<p>Long-term surveys for monitoring the demographics of a population can shed light on reproductive success and survival, as well as identify when changes may have occurred. For many species, collecting this data can be time consuming and labour intensive but for harvestable species we are able to collect this data more easily and use demographic patterns in the harvested population as a proxy for demographic changes in the overall population. Here, we use a 37-year dataset of harvested wildfowl wings to explore the demographic changes in two popular quarry species, Eurasian teal <i>Anas crecca</i> and Eurasian wigeon <i>Mareca penelope</i>. Generalised linear models revealed a significant decline in the reproductive index (juveniles per adult female) of Eurasian teal, which has halved over the study period alongside a decline in the proportion of females within the adult population. These declines are somewhat at odds with the long-term increasing wintering population trend within the northwest European flyway. Similarly, for Eurasian wigeon we found a significant decline in the proportion of adult females and reproductive index, the latter of which has dropped by a third. After notable wintering population declines across the northwest European flyway over the past decade, this population is now increasing and beginning to recover. Our results demonstrate that demographic changes observed in national datasets may not always align with flyway‑level population trends and express the need for collaborative, long-term monitoring of our migratory species across their flyway to better inform sustainable harvest.</p>