By Paul Rincon
BBC News science reporter
Counting the nests of orang-utans from the air helps give very accurate estimates of the endangered great apes' numbers, an international team reports.
Orang-utans once ranged throughout southeast Asia
Marc Ancrenaz and colleagues combined aerial surveys from a helicopter and ground surveys to count ape nests in the Malaysian state of Sabah on Borneo.
They estimate that the entire orang-utan population in the state has dropped by 35% in the past 20 years.
The team reports its results in the open-access journal Plos Biology.
The latest findings suggest that there were around 11,000 orang-utans in Sabah at the time of the survey. Numbers had been expected to be lower, perhaps around 2,000.
More than 60% of orang-utans in Sabah live outside protected areas, the researchers tell the journal.
"It's more orang-utans than we thought. This increases the world population. But the orang-utan population 20 years ago was probably much more than 20,000," co-author Dr Benoît Goossens, of Cardiff University, UK, told the BBC News website.
"So even though it's more than we thought, [orang-utan numbers] are still decreasing on a bigger scale."
One flew over the orang nest
Densities of orang-utans and other great apes are usually obtained by counting nests from the ground.
Then, in order to obtain final estimates of ape population sizes, these densities are extrapolated to large forest blocks identified from maps as being suitable habitats for apes.
But, say the researchers, sample sizes are often quite small and these surveys do not take account of other factors which can affect orang-utan populations, such as poaching, logging and mining.
Orang-utan numbers are decreasing overall
"Ground surveys are very difficult to carry out, because it's not easy to cover vast areas in dense jungle," said Dr Goossens.
"It's easy in degraded forest to see nests from a helicopter. So we wanted to show that we could survey a population of great apes in a short space of time by doing an aerial survey.
"Renting the helicopter is expensive, but you save time, equipment and it works out at a lower cost in the end."
The team found that orang-utans were patchily distributed throughout their range in Sabah, occurring mainly in the eastern and central parts of the state.
Only two small, significant populations were found, in the Crocker Range National Park and Mount Kinabalu National Park.
The researchers said aerial surveys could be applied to making estimates of great ape numbers in Africa.
However, they add, it may be more difficult to detect the nests of African great apes because they tend to be lower in the canopy and it may be impossible to distinguish between gorilla and chimp nests.