For 20 years our ‘bird counters’ have been flying into the remote Landsborough valley in South Westland to monitor the effects of sustained predator control on bird life. This work has contributed to a long-term picture of the health of the bird community in Landsborough Valley that includes more than a dozen native species. Christchurch-based Media Advisor Fiona Oliphant chats to scientist Colin O’Donnell and contractor Paul van Klink about the art of counting birds and the results of their work.
Our scientists set out to chart the recovery of forest birds in the Landsborough valley after trapping and aerial pest control began there in 1998. This large, forest-clad valley runs for 50 kilometres, from north to south, shadowing the Southern Alps/Kā Tiritiri o Te Moana before joining the mighty Haast River. Apart from rafters on guided trips and hunting parties, few people venture there.
The Landsborough has one of the most diverse native bird communities in Westland and is a stronghold for the rare mohua or yellowhead. But in the early 1990s bird numbers were seen to be steadily falling due to the regular spike in rat and stoat numbers following beech forest seeding. Possums had also made inroads.
It was assumed that controlling these pests would benefit the bird populations, but our scientists wanted to measure this response.
Each spring a team of four expert bird observers camp in the forest, in view of the Dechen Glacier, and spend fine days doing five-minute bird counts at regular points along set lines on one side of the river.
The five-minute bird count is the most commonly used bird counting method and gives an index of bird activity and relative bird numbers. It may sound easy but it’s demanding work, says Colin O’Donnell, who leads the programme and has done about half of the annual surveys.
“Each observer must identify the calls of all individual birds in the forest around them in a five-minute period and record this on a notepad.”
“They need to be able to distinguish the range of calls each species makes as well as differences between male and female birds.”
Birds are most vocal at dawn, but the observers start at nine in the morning to avoid being drowned in bird sound. Together they do 175 bird counts, which represents more than 14 hours of concentrated listening.
Paul van Klink has been counting birds in the Landsborough for 19 out of the last 20 years, the lure of this “legendary valley” drawing him back each time. The challenge when counting, Paul says, is to be truly in the moment; tuning in to every bird call in the area around you. It can be intense and takes a while to come back into the zone when you start out, he says.
“Some days—when it’s cloudy and not too hot—the birds are really going for it and it’s difficult to work out who’s who and where they are.”
In high counts there might be 30-40 individual birds identified within a 5-minute period.
Paul says he’s a “conservative” counter, which Colin confirms. But it’s consistency that’s important, which is why the same four observers have been used to carry out 90% of the bird counts.
The results of this painstaking monitoring are impressive. Overall, native bird numbers have doubled since 1998 when the first trapping and aerial 1080 operation began.
In this time the predator-sensitive mohua—the ‘canary of the coalmine’—has steadily increased in the monitoring area from 14 birds to 338. Prior to pest control, this vulnerable tree hole-nester, had declined significantly in the valley.
More common birds—tuī, bellbird, brown creeper, rifleman, grey warbler and kākāriki—are also on an upward trend.
Other species—kākā, fantail, tomtit and kereru—have stayed stable.
Two native species—silvereye and the migratory long-tailed cuckoo—have declined, although it’s not sure why. Silvereyes may be being outcompeted for food by tuī and bellbirds, which have swelled in numbers.
Interestingly, introduced birds like chaffinch, blackbird and redpoll finch have also declined, again perhaps due to competition for food from more native birds.
The results are heartening and show the benefits to this native bird community of decades of careful and consistent predator management, says Colin.
“But the exciting thing is that we don’t know how numerous the bird populations will get.
“Each year I look forward to analysing the results and seeing how they contribute to the trends.”