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Bird Populations Declining in Tropical Forest, Study Finds

Loss of birds from any habitat can threaten the integrity of the entire ecosystem, according to researchers.

Tropical Forest

Bird populations in a Central American rainforest are declining dramatically, with potential causes including climate degradation and habitat loss.

To determine how populations have changed between 1977 and 2020, scientists from the University of Illinois tracked bird species in a protected forest in central Panama.

A new study published in the Proceedings of the National Academy of Sciences has found that specimens of most species have declined significantly, many severely.

Twice a year for four decades, the researchers set up mist nets at several study sites, identifying and banding thousands of birds. The researchers modelled populations and estimated changes in the abundance of 57 species.

Of the 40 threatened species sampled, 35 have lost more than 50% of their initial abundance. Only two species have increased in number.

‘It was very surprising,’ said study author Henry Pollock at the Department of Natural Resources and Environmental Sciences (NRES) of the University of Illinois.

‘Many of these are species you would expect to be doing fine in a 22,000-hectare national park that has experienced no major land-use change for at least 50 years.’

Co-author Jeff Brawn, who has investigated birds at Parque Nacional Soberanía for more than 30 years, called the findings ‘concerning’.

‘This is one of the longest, if not the longest, study of its kind in the Neotropics,’ Brawn said.

‘Of course, it’s only one park. We can’t necessarily generalise to the whole region and say the sky is falling, but it’s quite concerning.’

Another reason for the drop could be an indirect effect of the climate emergency. Insects that birds eat are sensitive to changes in temperature and rainfall, and drought and erratic rainfall can affect the seasonal availability of fruit and nectar. There are also fears that climate change will benefit parasites that weaken birds.

Scientists said the damage was “horrendous”, affecting several species including the red-headed manakin, the most frugivorous bird specimen and extensive seed dispersal. In 1977, 23 were detected, but in 2020 only nine were caught.

The researchers said that rainforests are often seen as “reservoirs of biodiversity”, suggesting a worrying decline in the species’ population.

They concluded more studies are needed, writing: “The next logical step toward understanding and possibly preventing further declines is identifying the underlying ecological mechanisms. To accomplish this, intensive, long-term studies of individual species will likely be needed to drill down to the factors.”

As the world is getting warmer, India is also facing a decline in the bird population. According to the State of India’s birds 2020 report, the first comprehensive study of its kind, made two assessments: the drop in bird population over the last 25 years, and over the last five years.

“In the long-term trend assessment, there was appropriate data available only for 261 species, of which 52% had declined [in numbers]. For current trends, there was data only for 146 species, of which [numbers of] nearly 80% were declining,” said MD Madhusudan, co-founder of Nature Conservation Foundation.

It’s based on more than 10 million observations, drawn from sightings and meticulous notes made by professional birdwatchers.

The data was then collated on eBird, a global crowdsourced database that has real-time data on the distribution and abundance of birds.

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