Tropical bird populations are rapidly shrinking - and now scientists know why

Tropical bird populations have fallen hard in recent decades. A new analysis reports that human-driven heat extremes cut tropical bird abundance by 25 to 38 percent between 1950 and 2020 when compared to a world without warming.

The effects ripple far beyond any single forest. Tropical forests hold the largest share of the world’s bird diversity, with research indicating that about 72 percent of bird species live in these ecosystems.

Tropical bird declining numbers

To investigate, Maximilian Kotz worked with colleagues at the Barcelona Supercomputing Center (BSC) and the Potsdam Institute for Climate Impact Research (PIK), alongside James E. M. Watson and Tatsuya Amano at the University of Queensland (UQ).

The team coupled long term bird population records from the Living Planet Index with daily weather records to test how temperature spikes and rainfall shifts affect annual changes in abundance.

The researchers used ERA5, a global reanalysis that blends observations and models to reconstruct historical weather, to ensure consistent daily data across decades.

They analyzed the data to determine how much of the extreme heat exposure could be traced directly to human-driven warming. “As far as we are aware, this is the first animal climate attribution study,” said Kotz.

Across the tropics, the frequency of dangerously hot days has surged. Birds there now experience roughly ten times as many extreme heat days as they did several decades ago – a shift that lines up with the observed declines.

Heat kills tropical birds

Birds maintain high body temperatures and have tight thermal limits. When air temperatures spike, the path to hyperthermia is short, and the main emergency response is evaporative cooling through rapid breathing and related behaviors that trade water for heat loss.

That trade-off gets riskier as hot periods lengthen. Evidence shows that humidity pushes bodies toward lethal overheating faster, shrinking the margin for survival during hot spells in warm climates.

Heat stress also carries delayed costs. Birds that avoid immediate mortality can leave the breeding season lighter and less fit, which reduces nesting success and the number of young that reach independence.

These mechanisms line up with the new estimates. The analysis found that short bursts of extreme heat, not just small changes in seasonal averages, were the strongest predictors of reduced annual growth in tropical bird populations.

Tropical birds in protected areas

For years, biologists reported declines in seemingly intact tropical forests without a clear culprit. In central Amazonia, long term sampling reveals large falls in ground and near ground birds since the early 1980s, even in undisturbed sites.

In Panama’s protected Barro Colorado Island, a century of data shows a persistent erosion of tropical bird diversity. This trend cannot be chalked up to recent habitat destruction within the reserve.

These records are consistent with the new study’s estimates. The magnitude of decline in some intact sites resembles the size of the heat-attributed reductions reported for tropical populations overall.

The link does not claim a single cause for every place. It shows that the intensification of heat extremes is a powerful driver in regions where most birds already live near their thermal limits.

Heat damage beats other threats

Disentangling climate from direct human pressures has been hard. Here, the attribution framework indicates that in the observed tropical populations, the cumulative impact of heat extremes driven by climate change has been larger than direct human pressures across the same time window.

That does not minimize the damage from land clearing, logging, or mining. It highlights a second, already active threat that operates even when forests still stand.

The method distinguishes between average warming and extremes. The signal points to spikes of heat as the primary stressor for bird abundance in lower latitude regions.

This finding matters for planning and forecasting. Management that only tracks mean temperatures will understate the risk posed by a cluster of very hot days to small, localized populations.

Protecting tropical birds from heat

The IPCC’s latest assessment demonstrates that climate change is already a major risk to ecosystems and biodiversity, with escalating losses expected without rapid emissions cuts and stronger adaptation.

Reducing greenhouse gas emissions will blunt the rise in extreme heat that the new analysis identifies as the main driver of tropical bird declines. That is the foundation for any long term recovery of abundance in the hottest regions.

Ultimately, protecting tropical birds means preserving the forests they depend on and reducing the heat they cannot escape.

“On the conservation side, this work tells us that in addition to protected areas and stopping deforestation, we urgently need to look into strategies for species who are more vulnerable to heat extremes to maximize their adaptation potential,” said Amano.

“That might mean ex-situ conservation work, so working with some populations in other locations.”

The study is published in the journal Nature Ecology & Evolution.

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