This is a transcript from The World Today. The program is broadcast around Australia at 12:10pm on ABC Local Radio.

ELEANOR HALL: In Western Australia, the mystery surrounding the deaths of thousands of native birds could remain just that.

Around 4,000 birds were found dead near the south coast town of Esperance in December and January.

The Department of Environment has just completed its post-mortem examinations and has ruled out viruses or bacteria as the cause of death.

Now wildlife officials are planning to kill 15 healthy birds, to see if that can throw any light on what caused so many birds to suddenly drop from the sky.

In Perth, David Weber reports.

DAVID WEBER: The death toll includes wattle birds, honeyeaters, silvereyes, pigeons, seagulls and yellow-throated miners.

About 500 were found in one group, others were scattered around Esperance in the final weeks of last year.

The Department of Environment's Nature Protection Branch Manager, David Mell.

DAVID MELL: We've tested a range of toxins, which primarily are heavy metals and pesticides. There are a number of figures in there that are intriguing in terms of their levels. But what we don't know is whether those levels are normal for birds in that vicinity.

DAVID WEBER: There was a suggestion earlier that the deaths could have been caused by heavy metal contamination.

Can that now be ruled out?

DAVID MELL: It can't be entirely at this point in time, no.

DAVID WEBER: What about contamination into the food chain from algal blooms?

DAVID MELL: Yes, there's no evidence to suggest that algal blooms are, or similar events like botulism, have had an impact. You would expect to see an impact on waterfowl. That hasn't been the case. The principal group of birds that have been affected are birds that feed on nectar, but they also feed on insects.

DAVID WEBER: And those kinds of birds have been returning to Esperance in numbers.

DAVID MELL: Those kinds of birds, my understanding, have begun returning, so things like New Holland honeyeaters, silvereyes, red wattle birds and mynas, which are the birds that were mostly affected appear to be starting to move back into the area.

DAVID WEBER: The Department of Environment will collect more birds from around the Esperance area, and take samples.

It will compare them with those taken from the birds that died in December. It's hoped that this will determine what is a safe level of toxicity.

David Mell says he's never seen an event like this before. He says the deaths of about 200 tree martins in the wheatbelt town of Narembeen last month was also surprising.

But Mr Mell says the Department has ruled out any link.

DAVID MELL: What we've been able to do, we have an understanding of the biology of that particular species, and the coincidence of the sudden weather change is likely to be the cause of death of those birds and we're confident that the incident in Narembeen is not related to the Esperance incident.

DAVID WEBER: The deaths of the birds in Esperance came some time before the weather changed, because it preceded the cyclone by some several weeks.

DAVID MELL: That's right. So what we're saying is that the weather change, that is not one of the factors we are considering in terms of the bird deaths at Esperance and consequently we've also ruled out diseases - bird flu, a range of things. Histopathologies showed no evidence of viral or bacterial infections and so the area that we're focusing on at the moment is toxins.

DAVID WEBER: How confident are you that a cause will be able to be determined?

DAVID MELL: That's not possible for me to answer at this point in time because of the range of variables that are persistent at the moment.

ELEANOR HALL: That's the Western Australian Department of Environment's David Mell speaking to David Weber in Perth.