Those migratory flocks, heading inland toward late-winter breeding grounds in the salmon gum woodlands, were estimated to number into the tens of thousands. The local Noongar people called these noisy, gregarious birds ''ngo-larks'' or ''gnulars'' - names evoking the inflection of their raucous calls. The colonial settlers called them ''rainbirds'', because their movements seemed to coincide with seasonal rains, and the flocks of black cockatoos were considered omens of good luck.

But less than 200 years later, Carnaby's black cockatoo (Calyptorhynchus latirostris) is in serious trouble. The big, rowdy flocks are now a distant memory, and a recent survey suggests population numbers in the Swan River region could be as low as just 5000 birds.

The WA government has listed these playful, intelligent birds as ''rare or likely to become extinct'' - and, as the birds only occur in WA, that makes them one of Australia's most seriously endangered birds.

The West Australian Conservation Council is vigorously lobbying for tougher laws to ban clearing of woodlands for forestry, farms and urban development. The council has also warned that loss of food trees has been so severe that the birds are in danger of starving. Cases of poisoning have also been reported, with starving cockatoos feeding on almond trees that are toxic to their system.

Perth filmmaker Leighton De Barros has spent five years researching and crafting a documentary on these endangered birds. The result, On A Wing and A Prayer, narrated by actor William McInnes, screens tomorrow on ABC 1 at 8.30pm. ''It's an extraordinary documentary,'' says Canberra scientist Denis Saunders, who has spent more than 40 years studying Carnaby's black cockatoo nesting sites in WA's wheat belt.

''It's not only a rare film that respects the science, but it's helped reveal new information about the species. There's some remarkable footage, taken inside a nesting hollow, that's shed a new light on nesting behaviour. It really is a wonderful film that genuinely breaks new ground. It deserves to become a classic.''

And for De Barros, an experienced filmmaker who's shot local footage for the BBC natural history unit and National Geographic, it's a departure from his usual ocean habitat as a marine cinematographer.

''Yeah, I'm normally out on the water, or in it, filming dolphins and whales,'' he says.

De Barros has been nominated for four Emmy Awards for his marine documentaries, including Ocean Giants and Dolphins of Shark Bay for the BBC. Remember the famous aqua-planing dolphin footage in the latter? That was De Barros at work.

So, why the switch to dry land? De Barros says he became fascinated by the cockatoos after meeting Rick Dawson, a wildlife officer working with the WA department of environment and conservation. They met while De Barros was filming a documentary on a WA government ocean patrol that cuts entangled whales loose from discarded fishing gear.

Dawson, a former naval policeman, was one of the whale patrollers and also involved in recovery research for Carnaby's black cockatoo.

''He inspired me to think about making a film because he was so passionate about these birds,'' says De Barros.

''It was completely new territory for me. I'd never made a documentary about birds, so it was a challenge to read up on all the research and plan the logistics of the shoot.There's a lot to think about and you have to get the timing right. Some behaviour is seasonal, so if your timing is out, then it means waiting another year to get footage.''

All up, the film took around five years to plan, film and edit. De Barros estimates it took ''maybe 12 to 15 research trips'' into the WA wheat belt to scope out locations of nesting hollows. He also worked with camera assistant and grip Dean Riegner ''a good mate who can build anything'', to design and build remotely-controlled cameras that could film inside the hollows. The result was ''about the size of a smart phone, but a bit thicker,'' he says.

''It took us a month to make and install them, and we were worried we wouldn't get them finished in time for the breeding season. But in the end, it all came together.''

De Barros asked Saunders - a former CSIRO chief research scientist - to act as the film's technical adviser, which meant advising on locations and sifting through post-production scripts and rushes to spot any potential science glitches. Saunders says he was impressed by the filmmaker's dogged insistence on getting the science right, and delighted by the footage obtained from the nesting hollows. That was an unexpected bonus, he says.

''When I started my research in 1969, we didn't have the technology that would allow us to film inside the hollows. And we couldn't just climb up a ladder to see what was happening, because that disturbs the birds.''

New behavioural information revealed by the film includes footage of the nesting female cockatoo gently rolling the eggs, possibly to stop the yolk sticking to the sides. The female is also filmed feeding a seed to the hatchling, and helping it emerge from the egg. There's also footage - not shown in the documentary - of a second hatchling being jostled by the chick when the mother leaves the nesting hollow. The second chick dies, and the footage suggests sibling rivalry could have deadly consequences.

''These are all new things we didn't know previously,'' says Saunders.

De Barros says a personal highpoint was capturing the cockatoo fledgling taking flight from the hollow. ''I was sweating on getting that, because once they fly out from the hollow, they don't go back. So if I'd missed it, that would have meant coming back next year.''

But the most confronting aspect of the film was participating in a covert operation with Dawson to film an orchardist illegally shooting Baudin's black cockatoos (a separate species, but once thought to be the same species as Carnaby's). De Barros says he found it ''a bit hard to take, filming such beautiful birds being shot, but I knew we had to do it to get evidence that such a terrible thing is still going on''.

Back in the 1970s, when Saunders was studying the cockatoos, they were shot as agricultural pests because the birds damaged trees in exotic pine plantations when they alighted to feed on the pine cones. Land clearing and urban sprawl have removed food trees and nesting hollows, and the birds have also become common roadkill casualties because they congregate to feed on roadside verges. The film highlights their plight, showing the work being done by local vets and community groups to treat and rehabilitate injured cockatoos.

De Barros is hopeful the film will find a sympathetic audience, both nationally and overseas.

''I'm pretty proud of it, and I hope it will make people think about our wildlife,'' he says. ''You can see plenty of good films about endangered wildlife in other countries, and it's easy to focus on what's happening overseas. But our wildlife is just as endangered, and we need to wake up to that before it's too late.''

■ On A Wing and A Prayer screens tomorrow night on ABC 1 at 8.30pm.

■ Rosslyn Beeby is science and environment reporter