Scientists at the University of Illinois at Urbana-Champaign monitored bird species in a protected forest sanctuary in central Panama to determine if and how populations had changed from 1977 to 2020. There are very few long-term studies on tropical bird population trends and the report provides information on how the species copes with habitat loss and the climate crisis. A new study, published in the Proceedings of the National Academy of Sciences, found that most of the sample species had dwindled in abundance, many of them severely. Twice a year for four decades, the authors developed fog nets at multiple study sites, locating and capturing thousands of birds. The authors then modeled populations and calculated changes in the abundance of 57 species. Of the declining species sampled, 35 of the 40 lost more than 50% of their original abundance. Only two species increased in number. The reductions extended to different families of birds and were generally independent of ecological characteristics such as body mass, type of food search or the initial abundance of the species. According to the authors, creating decline and identifying the underlying ecological mechanisms should be a conservation priority. The scientists said that human activity is likely to lead to falls in numbers, with changing rainfall patterns, rising temperatures and deforestation causing stress to bird populations. They said: “Deforestation in the humid tropics accelerated by 62% between 1990 and 2010, resulting in a net loss of about 8 million hectares (20 hectares) during this time period and losses have continued to increase in recent years. “Rising temperatures are leading to upward shifts in tropical plain species. [taxonomic groups] Both theoretical and empirical evidence suggest that the tropics are more strongly influenced by their respective temperate regions. “Changing rainfall regimes in the tropics are also expected to greatly affect species distribution and population dynamics.” Another factor in the downturn could be the indirect impact of the climate emergency. The insects that birds feed on are sensitive to changes in temperature and rainfall, and drought as well as irregular rainfall can affect the seasonal availability of fruit and nectar. There are also concerns that climate change could benefit parasites that weaken birds. The scientists said the losses were “alarming”, with a number of species affected, including the red-hat mannequin, the most abundant fruit-eating bird sampled and a major seed disperser. In 1977 23 were identified, but in 2020 only nine were arrested. The researchers said that although rainforests are often thought to be “biodiversity reservoirs”, this study suggests that there are worrying reductions in the species’ populations. They concluded that more studies were needed, writing: “The next logical step towards understanding and possibly preventing further decline is to identify the underlying ecological mechanisms. “To achieve this, intensive, long-term studies of individual species will probably be needed to investigate the factors.”
