In North America and Europe, scientists have long warned bird numbers are falling. As cities and farms expand, forests around them become fragments, animal habitats shrink, pollution contaminates rivers, pesticides and fertilisers kill off insects. Even pets are a factor – in the US, domestic cats are killing up to an estimated 4 billion birds a year. Tiputini, however, is one of the few patches of the planet not directly feeling those pressures: no nearby farms, no polluting factories, no encroaching loggers, no roads in. Yet, their birds were dying.
At other remote sites around the world, scientists had been starting to observe similar trends. In Brazil, the Biological Dynamics of Forest Fragments Project (BDFFP) is an ecological study located deep in primary Amazon forest, unreachable by road. These regions hold some of the oldest living forests on the planet – they evaded the ice age events that remade forests in the US and Europe with the growth and retreat of glaciers. “In the Amazon, we’ve had pockets of stable forests over millions of years,” says ecologist Jared Wolfe, one of the project’s research scientists. “The site is truly amazing.”
But in 2020, when researchers there compared bird numbers with the 1980s, they found a number of species in deep decline. At another site in Panama, scientists working in a 22,000-hectare (54,000-acre) stretch of intact forest had been gathering bird data since the mid-1970s. By 2020, their numbers had gone off a cliff: 70% of species had declined, most of them severely; 88% had lost more than half their population. At some sites, scientists are beginning to observe “almost complete community collapse”, says Wolfe. “This is occurring in pristine environments, which is really unsettling.”
This week, Wolfe and collaborators published new work directly linking rising temperatures to bird declines. Their research, published in Science Advances, tracked birds living in the forest understory at the BDFFP against detailed climate data. They found that harsher dry seasons significantly reduced the survival of 83% of species. A 1˚C increase in dry season temperature would reduce the average survival of birds by 63%.
Exactly how the heat is causing bird numbers to decline is tricky to pinpoint, Wolfe says, but “these birds are intrinsically linked to small, small changes in temperature and precipitation”. One of the most immediate ways a heating planet hurts wildlife is by putting them out of step with their food sources: when fewer insects survive dry seasons, or leaves bloom and fruit ripens at different times, birds find themselves unable to forage and feed their young. Their nests begin to fail. Within a few generations, their numbers fall.
Most western conservation works by sectioning off wilderness, as national parks or reserves. These places are like arks: reservoirs of wildlife that we hope will be saved, even as people transform the land around them. But what the researchers were seeing with birds suggested that these arks are far more fragile than first thought.