In the wake of recent studies showing widespread declines in rainforest bird populations, research in one part of the Amazon offers more optimistic news.
A 97-ha plot of the Amazon in south-eastern Peru was visited in 2018 to repeat the methodologies of a 1982 research trip to the same land. The aim was to shed light on the changes to the bird community over nearly four decades.
Covering intact and mature Amazionian floodplain forest at the Cocha Cashu Biological Station, the groundbreaking original survey work documented the structure of the area's bird community for the first time, when Neotropical bird identification was still a rapidly growing area of study. The area has remained untouched ever since.
Black-capped Donacobius is among the 379 species encountered at the plot during the census visits (Lukasz Pulawski).
The return visit to the area in 2018 repeated the exact same methodologies, allowing scientists to estimate population sizes across the whole bird community. These included spot-mapping, mist-netting, counting colonial birds and plotting the ranges of mixed-species flocks.
This work revealed that although two foraging guilds (groups of bird species that share feeding habitats) had declined over the 36 years, five foraging guilds had seen increases and a further seven had remained stable.
A total of 379 species were recorded in the plot either during the original census or the repeat visit, though 104 of these were not included in analyses because they were not encountered through formal survey methods.
This contrasts with findings published in 2022 from primary forest in Panama, where most species were in decline, and the majority of declining species had crashed by more than half in 44 years. Similar findings have been made in Tiputini Biodiversity Station in Ecuador.
At Cocha Cashu Biological Station, it was the larger and non-social species, river-edge specialists, insectivores, arboreal frugivores and raptors that showed increases. On the other hand, declines were found in canopy-flocking species and seed-eaters meant that the researchers had proven significant turnover in the overall bird community. Intriguingly, understorey-flocking species remained stable, perhaps due to a lesser reliance on the fruiting parts of a tree than birds that flock in the canopy.
Nectarivores, such as Fork-tailed Woodnymph, have remained stable at the study site (Josh Jones).
It came as a surprise to the researchers that insectivores, known to be particularly vulnerable to declines at both untouched and fragmented sites, were among the guilds showing an increase since 1982. It is thought that the unpredictable nature of food sources for seed-eaters may have driven the declines seen in granivores, and it is possible that these species may bounce back as food availability changes, or may simply have travelled beyond the plot to find food.
The trends contrasted with the recent results from Panama and Ecuador, both of which broadly witnessed declines, in eight out of 10 ecological guilds. Only terrestrial frugivores and aquatic species remained stable in Panama, Ecuador and at Cocha Cashu.
It could be that large reserves, like Manu National Park, continue to offer a defence against some of the global pressures on forest bird communities, the researchers suggested. Manu has the advantage of being a floodplain forest, buffering the effect of reduced rainfall on the water table and armouring its micro-environmental conditions. Drier precipitation patterns are thought to have contributed to declines in 'tierra firme' forests elsewhere.
To explain the apparent resilience of the bird community at Cocha Cashu, the researchers said that evaluating forest structural changes and gathering long-term data will be vital.
Reference
Martínez, A, Ponciano, J M, et al. 2023. The structure and organisation of an Amazonian bird community remains little changed after nearly four decades in Manu National Park. Ecology Letters. DOI: https://doi.org/10.1111/ele.14159
