Birds currently inhabiting many territories across Africa, Latin America and Asia are, on average, considerably smaller than those that predominated in 1940. This is the conclusion of an international study led by the Institute of Environmental Science and Technology at the Universitat Autònoma de Barcelona (ICTA-UAB), which documents—drawing on the collective ecological memory of ten Indigenous Peoples and local communities—a reduction of up to 72% in the mean body mass of the bird species present in their territories between 1940 and 2020.

The research is based on a globally coordinated survey involving 1,434 adult participants from 10 place-based communities across three continents. In total, the team compiled 6,914 unique bird reports corresponding to 283 bird species, comparing the bird species most commonly reported during participants’ childhood with those currently reported in their territories.

The analysis reveals a consistent pattern: large-bodied bird species have progressively disappeared from local environments, being replaced by smaller-bodied species. Whereas in the 1940s the mean body mass of reported bird species exceeded 1,500 grams, by the 2020s the average stands at around 535 grams. Overall, statistical models point to a decline of approximately 72% in the mean body mass of the bird assemblages observed by these communities over eight decades.

According to Álvaro Fernández-Llamazares, senior researcher at ICTA-UAB and lead author of the study, this shift may reflect both the local extinction of large-bodied species—more vulnerable to hunting, habitat loss and infrastructure development—and profound social transformations that have altered the relationships between communities and their local ecologies.

The study also underscores that the global avian extinction crisis, widely documented in scientific literature, is equally perceptible in the collective memory of communities with long-term, place-based connections to their territories. Rather than treating Indigenous and local knowledge as merely complementary data, “we advocate for a respectful and equitable dialogue between scientific and Indigenous knowledge systems, recognizing their inherent value and their potential to strengthen biodiversity policy and conservation practice,” says Dr Fernández-Llamazares.

The loss of large-bodied birds has not only ecological implications—many play key functional roles in ecosystems—but also cultural ones, as these species form part of the identity, memory and traditional practices of numerous communities worldwide.
The study demonstrates that biodiversity change is recorded not only in scientific datasets, but also in the lived experience of people who have maintained close relationships with the nature in their territories across generations.