The United Kingdom is an important wintering ground for millions of waterbirds. Most U.K. wintering waterbird populations increased between 1970 and the mid-1990s, but declined thereafter. We examined U.K. population indices in 46 wintering waterbird species in two 25-year periods, 1970–1994 and 1995–2019, to identify which waterbird groups were vulnerable to declines. We modeled abundance to examine whether spatial patterns in population change were consistent with changes in habitat quality, climate, or phenology. We used the resulting model outputs to test whether there were patterns in these relationships across all monitored wintering waterbirds and within groups based on ecology, migratory strategy, and breeding location. Waterbird populations in the United Kingdom increased from 1970 to 1994, particularly in estuaries, but then showed mixed trends from 1995 to 2019: geese and swan populations increased, and waders declined. Wader declines were least apparent, although still declining, in estuaries and most apparent in reservoirs. Increased breeding season temperatures were linked to population increases for most species in both periods, although there was no beneficial impact of climate warming for migrants from 1995 to 2019. We found no evidence that apparent declines were due to phenological changes. These results highlight the need for more research into why U.K. wintering waterbird trends have become more negative over the past 25 years.