Indigenous Peoples and local communities report a consistent decline in the body mass of birds across three continents

Increasing evidence shows that genuine collaboration between scientists and Indigenous Peoples and local communities can deepen global understanding of species’ ecological distribution ranges, baselines and trends. In this study, we explore trends in bird body mass as reflected in the collective biocultural memory of 10 place-based communities on three continents. To do so, we conducted a globally coordinated survey, asking 1,434 adult participants about the most common bird species around their territories both at present and during their childhood. The survey resulted in 6,914 unique bird reports, corresponding to 283 bird species and covering an 80-year period (1940–2020). By combining our ethno-ornithological dataset with scientific data on species’ body mass, we assessed whether the composition of locally abundant bird assemblages has shifted towards smaller-bodied species over time. Our results show a general shift in the bird species observed over the participants’ lifetimes, with recent species assemblages being composed of species with smaller body sizes than those in the past. Despite variation among sites, we find an overall statistically significant body mass reduction of 72% across all sites over 80 years. This work illustrates that the depth of the current avian extinction crisis, which has been well documented by scientists, is also widely acknowledged by Indigenous Peoples and local communities through their deep ecological knowledge and place-based observations. It highlights the substantial benefits of establishing meaningful collaborations across different knowledge systems to increase the evidence basis that underpins biodiversity policy and practice.