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. The co-creation of knowledge between science-based wildlife monitoring techniques and local ecological knowledge can significantly improve biodiversity monitoring compared to scientific methods alone. In this study, we explore trends in bird body mass as reflected in the collective biocultural memory of 10 distinct place-based communities from three different continents. To do so, we conducted a globally coordinated survey asking 1434 adult participants about the most common bird species around their territories both at present and during their childhood. Such survey resulted in 6914 unique bird reports, corresponding to 283 different bird species and covering an 80-year period (1940-2020). Complementing this ethno-ornithological dataset with scientific data on bird species body mass, we assessed if the assemblages of bird species locally perceived to be most abundant have experienced compounded changes in body size 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 the last 80 years. This work illustrates that the depth of the current avian extinction crisis, which has been thoroughly documented by scientists, is also widely acknowledged by Indigenous Peoples and local communities through their local ecological knowledge. It highlights the substantial benefits of establishing meaningful collaborations across different knowledge systems to increase the evidence basis that underpins biodiversity knowledge, policy and practice.