The black-fronted tern (Chlidonias albostriatus) feeds on insects found in braided rivers as well as native fish and lizards.
Each bird species on braided rivers has evolved to feed on insects in distinct ways. Specialisation minimises competition for food between the bird species.
Black-fronted terns and black-billed gulls feed on the wing over main channels, catching insects in the air or scooping fish and insects from the water’s surface. They sometimes feed on insects from surrounding farmland.
In a braided river the availability of food is always unpredictable. During lean times, the birds must range from the riverbed into stable side channels and pond areas to find food.
Image: Peter McIntosh ©
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.
Terns and gulls nest in colonies on open shingle. This gives parent birds a better chance of noticing approaching predators. Groups of birds are better able to scare them off.
Their eggs and chicks are also well camouflaged. But, unlike other species, young gulls and terns must remain near the nest, relying on parents to bring food until they can fly and hunt for themselves.
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. Terns, gulls and oystercatchers may dive-bomb and call loudly.
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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Help braided rivers and the plants and animals that live in them.