When Leslie Kelly heard an unusual bird call in her western Sydney garden, it was a delightful surprise to find it coming from a critically endangered bird released hundreds of kilometres away.

Key points: Three hundred birds have been part of the regent honeyeater captive release program, but researchers are still unsure of the path the birds are taking

Three hundred birds have been part of the regent honeyeater captive release program, but researchers are still unsure of the path the birds are taking Bird 'backpacks' with satellite tracking tech weighing just two grams are about to be deployed

Bird 'backpacks' with satellite tracking tech weighing just two grams are about to be deployed Researchers needed to wait for transmitter technology to develop to fit a bird which weighs between 35 to 50 grams

The sighting of a male regent honeyeater earlier this month, nestled among backyard grevillea and callistemon at Oxley Park, is another promising sign of recovery for the species being closely watched following a captive release program in north-east Victoria.

A photo of the Oxley Park bird capturing banding on its legs shows it had travelled at least 463 kilometres since its 2017 release in the Chiltern-Mount Pilot National Park.

This bird now boasts the fourth longest movement recorded for the captive release program, of which over 300 birds have been part of.

But researchers are still unsure of the path these small birds are taking to achieve such long journeys.

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Watch Duration: 1 minute 14 seconds 1 m 14 s A behind-the-scenes look at the regent honeyeater breeding program (Photo supplied: Mick Roderick) ( Suppied: Taronga Conservation Society )

Satellite 'backpacks' for birds

Conservationists are now hoping to use cutting-edge technology to track the flight of the regent honeyeaters as early as this year.

The release program currently relies on 'colour banding' to identify the birds when they are spotted in the wild.

While it is successful in identifying the length of their journey, BirdLife Australia's national regent honeyeater recovery coordinator Dean Ingwersen said there were still lots of questions that needed to be answered.

"At the moment we don't know how they get from point A to point B, [or] whether they travel in a straight line, or have important stop over points along the way there," he said.

"One of the issues we've got is we've worked on this species for a couple of decades now as a recovery team and we have a gap every year.

"From January through until March and April they just disappear. We don't know where they go.

A satellite tracker fitted to a mounted specimen — not a live bird — with researchers hoping to be able to better monitor the endangered species soon. ( Supplied: Dean Ingwersen )

"One of our concerns is we missing really important parts of the landscape we should be looking after or managing to make sure they're safe."

The species' numbers have been estimated to be as low as 400 in the wild due to the clearing of their woodland habitat, among other threats.

But developments in technology now mean conservationists can start following the birds using satellite tracking technology.

With it, the birds can be traced as they move through the landscape on a daily basis, allowing researchers to follow them in real time.

A satellite transmitter is specially designed using a 3D printed plastic harness which is placed on the bird like a backpack.

"So if you imagine a person wearing a backpack with straps over their shoulders, we do basically the same thing," Mr Ingerwesen said.

"The transmitter sits in the middle of the bird's back and the harness goes over the head and across the breast.

"It's a good centre of gravity when they fly and it doesn't load the wings too much, so it doesn't impact on their ability to fly."

The fit has been tested on a model specimen, and aviary trials are about to go ahead with live regent honeyeaters at Taronga Zoo.

Small birds, big tech hurdles

Until just two years ago, the smallest transmitter available for tracking birds weighed five grams — too heavy for smaller species like the regent honeyeater.

The new design has been scaled back to weigh just two grams, and the practice is being controlled by strict ethical procedures.

"There's a five per cent limit in which you're not allowed to put something on a bird, so two grams is just under the threshold for a regent honeyeater," Mr Ingerwesen said.

A tracking device small enough to fit on the regent honeyeater is being tested on the back of a mounted specimen. ( Supplied: Dean Ingwersen )

"That's why we've had to redesign the harness attachment to create a harness that's as light as possible to get us under that safe figure.

"Even having done that we will be limited to larger, heavier birds we can find. So predominantly males that we'll get in the landscape."

Most of the satellite technology being used for tracking bird movement in Australia currently monitor migratory shore birds such eastern curlews and oriental plovers, which are bigger species able to be fitted with larger transmitters.

Finding a transmitter to fit a regent honeyeater, which weigh between 35 to 50 grams when mature, has taken longer.

Conservationists are now hoping their equipment will take flight on a captive release bird this year, with remaining trackers eventually being placed on wild honeyeaters.

The team have five transmitters that are manufactured in North America and funded by the Australian Threatened Species Recovery Fund, and are worth around $6,000 each.