Bird populations in a Central American tropical rainforest are suffering severe declines, with likely factors including climate breakdown and habitat loss.
Scientists from the University of Illinois tracked species of birds in a protected forest reserve in central Panama to determine if and how populations had changed from 1977 to 2020.
There are very few long-term studies on population trends for tropical birds, and the report provides insights into how species are coping with habitat loss and the climate crisis.
A new study, published in Proceedings of the National Academy of Sciences, found that the majority of sampled species had declined in abundance, many of them severely.
Twice a year over four decades, the authors deployed mist nets in multiple study sites, identifying and banding thousands of birds. Next, the authors modeled populations and estimated changes in the abundance of 57 species.
Of the declining species sampled, 35 out of 40 lost more than 50% of their initial abundance. Only two species increased in numbers.
The declines extended across different bird families and were generally independent of ecological traits such as body mass, foraging type, or the species’ initial abundance. According to the authors, establishing declines and identifying the underlying ecological mechanisms should be a conservation priority.
The scientists said human activity was likely to be driving the drops in numbers, with changing rainfall patterns, increasing temperatures and deforestation causing stress to bird populations. They said: “Deforestation in the humid tropics accelerated 62% between 1990 and 2010, resulting in a net loss of about 8m hectares (20m acres) during this time period, and losses have continued to accrue in recent years.
“Increasing temperatures are driving upslope range shifts of lowland tropical species across [taxonomic groups] and both theoretical and empirical evidence indicate that tropical biotas are more strongly affected than their temperate counterparts. Changing rainfall regimes in the tropics are also expected to strongly impact species’ distributions and population dynamics.”
Another factor in the decline could be the indirect impact of the climate emergency. The insects the birds feed on are sensitive to changing temperature and rainfall, and droughts as well as irregular rainfall can affect the seasonal availability of fruit and nectar. There are also concerns the change in climate could be benefitting parasites that weaken the birds.
The scientists said the losses were “alarming”, with a range of species affected, including the red-capped manakin, the most abundant fruit-eating bird sampled and an important seed disperser. In 1977, 23 were spotted, but in 2020, just nine were captured.
Researchers said that though tropical forests are often thought to be “reservoirs of biodiversity”, this study suggests there are worrying declines in species populations.
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.”