When Rachel Carson's book Silent Spring was published in 1962, it caused a national outcry and breathed life into the environment movement in the US.
The title was taken from a line in a Keats poem: "The sedge has withered from the lake, and no birds sing."
The book warned of the silence that would fall if the US kept spraying DDT, which was knocking out insects and birds at an alarming rate.
DDT was banned soon after, but the natural world is quieter today than we've ever known.
There are now 2.9 billion — about 29 per cent — fewer birds in North America than in 1970, according to research published on Friday in the journal Science.
And that's a trend across much of the globe that is going hand-in-hand with a rapid decline in insects.
Most of the worst-affected birds are insect eaters, according to the research.
A silent spring would have consequences for all life on earth, and we need to heed the warning, said researcher Mike Parr, president of the American Bird Conservancy.
"Birds are both predator and prey — buyers and sellers — and they are numerous in all ecosystems," he said.
"This isn't a good sign. We need to pay attention."
Like bees and other insects, birds pollinate our crops, distribute seeds, are food for predators, and perform myriad ecosystem services we're not yet aware of.
They help to regulate the environment, and even control many of the pests that impact agriculture.
"Everything in nature is connected — birds are abundant and highly diversified — so they are both good indicators of problems and, unfortunately, a problem themselves when they disappear," Mr Parr said.
His research team were shocked not only by the scale of bird loss they found in the US, but that even introduced bird species were disappearing.
"If we've created an environment that's so unwelcoming or toxic … even for those introduced species, that's pretty worrying," he said.
"Something is going on with the environment."
'How will we know until it's too late?'
Most research on the decline of birds and insects has focused on Europe and the US.
German research published in Nature in 2017 found that nearly 77 per cent of that country's insects have disappeared since 1989.
We know that some birds and insects are declining in Australia but we don't know if it's on a comparable scale, according to Maggie Watson from Charles Sturt University, who wasn't involved in the study.
"In Australia, who's going to notice that these declines are happening?" Dr Watson said.
"Because we're not doing the monitoring."
Many species of birds and insects go through boom-bust cycles, especially in Australia where rains are inconsistent.
So differentiating between a cyclical bust and an actual population collapse is difficult.
"We're seeing it with the Bogong moths right now. With the boom-bust scenario, how will we know until it's too late?" Dr Watson said.
Pesticides kill food, affect immune system
Although species extinctions tend to get a lot of attention, the researchers found that it's the really common bird species that have been slipping away under our noses.
Ninety per cent of the decline in the US has come from just 12 bird families; things like blackbirds, warblers, sparrows and finches.
The causes of the crashes vary between the United States, Europe and elsewhere, and researchers are only beginning to build enough data to offer reasons, Mr Parr said.
"Western Europe is becoming a biological desert from intensified agriculture," he said.
"There are several studies that link this to pesticides, but habitat loss and extreme levels of songbird hunting around the Mediterranean are also factors there.
"In the Americas we don’t see as much songbird hunting, but forest loss on wintering grounds to our south is running at 50 per cent in some areas."
Some waterbirds showed increased numbers in the US since 1970.
Ironically, they're the beneficiaries of land conversion in the southern US from grassland and forest to irrigated rice farms, according to Dr Watson.
Neonicotinoid pesticides have been implicated in colony collapse in bees, which has led to bans in regions of Europe.
US President Donald Trump has eased bans on some neonicotinoids introduced during the Obama presidency.
The stated purpose of insecticides is to kill insects, and they work. It's inevitable that birds that rely on those insects for food will also decline.
But there's increasing evidence that insecticides — neonicotinoids and organophosphates — are affecting birds more directly.
Often crop seeds are pre-coated in insecticide for planting, regardless of the presence of an insect threat.
Research published in Science last week found that migratory songbirds that eat those seeds at sub-lethal doses delay their migration, which can lower breeding success.
While the US somewhat regulates its use of neonicotinoids, many of their birds winter in parts of Central and South America, where regulations on pesticide use are even more lax, according to Dr Watson.
"Not only do we have issues with starvation because there are not enough insects, but also because there's a build-up of insecticides [in the birds] that destroys the immune system."
Forest clearing is also destroying migratory bird habitat across the southern border of the US.
In Australia, it's a similar story, according to Dr Watson.
"A lot of waterbirds are going downhill because of over extraction [of water]," she said.
"And we're destroying seabird habitat left right and centre. They built a golf course in Tasmania and destroyed thousands of seabird burrows, for example."
Around 12,000 mutton bird burrows were bulldozed for construction of a golf course on King Island off Tasmania in 2013.
Shifting Baseline Syndrome: boiling frogs
There's an apocryphal tale of a science experiment in which a live frog is dropped into a pot of boiling water.
The scalded frog leaps out the second it feels the heat.
A second frog is put into a pot of cool water, and the temperature is slowly increased.
It adjusts to the incremental increase in temperature, and sits in the pot until it's boiled alive.
The tale may be tall but the phenomena is real. In ecology it's called Shifting Baseline Syndrome.
According to researchers Masashi Soga and Kevin Gaston, it's "one of the fundamental obstacles to addressing a wide range of today's global environmental issues".
In essence, like the frog adjusting to the heat, we adjust our baselines of what we perceive to be "normal", over time.
The phenomenon was originally recognised in fisheries, said Global Ecology fellow Corey Bradshaw from Flinders University.
"The concept of shifting baselines is more of a psychological one," Professor Bradshaw said.
"So grandpa used to catch big whoppers, dad used to catch slightly smaller big whoppers, and the fish you call whoppers today, grandpa would have thrown back."
Shifting baselines happen across generations but also within our own lifetime.
"Because people tend to remember things that were more recent, there's a constant push toward downwardly biasing the trends of change," Professor Bradshaw said.
"We don't recognise changes in things unless they happen overnight — say a forest is bulldozed, we might speak up about that.
"But then your kids come along and to them that forest was always bulldozed."
For kids born today, having 3 billion fewer birds in the US is their baseline.
The same goes for other environmental problems, most notably climate change.
People who doubt the science of climate change will often say that we've always had droughts or we've always had heatwaves.
And they're right.
But though it might threaten our sense of identity, memories and personal experience aren't as reliable as we like to think they are.
Which is why having data over time is so important.
The data shows that the heatwaves are worse, droughts are longer and the climate is getting warmer than when we were kids.
The data offers us the chance to avoid suffering the fate of the doomed frog, Professor Bradshaw said.
"The accuracy and precision of your recollection tends to move and shift with your ideology," he said.
"That's the whole reason that science exists. Science is the pursuit of subjectivity-reduction, because objectivity doesn't exist."