviours. The chicks are active soon after hatching, leaving the nest to follow the parents as they forage for food. Within hours, newly-hatched chicks can hunt for food and swim if necessary.
Birds that find and hold good nesting sites are more likely to raise chicks successfully. The best nest sites have:
- islands surrounded by a moat of water for protection from predators
- high points which are less flood prone
- little or no vegetation for all round visibility
- a good food supply close at hand
- little or no disturbance.
Swamp harriers/kāhu and black-backed gulls/karoro are natural predators of braided river birds. However, these predators have taken advantage of changes made by humans and their numbers have increased dramatically. More predators in an area pose a greater threat to ground-nesting river birds trying to protect their eggs and chicks.
Over time braided river birds have developed good camouflage to cope with native avian predators. Nests are hardly more than a slight hollow in the gravel, and the eggs blend well with the stones. The speckled chicks are also difficult to see and have a natural instinct to freeze when alarmed. Adults’ feathers are greys and browns to blend with the river environment they inhabit.
Parent birds defend eggs and chicks by distracting the predator away from the nest. Wrybills, oystercatchers and dotterels often pretend to have a broken wing to lead predators away.
New Zealand’s ground-nesting birds have evolved without mammalian predators that move quickly and have a keen sense of smell to target prey. The defences our native birds have developed against avian predators are little use against introduced predators such as cats, stoats, ferrets, rats and hedgehogs.
Ensuring the survival of the birds’ natural open habitat is important in combating predation.
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